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diff --git a/contrib/perl5/pod/perlfunc.pod b/contrib/perl5/pod/perlfunc.pod deleted file mode 100644 index e959abc..0000000 --- a/contrib/perl5/pod/perlfunc.pod +++ /dev/null @@ -1,5885 +0,0 @@ -=head1 NAME - -perlfunc - Perl builtin functions - -=head1 DESCRIPTION - -The functions in this section can serve as terms in an expression. -They fall into two major categories: list operators and named unary -operators. These differ in their precedence relationship with a -following comma. (See the precedence table in L<perlop>.) List -operators take more than one argument, while unary operators can never -take more than one argument. Thus, a comma terminates the argument of -a unary operator, but merely separates the arguments of a list -operator. A unary operator generally provides a scalar context to its -argument, while a list operator may provide either scalar or list -contexts for its arguments. If it does both, the scalar arguments will -be first, and the list argument will follow. (Note that there can ever -be only one such list argument.) For instance, splice() has three scalar -arguments followed by a list, whereas gethostbyname() has four scalar -arguments. - -In the syntax descriptions that follow, list operators that expect a -list (and provide list context for the elements of the list) are shown -with LIST as an argument. Such a list may consist of any combination -of scalar arguments or list values; the list values will be included -in the list as if each individual element were interpolated at that -point in the list, forming a longer single-dimensional list value. -Elements of the LIST should be separated by commas. - -Any function in the list below may be used either with or without -parentheses around its arguments. (The syntax descriptions omit the -parentheses.) If you use the parentheses, the simple (but occasionally -surprising) rule is this: It I<looks> like a function, therefore it I<is> a -function, and precedence doesn't matter. Otherwise it's a list -operator or unary operator, and precedence does matter. And whitespace -between the function and left parenthesis doesn't count--so you need to -be careful sometimes: - - print 1+2+4; # Prints 7. - print(1+2) + 4; # Prints 3. - print (1+2)+4; # Also prints 3! - print +(1+2)+4; # Prints 7. - print ((1+2)+4); # Prints 7. - -If you run Perl with the B<-w> switch it can warn you about this. For -example, the third line above produces: - - print (...) interpreted as function at - line 1. - Useless use of integer addition in void context at - line 1. - -A few functions take no arguments at all, and therefore work as neither -unary nor list operators. These include such functions as C<time> -and C<endpwent>. For example, C<time+86_400> always means -C<time() + 86_400>. - -For functions that can be used in either a scalar or list context, -nonabortive failure is generally indicated in a scalar context by -returning the undefined value, and in a list context by returning the -null list. - -Remember the following important rule: There is B<no rule> that relates -the behavior of an expression in list context to its behavior in scalar -context, or vice versa. It might do two totally different things. -Each operator and function decides which sort of value it would be most -appropriate to return in scalar context. Some operators return the -length of the list that would have been returned in list context. Some -operators return the first value in the list. Some operators return the -last value in the list. Some operators return a count of successful -operations. In general, they do what you want, unless you want -consistency. - -An named array in scalar context is quite different from what would at -first glance appear to be a list in scalar context. You can't get a list -like C<(1,2,3)> into being in scalar context, because the compiler knows -the context at compile time. It would generate the scalar comma operator -there, not the list construction version of the comma. That means it -was never a list to start with. - -In general, functions in Perl that serve as wrappers for system calls -of the same name (like chown(2), fork(2), closedir(2), etc.) all return -true when they succeed and C<undef> otherwise, as is usually mentioned -in the descriptions below. This is different from the C interfaces, -which return C<-1> on failure. Exceptions to this rule are C<wait>, -C<waitpid>, and C<syscall>. System calls also set the special C<$!> -variable on failure. Other functions do not, except accidentally. - -=head2 Perl Functions by Category - -Here are Perl's functions (including things that look like -functions, like some keywords and named operators) -arranged by category. Some functions appear in more -than one place. - -=over 4 - -=item Functions for SCALARs or strings - -C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, -C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, -C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> - -=item Regular expressions and pattern matching - -C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> - -=item Numeric functions - -C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, -C<sin>, C<sqrt>, C<srand> - -=item Functions for real @ARRAYs - -C<pop>, C<push>, C<shift>, C<splice>, C<unshift> - -=item Functions for list data - -C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> - -=item Functions for real %HASHes - -C<delete>, C<each>, C<exists>, C<keys>, C<values> - -=item Input and output functions - -C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, -C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, -C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, -C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, -C<warn>, C<write> - -=item Functions for fixed length data or records - -C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> - -=item Functions for filehandles, files, or directories - -C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, -C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, -C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, -C<unlink>, C<utime> - -=item Keywords related to the control flow of your perl program - -C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, -C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> - -=item Keywords related to scoping - -C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use> - -=item Miscellaneous functions - -C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>, -C<scalar>, C<undef>, C<wantarray> - -=item Functions for processes and process groups - -C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, -C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, -C<times>, C<wait>, C<waitpid> - -=item Keywords related to perl modules - -C<do>, C<import>, C<no>, C<package>, C<require>, C<use> - -=item Keywords related to classes and object-orientedness - -C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, -C<untie>, C<use> - -=item Low-level socket functions - -C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, -C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, -C<socket>, C<socketpair> - -=item System V interprocess communication functions - -C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, -C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> - -=item Fetching user and group info - -C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, -C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, -C<getpwuid>, C<setgrent>, C<setpwent> - -=item Fetching network info - -C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, -C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, -C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, -C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, -C<setnetent>, C<setprotoent>, C<setservent> - -=item Time-related functions - -C<gmtime>, C<localtime>, C<time>, C<times> - -=item Functions new in perl5 - -C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, -C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>, -C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, -C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> - -* - C<sub> was a keyword in perl4, but in perl5 it is an -operator, which can be used in expressions. - -=item Functions obsoleted in perl5 - -C<dbmclose>, C<dbmopen> - -=back - -=head2 Portability - -Perl was born in Unix and can therefore access all common Unix -system calls. In non-Unix environments, the functionality of some -Unix system calls may not be available, or details of the available -functionality may differ slightly. The Perl functions affected -by this are: - -C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, -C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, -C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, -C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, -C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, -C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, -C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, -C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, -C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, -C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, -C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, -C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, -C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, -C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>, -C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>, -C<utime>, C<wait>, C<waitpid> - -For more information about the portability of these functions, see -L<perlport> and other available platform-specific documentation. - -=head2 Alphabetical Listing of Perl Functions - -=over 8 - -=item I<-X> FILEHANDLE - -=item I<-X> EXPR - -=item I<-X> - -A file test, where X is one of the letters listed below. This unary -operator takes one argument, either a filename or a filehandle, and -tests the associated file to see if something is true about it. If the -argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. -Unless otherwise documented, it returns C<1> for true and C<''> for false, or -the undefined value if the file doesn't exist. Despite the funny -names, precedence is the same as any other named unary operator, and -the argument may be parenthesized like any other unary operator. The -operator may be any of: -X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> -X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> - - -r File is readable by effective uid/gid. - -w File is writable by effective uid/gid. - -x File is executable by effective uid/gid. - -o File is owned by effective uid. - - -R File is readable by real uid/gid. - -W File is writable by real uid/gid. - -X File is executable by real uid/gid. - -O File is owned by real uid. - - -e File exists. - -z File has zero size (is empty). - -s File has nonzero size (returns size in bytes). - - -f File is a plain file. - -d File is a directory. - -l File is a symbolic link. - -p File is a named pipe (FIFO), or Filehandle is a pipe. - -S File is a socket. - -b File is a block special file. - -c File is a character special file. - -t Filehandle is opened to a tty. - - -u File has setuid bit set. - -g File has setgid bit set. - -k File has sticky bit set. - - -T File is an ASCII text file. - -B File is a "binary" file (opposite of -T). - - -M Age of file in days when script started. - -A Same for access time. - -C Same for inode change time. - -Example: - - while (<>) { - chomp; - next unless -f $_; # ignore specials - #... - } - -The interpretation of the file permission operators C<-r>, C<-R>, -C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode -of the file and the uids and gids of the user. There may be other -reasons you can't actually read, write, or execute the file. Such -reasons may be for example network filesystem access controls, ACLs -(access control lists), read-only filesystems, and unrecognized -executable formats. - -Also note that, for the superuser on the local filesystems, the C<-r>, -C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 -if any execute bit is set in the mode. Scripts run by the superuser -may thus need to do a stat() to determine the actual mode of the file, -or temporarily set their effective uid to something else. - -If you are using ACLs, there is a pragma called C<filetest> that may -produce more accurate results than the bare stat() mode bits. -When under the C<use filetest 'access'> the above-mentioned filetests -will test whether the permission can (not) be granted using the -access() family of system calls. Also note that the C<-x> and C<-X> may -under this pragma return true even if there are no execute permission -bits set (nor any extra execute permission ACLs). This strangeness is -due to the underlying system calls' definitions. Read the -documentation for the C<filetest> pragma for more information. - -Note that C<-s/a/b/> does not do a negated substitution. Saying -C<-exp($foo)> still works as expected, however--only single letters -following a minus are interpreted as file tests. - -The C<-T> and C<-B> switches work as follows. The first block or so of the -file is examined for odd characters such as strange control codes or -characters with the high bit set. If too many strange characters (>30%) -are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file -containing null in the first block is considered a binary file. If C<-T> -or C<-B> is used on a filehandle, the current stdio buffer is examined -rather than the first block. Both C<-T> and C<-B> return true on a null -file, or a file at EOF when testing a filehandle. Because you have to -read a file to do the C<-T> test, on most occasions you want to use a C<-f> -against the file first, as in C<next unless -f $file && -T $file>. - -If any of the file tests (or either the C<stat> or C<lstat> operators) are given -the special filehandle consisting of a solitary underline, then the stat -structure of the previous file test (or stat operator) is used, saving -a system call. (This doesn't work with C<-t>, and you need to remember -that lstat() and C<-l> will leave values in the stat structure for the -symbolic link, not the real file.) Example: - - print "Can do.\n" if -r $a || -w _ || -x _; - - stat($filename); - print "Readable\n" if -r _; - print "Writable\n" if -w _; - print "Executable\n" if -x _; - print "Setuid\n" if -u _; - print "Setgid\n" if -g _; - print "Sticky\n" if -k _; - print "Text\n" if -T _; - print "Binary\n" if -B _; - -=item abs VALUE - -=item abs - -Returns the absolute value of its argument. -If VALUE is omitted, uses C<$_>. - -=item accept NEWSOCKET,GENERICSOCKET - -Accepts an incoming socket connect, just as the accept(2) system call -does. Returns the packed address if it succeeded, false otherwise. -See the example in L<perlipc/"Sockets: Client/Server Communication">. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptor, as determined by the -value of $^F. See L<perlvar/$^F>. - -=item alarm SECONDS - -=item alarm - -Arranges to have a SIGALRM delivered to this process after the -specified number of seconds have elapsed. If SECONDS is not specified, -the value stored in C<$_> is used. (On some machines, -unfortunately, the elapsed time may be up to one second less than you -specified because of how seconds are counted.) Only one timer may be -counting at once. Each call disables the previous timer, and an -argument of C<0> may be supplied to cancel the previous timer without -starting a new one. The returned value is the amount of time remaining -on the previous timer. - -For delays of finer granularity than one second, you may use Perl's -four-argument version of select() leaving the first three arguments -undefined, or you might be able to use the C<syscall> interface to -access setitimer(2) if your system supports it. The Time::HiRes module -from CPAN may also prove useful. - -It is usually a mistake to intermix C<alarm> and C<sleep> calls. -(C<sleep> may be internally implemented in your system with C<alarm>) - -If you want to use C<alarm> to time out a system call you need to use an -C<eval>/C<die> pair. You can't rely on the alarm causing the system call to -fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to -restart system calls on some systems. Using C<eval>/C<die> always works, -modulo the caveats given in L<perlipc/"Signals">. - - eval { - local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required - alarm $timeout; - $nread = sysread SOCKET, $buffer, $size; - alarm 0; - }; - if ($@) { - die unless $@ eq "alarm\n"; # propagate unexpected errors - # timed out - } - else { - # didn't - } - -=item atan2 Y,X - -Returns the arctangent of Y/X in the range -PI to PI. - -For the tangent operation, you may use the C<Math::Trig::tan> -function, or use the familiar relation: - - sub tan { sin($_[0]) / cos($_[0]) } - -=item bind SOCKET,NAME - -Binds a network address to a socket, just as the bind system call -does. Returns true if it succeeded, false otherwise. NAME should be a -packed address of the appropriate type for the socket. See the examples in -L<perlipc/"Sockets: Client/Server Communication">. - -=item binmode FILEHANDLE, DISCIPLINE - -=item binmode FILEHANDLE - -Arranges for FILEHANDLE to be read or written in "binary" or "text" mode -on systems where the run-time libraries distinguish between binary and -text files. If FILEHANDLE is an expression, the value is taken as the -name of the filehandle. DISCIPLINE can be either of C<":raw"> for -binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is -omitted, it defaults to C<":raw">. - -binmode() should be called after open() but before any I/O is done on -the filehandle. - -On many systems binmode() currently has no effect, but in future, it -will be extended to support user-defined input and output disciplines. -On some systems binmode() is necessary when you're not working with a -text file. For the sake of portability it is a good idea to always use -it when appropriate, and to never use it when it isn't appropriate. - -In other words: Regardless of platform, use binmode() on binary -files, and do not use binmode() on text files. - -The C<open> pragma can be used to establish default disciplines. -See L<open>. - -The operating system, device drivers, C libraries, and Perl run-time -system all work together to let the programmer treat a single -character (C<\n>) as the line terminator, irrespective of the external -representation. On many operating systems, the native text file -representation matches the internal representation, but on some -platforms the external representation of C<\n> is made up of more than -one character. - -Mac OS and all variants of Unix use a single character to end each line -in the external representation of text (even though that single -character is not necessarily the same across these platforms). -Consequently binmode() has no effect on these operating systems. In -other systems like VMS, MS-DOS and the various flavors of MS-Windows -your program sees a C<\n> as a simple C<\cJ>, but what's stored in text -files are the two characters C<\cM\cJ>. That means that, if you don't -use binmode() on these systems, C<\cM\cJ> sequences on disk will be -converted to C<\n> on input, and any C<\n> in your program will be -converted back to C<\cM\cJ> on output. This is what you want for text -files, but it can be disastrous for binary files. - -Another consequence of using binmode() (on some systems) is that -special end-of-file markers will be seen as part of the data stream. -For systems from the Microsoft family this means that if your binary -data contains C<\cZ>, the I/O subsystem will regard it as the end of -the file, unless you use binmode(). - -binmode() is not only important for readline() and print() operations, -but also when using read(), seek(), sysread(), syswrite() and tell() -(see L<perlport> for more details). See the C<$/> and C<$\> variables -in L<perlvar> for how to manually set your input and output -line-termination sequences. - -=item bless REF,CLASSNAME - -=item bless REF - -This function tells the thingy referenced by REF that it is now an object -in the CLASSNAME package. If CLASSNAME is omitted, the current package -is used. Because a C<bless> is often the last thing in a constructor, -it returns the reference for convenience. Always use the two-argument -version if the function doing the blessing might be inherited by a -derived class. See L<perltoot> and L<perlobj> for more about the blessing -(and blessings) of objects. - -Consider always blessing objects in CLASSNAMEs that are mixed case. -Namespaces with all lowercase names are considered reserved for -Perl pragmata. Builtin types have all uppercase names, so to prevent -confusion, you may wish to avoid such package names as well. Make sure -that CLASSNAME is a true value. - -See L<perlmod/"Perl Modules">. - -=item caller EXPR - -=item caller - -Returns the context of the current subroutine call. In scalar context, -returns the caller's package name if there is a caller, that is, if -we're in a subroutine or C<eval> or C<require>, and the undefined value -otherwise. In list context, returns - - ($package, $filename, $line) = caller; - -With EXPR, it returns some extra information that the debugger uses to -print a stack trace. The value of EXPR indicates how many call frames -to go back before the current one. - - ($package, $filename, $line, $subroutine, $hasargs, - $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); - -Here $subroutine may be C<(eval)> if the frame is not a subroutine -call, but an C<eval>. In such a case additional elements $evaltext and -C<$is_require> are set: C<$is_require> is true if the frame is created by a -C<require> or C<use> statement, $evaltext contains the text of the -C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, -$filename is C<(eval)>, but $evaltext is undefined. (Note also that -each C<use> statement creates a C<require> frame inside an C<eval EXPR>) -frame. C<$hasargs> is true if a new instance of C<@_> was set up for the -frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller -was compiled with. The C<$hints> and C<$bitmask> values are subject to -change between versions of Perl, and are not meant for external use. - -Furthermore, when called from within the DB package, caller returns more -detailed information: it sets the list variable C<@DB::args> to be the -arguments with which the subroutine was invoked. - -Be aware that the optimizer might have optimized call frames away before -C<caller> had a chance to get the information. That means that C<caller(N)> -might not return information about the call frame you expect it do, for -C<< N > 1 >>. In particular, C<@DB::args> might have information from the -previous time C<caller> was called. - -=item chdir EXPR - -Changes the working directory to EXPR, if possible. If EXPR is omitted, -changes to the directory specified by C<$ENV{HOME}>, if set; if not, -changes to the directory specified by C<$ENV{LOGDIR}>. If neither is -set, C<chdir> does nothing. It returns true upon success, false -otherwise. See the example under C<die>. - -=item chmod LIST - -Changes the permissions of a list of files. The first element of the -list must be the numerical mode, which should probably be an octal -number, and which definitely should I<not> a string of octal digits: -C<0644> is okay, C<'0644'> is not. Returns the number of files -successfully changed. See also L</oct>, if all you have is a string. - - $cnt = chmod 0755, 'foo', 'bar'; - chmod 0755, @executables; - $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to - # --w----r-T - $mode = '0644'; chmod oct($mode), 'foo'; # this is better - $mode = 0644; chmod $mode, 'foo'; # this is best - -You can also import the symbolic C<S_I*> constants from the Fcntl -module: - - use Fcntl ':mode'; - - chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; - # This is identical to the chmod 0755 of the above example. - -=item chomp VARIABLE - -=item chomp LIST - -=item chomp - -This safer version of L</chop> removes any trailing string -that corresponds to the current value of C<$/> (also known as -$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total -number of characters removed from all its arguments. It's often used to -remove the newline from the end of an input record when you're worried -that the final record may be missing its newline. When in paragraph -mode (C<$/ = "">), it removes all trailing newlines from the string. -When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is -a reference to an integer or the like, see L<perlvar>) chomp() won't -remove anything. -If VARIABLE is omitted, it chomps C<$_>. Example: - - while (<>) { - chomp; # avoid \n on last field - @array = split(/:/); - # ... - } - -If VARIABLE is a hash, it chomps the hash's values, but not its keys. - -You can actually chomp anything that's an lvalue, including an assignment: - - chomp($cwd = `pwd`); - chomp($answer = <STDIN>); - -If you chomp a list, each element is chomped, and the total number of -characters removed is returned. - -=item chop VARIABLE - -=item chop LIST - -=item chop - -Chops off the last character of a string and returns the character -chopped. It is much more efficient than C<s/.$//s> because it neither -scans nor copies the string. If VARIABLE is omitted, chops C<$_>. -If VARIABLE is a hash, it chops the hash's values, but not its keys. - -You can actually chop anything that's an lvalue, including an assignment. - -If you chop a list, each element is chopped. Only the value of the -last C<chop> is returned. - -Note that C<chop> returns the last character. To return all but the last -character, use C<substr($string, 0, -1)>. - -=item chown LIST - -Changes the owner (and group) of a list of files. The first two -elements of the list must be the I<numeric> uid and gid, in that -order. A value of -1 in either position is interpreted by most -systems to leave that value unchanged. Returns the number of files -successfully changed. - - $cnt = chown $uid, $gid, 'foo', 'bar'; - chown $uid, $gid, @filenames; - -Here's an example that looks up nonnumeric uids in the passwd file: - - print "User: "; - chomp($user = <STDIN>); - print "Files: "; - chomp($pattern = <STDIN>); - - ($login,$pass,$uid,$gid) = getpwnam($user) - or die "$user not in passwd file"; - - @ary = glob($pattern); # expand filenames - chown $uid, $gid, @ary; - -On most systems, you are not allowed to change the ownership of the -file unless you're the superuser, although you should be able to change -the group to any of your secondary groups. On insecure systems, these -restrictions may be relaxed, but this is not a portable assumption. -On POSIX systems, you can detect this condition this way: - - use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); - $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); - -=item chr NUMBER - -=item chr - -Returns the character represented by that NUMBER in the character set. -For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and -chr(0x263a) is a Unicode smiley face (but only within the scope of -a C<use utf8>). For the reverse, use L</ord>. -See L<utf8> for more about Unicode. - -If NUMBER is omitted, uses C<$_>. - -=item chroot FILENAME - -=item chroot - -This function works like the system call by the same name: it makes the -named directory the new root directory for all further pathnames that -begin with a C</> by your process and all its children. (It doesn't -change your current working directory, which is unaffected.) For security -reasons, this call is restricted to the superuser. If FILENAME is -omitted, does a C<chroot> to C<$_>. - -=item close FILEHANDLE - -=item close - -Closes the file or pipe associated with the file handle, returning true -only if stdio successfully flushes buffers and closes the system file -descriptor. Closes the currently selected filehandle if the argument -is omitted. - -You don't have to close FILEHANDLE if you are immediately going to do -another C<open> on it, because C<open> will close it for you. (See -C<open>.) However, an explicit C<close> on an input file resets the line -counter (C<$.>), while the implicit close done by C<open> does not. - -If the file handle came from a piped open C<close> will additionally -return false if one of the other system calls involved fails or if the -program exits with non-zero status. (If the only problem was that the -program exited non-zero C<$!> will be set to C<0>.) Closing a pipe -also waits for the process executing on the pipe to complete, in case you -want to look at the output of the pipe afterwards, and -implicitly puts the exit status value of that command into C<$?>. - -Prematurely closing the read end of a pipe (i.e. before the process -writing to it at the other end has closed it) will result in a -SIGPIPE being delivered to the writer. If the other end can't -handle that, be sure to read all the data before closing the pipe. - -Example: - - open(OUTPUT, '|sort >foo') # pipe to sort - or die "Can't start sort: $!"; - #... # print stuff to output - close OUTPUT # wait for sort to finish - or warn $! ? "Error closing sort pipe: $!" - : "Exit status $? from sort"; - open(INPUT, 'foo') # get sort's results - or die "Can't open 'foo' for input: $!"; - -FILEHANDLE may be an expression whose value can be used as an indirect -filehandle, usually the real filehandle name. - -=item closedir DIRHANDLE - -Closes a directory opened by C<opendir> and returns the success of that -system call. - -DIRHANDLE may be an expression whose value can be used as an indirect -dirhandle, usually the real dirhandle name. - -=item connect SOCKET,NAME - -Attempts to connect to a remote socket, just as the connect system call -does. Returns true if it succeeded, false otherwise. NAME should be a -packed address of the appropriate type for the socket. See the examples in -L<perlipc/"Sockets: Client/Server Communication">. - -=item continue BLOCK - -Actually a flow control statement rather than a function. If there is a -C<continue> BLOCK attached to a BLOCK (typically in a C<while> or -C<foreach>), it is always executed just before the conditional is about to -be evaluated again, just like the third part of a C<for> loop in C. Thus -it can be used to increment a loop variable, even when the loop has been -continued via the C<next> statement (which is similar to the C C<continue> -statement). - -C<last>, C<next>, or C<redo> may appear within a C<continue> -block. C<last> and C<redo> will behave as if they had been executed within -the main block. So will C<next>, but since it will execute a C<continue> -block, it may be more entertaining. - - while (EXPR) { - ### redo always comes here - do_something; - } continue { - ### next always comes here - do_something_else; - # then back the top to re-check EXPR - } - ### last always comes here - -Omitting the C<continue> section is semantically equivalent to using an -empty one, logically enough. In that case, C<next> goes directly back -to check the condition at the top of the loop. - -=item cos EXPR - -=item cos - -Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, -takes cosine of C<$_>. - -For the inverse cosine operation, you may use the C<Math::Trig::acos()> -function, or use this relation: - - sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } - -=item crypt PLAINTEXT,SALT - -Encrypts a string exactly like the crypt(3) function in the C library -(assuming that you actually have a version there that has not been -extirpated as a potential munition). This can prove useful for checking -the password file for lousy passwords, amongst other things. Only the -guys wearing white hats should do this. - -Note that C<crypt> is intended to be a one-way function, much like breaking -eggs to make an omelette. There is no (known) corresponding decrypt -function. As a result, this function isn't all that useful for -cryptography. (For that, see your nearby CPAN mirror.) - -When verifying an existing encrypted string you should use the encrypted -text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This -allows your code to work with the standard C<crypt> and with more -exotic implementations. When choosing a new salt create a random two -character string whose characters come from the set C<[./0-9A-Za-z]> -(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). - -Here's an example that makes sure that whoever runs this program knows -their own password: - - $pwd = (getpwuid($<))[1]; - - system "stty -echo"; - print "Password: "; - chomp($word = <STDIN>); - print "\n"; - system "stty echo"; - - if (crypt($word, $pwd) ne $pwd) { - die "Sorry...\n"; - } else { - print "ok\n"; - } - -Of course, typing in your own password to whoever asks you -for it is unwise. - -The L<crypt> function is unsuitable for encrypting large quantities -of data, not least of all because you can't get the information -back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories -on your favorite CPAN mirror for a slew of potentially useful -modules. - -=item dbmclose HASH - -[This function has been largely superseded by the C<untie> function.] - -Breaks the binding between a DBM file and a hash. - -=item dbmopen HASH,DBNAME,MASK - -[This function has been largely superseded by the C<tie> function.] - -This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a -hash. HASH is the name of the hash. (Unlike normal C<open>, the first -argument is I<not> a filehandle, even though it looks like one). DBNAME -is the name of the database (without the F<.dir> or F<.pag> extension if -any). If the database does not exist, it is created with protection -specified by MASK (as modified by the C<umask>). If your system supports -only the older DBM functions, you may perform only one C<dbmopen> in your -program. In older versions of Perl, if your system had neither DBM nor -ndbm, calling C<dbmopen> produced a fatal error; it now falls back to -sdbm(3). - -If you don't have write access to the DBM file, you can only read hash -variables, not set them. If you want to test whether you can write, -either use file tests or try setting a dummy hash entry inside an C<eval>, -which will trap the error. - -Note that functions such as C<keys> and C<values> may return huge lists -when used on large DBM files. You may prefer to use the C<each> -function to iterate over large DBM files. Example: - - # print out history file offsets - dbmopen(%HIST,'/usr/lib/news/history',0666); - while (($key,$val) = each %HIST) { - print $key, ' = ', unpack('L',$val), "\n"; - } - dbmclose(%HIST); - -See also L<AnyDBM_File> for a more general description of the pros and -cons of the various dbm approaches, as well as L<DB_File> for a particularly -rich implementation. - -You can control which DBM library you use by loading that library -before you call dbmopen(): - - use DB_File; - dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") - or die "Can't open netscape history file: $!"; - -=item defined EXPR - -=item defined - -Returns a Boolean value telling whether EXPR has a value other than -the undefined value C<undef>. If EXPR is not present, C<$_> will be -checked. - -Many operations return C<undef> to indicate failure, end of file, -system error, uninitialized variable, and other exceptional -conditions. This function allows you to distinguish C<undef> from -other values. (A simple Boolean test will not distinguish among -C<undef>, zero, the empty string, and C<"0">, which are all equally -false.) Note that since C<undef> is a valid scalar, its presence -doesn't I<necessarily> indicate an exceptional condition: C<pop> -returns C<undef> when its argument is an empty array, I<or> when the -element to return happens to be C<undef>. - -You may also use C<defined(&func)> to check whether subroutine C<&func> -has ever been defined. The return value is unaffected by any forward -declarations of C<&foo>. Note that a subroutine which is not defined -may still be callable: its package may have an C<AUTOLOAD> method that -makes it spring into existence the first time that it is called -- see -L<perlsub>. - -Use of C<defined> on aggregates (hashes and arrays) is deprecated. It -used to report whether memory for that aggregate has ever been -allocated. This behavior may disappear in future versions of Perl. -You should instead use a simple test for size: - - if (@an_array) { print "has array elements\n" } - if (%a_hash) { print "has hash members\n" } - -When used on a hash element, it tells you whether the value is defined, -not whether the key exists in the hash. Use L</exists> for the latter -purpose. - -Examples: - - print if defined $switch{'D'}; - print "$val\n" while defined($val = pop(@ary)); - die "Can't readlink $sym: $!" - unless defined($value = readlink $sym); - sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } - $debugging = 0 unless defined $debugging; - -Note: Many folks tend to overuse C<defined>, and then are surprised to -discover that the number C<0> and C<""> (the zero-length string) are, in fact, -defined values. For example, if you say - - "ab" =~ /a(.*)b/; - -The pattern match succeeds, and C<$1> is defined, despite the fact that it -matched "nothing". But it didn't really match nothing--rather, it -matched something that happened to be zero characters long. This is all -very above-board and honest. When a function returns an undefined value, -it's an admission that it couldn't give you an honest answer. So you -should use C<defined> only when you're questioning the integrity of what -you're trying to do. At other times, a simple comparison to C<0> or C<""> is -what you want. - -See also L</undef>, L</exists>, L</ref>. - -=item delete EXPR - -Given an expression that specifies a hash element, array element, hash slice, -or array slice, deletes the specified element(s) from the hash or array. -In the case of an array, if the array elements happen to be at the end, -the size of the array will shrink to the highest element that tests -true for exists() (or 0 if no such element exists). - -Returns each element so deleted or the undefined value if there was no such -element. Deleting from C<$ENV{}> modifies the environment. Deleting from -a hash tied to a DBM file deletes the entry from the DBM file. Deleting -from a C<tie>d hash or array may not necessarily return anything. - -Deleting an array element effectively returns that position of the array -to its initial, uninitialized state. Subsequently testing for the same -element with exists() will return false. Note that deleting array -elements in the middle of an array will not shift the index of the ones -after them down--use splice() for that. See L</exists>. - -The following (inefficiently) deletes all the values of %HASH and @ARRAY: - - foreach $key (keys %HASH) { - delete $HASH{$key}; - } - - foreach $index (0 .. $#ARRAY) { - delete $ARRAY[$index]; - } - -And so do these: - - delete @HASH{keys %HASH}; - - delete @ARRAY[0 .. $#ARRAY]; - -But both of these are slower than just assigning the empty list -or undefining %HASH or @ARRAY: - - %HASH = (); # completely empty %HASH - undef %HASH; # forget %HASH ever existed - - @ARRAY = (); # completely empty @ARRAY - undef @ARRAY; # forget @ARRAY ever existed - -Note that the EXPR can be arbitrarily complicated as long as the final -operation is a hash element, array element, hash slice, or array slice -lookup: - - delete $ref->[$x][$y]{$key}; - delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; - - delete $ref->[$x][$y][$index]; - delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; - -=item die LIST - -Outside an C<eval>, prints the value of LIST to C<STDERR> and -exits with the current value of C<$!> (errno). If C<$!> is C<0>, -exits with the value of C<<< ($? >> 8) >>> (backtick `command` -status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside -an C<eval(),> the error message is stuffed into C<$@> and the -C<eval> is terminated with the undefined value. This makes -C<die> the way to raise an exception. - -Equivalent examples: - - die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; - chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" - -If the value of EXPR does not end in a newline, the current script line -number and input line number (if any) are also printed, and a newline -is supplied. Note that the "input line number" (also known as "chunk") -is subject to whatever notion of "line" happens to be currently in -effect, and is also available as the special variable C<$.>. -See L<perlvar/"$/"> and L<perlvar/"$.">. - -Hint: sometimes appending C<", stopped"> to your message -will cause it to make better sense when the string C<"at foo line 123"> is -appended. Suppose you are running script "canasta". - - die "/etc/games is no good"; - die "/etc/games is no good, stopped"; - -produce, respectively - - /etc/games is no good at canasta line 123. - /etc/games is no good, stopped at canasta line 123. - -See also exit(), warn(), and the Carp module. - -If LIST is empty and C<$@> already contains a value (typically from a -previous eval) that value is reused after appending C<"\t...propagated">. -This is useful for propagating exceptions: - - eval { ... }; - die unless $@ =~ /Expected exception/; - -If C<$@> is empty then the string C<"Died"> is used. - -die() can also be called with a reference argument. If this happens to be -trapped within an eval(), $@ contains the reference. This behavior permits -a more elaborate exception handling implementation using objects that -maintain arbitrary state about the nature of the exception. Such a scheme -is sometimes preferable to matching particular string values of $@ using -regular expressions. Here's an example: - - eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; - if ($@) { - if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { - # handle Some::Module::Exception - } - else { - # handle all other possible exceptions - } - } - -Because perl will stringify uncaught exception messages before displaying -them, you may want to overload stringification operations on such custom -exception objects. See L<overload> for details about that. - -You can arrange for a callback to be run just before the C<die> -does its deed, by setting the C<$SIG{__DIE__}> hook. The associated -handler will be called with the error text and can change the error -message, if it sees fit, by calling C<die> again. See -L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and -L<"eval BLOCK"> for some examples. Although this feature was meant -to be run only right before your program was to exit, this is not -currently the case--the C<$SIG{__DIE__}> hook is currently called -even inside eval()ed blocks/strings! If one wants the hook to do -nothing in such situations, put - - die @_ if $^S; - -as the first line of the handler (see L<perlvar/$^S>). Because -this promotes strange action at a distance, this counterintuitive -behavior may be fixed in a future release. - -=item do BLOCK - -Not really a function. Returns the value of the last command in the -sequence of commands indicated by BLOCK. When modified by a loop -modifier, executes the BLOCK once before testing the loop condition. -(On other statements the loop modifiers test the conditional first.) - -C<do BLOCK> does I<not> count as a loop, so the loop control statements -C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. -See L<perlsyn> for alternative strategies. - -=item do SUBROUTINE(LIST) - -A deprecated form of subroutine call. See L<perlsub>. - -=item do EXPR - -Uses the value of EXPR as a filename and executes the contents of the -file as a Perl script. Its primary use is to include subroutines -from a Perl subroutine library. - - do 'stat.pl'; - -is just like - - scalar eval `cat stat.pl`; - -except that it's more efficient and concise, keeps track of the current -filename for error messages, searches the @INC libraries, and updates -C<%INC> if the file is found. See L<perlvar/Predefined Names> for these -variables. It also differs in that code evaluated with C<do FILENAME> -cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the -same, however, in that it does reparse the file every time you call it, -so you probably don't want to do this inside a loop. - -If C<do> cannot read the file, it returns undef and sets C<$!> to the -error. If C<do> can read the file but cannot compile it, it -returns undef and sets an error message in C<$@>. If the file is -successfully compiled, C<do> returns the value of the last expression -evaluated. - -Note that inclusion of library modules is better done with the -C<use> and C<require> operators, which also do automatic error checking -and raise an exception if there's a problem. - -You might like to use C<do> to read in a program configuration -file. Manual error checking can be done this way: - - # read in config files: system first, then user - for $file ("/share/prog/defaults.rc", - "$ENV{HOME}/.someprogrc") - { - unless ($return = do $file) { - warn "couldn't parse $file: $@" if $@; - warn "couldn't do $file: $!" unless defined $return; - warn "couldn't run $file" unless $return; - } - } - -=item dump LABEL - -=item dump - -This function causes an immediate core dump. See also the B<-u> -command-line switch in L<perlrun>, which does the same thing. -Primarily this is so that you can use the B<undump> program (not -supplied) to turn your core dump into an executable binary after -having initialized all your variables at the beginning of the -program. When the new binary is executed it will begin by executing -a C<goto LABEL> (with all the restrictions that C<goto> suffers). -Think of it as a goto with an intervening core dump and reincarnation. -If C<LABEL> is omitted, restarts the program from the top. - -B<WARNING>: Any files opened at the time of the dump will I<not> -be open any more when the program is reincarnated, with possible -resulting confusion on the part of Perl. - -This function is now largely obsolete, partly because it's very -hard to convert a core file into an executable, and because the -real compiler backends for generating portable bytecode and compilable -C code have superseded it. - -If you're looking to use L<dump> to speed up your program, consider -generating bytecode or native C code as described in L<perlcc>. If -you're just trying to accelerate a CGI script, consider using the -C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. -You might also consider autoloading or selfloading, which at least -make your program I<appear> to run faster. - -=item each HASH - -When called in list context, returns a 2-element list consisting of the -key and value for the next element of a hash, so that you can iterate over -it. When called in scalar context, returns only the key for the next -element in the hash. - -Entries are returned in an apparently random order. The actual random -order is subject to change in future versions of perl, but it is guaranteed -to be in the same order as either the C<keys> or C<values> function -would produce on the same (unmodified) hash. - -When the hash is entirely read, a null array is returned in list context -(which when assigned produces a false (C<0>) value), and C<undef> in -scalar context. The next call to C<each> after that will start iterating -again. There is a single iterator for each hash, shared by all C<each>, -C<keys>, and C<values> function calls in the program; it can be reset by -reading all the elements from the hash, or by evaluating C<keys HASH> or -C<values HASH>. If you add or delete elements of a hash while you're -iterating over it, you may get entries skipped or duplicated, so -don't. Exception: It is always safe to delete the item most recently -returned by C<each()>, which means that the following code will work: - - while (($key, $value) = each %hash) { - print $key, "\n"; - delete $hash{$key}; # This is safe - } - -The following prints out your environment like the printenv(1) program, -only in a different order: - - while (($key,$value) = each %ENV) { - print "$key=$value\n"; - } - -See also C<keys>, C<values> and C<sort>. - -=item eof FILEHANDLE - -=item eof () - -=item eof - -Returns 1 if the next read on FILEHANDLE will return end of file, or if -FILEHANDLE is not open. FILEHANDLE may be an expression whose value -gives the real filehandle. (Note that this function actually -reads a character and then C<ungetc>s it, so isn't very useful in an -interactive context.) Do not read from a terminal file (or call -C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such -as terminals may lose the end-of-file condition if you do. - -An C<eof> without an argument uses the last file read. Using C<eof()> -with empty parentheses is very different. It refers to the pseudo file -formed from the files listed on the command line and accessed via the -C<< <> >> operator. Since C<< <> >> isn't explicitly opened, -as a normal filehandle is, an C<eof()> before C<< <> >> has been -used will cause C<@ARGV> to be examined to determine if input is -available. - -In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to -detect the end of each file, C<eof()> will only detect the end of the -last file. Examples: - - # reset line numbering on each input file - while (<>) { - next if /^\s*#/; # skip comments - print "$.\t$_"; - } continue { - close ARGV if eof; # Not eof()! - } - - # insert dashes just before last line of last file - while (<>) { - if (eof()) { # check for end of current file - print "--------------\n"; - close(ARGV); # close or last; is needed if we - # are reading from the terminal - } - print; - } - -Practical hint: you almost never need to use C<eof> in Perl, because the -input operators typically return C<undef> when they run out of data, or if -there was an error. - -=item eval EXPR - -=item eval BLOCK - -In the first form, the return value of EXPR is parsed and executed as if it -were a little Perl program. The value of the expression (which is itself -determined within scalar context) is first parsed, and if there weren't any -errors, executed in the lexical context of the current Perl program, so -that any variable settings or subroutine and format definitions remain -afterwards. Note that the value is parsed every time the eval executes. -If EXPR is omitted, evaluates C<$_>. This form is typically used to -delay parsing and subsequent execution of the text of EXPR until run time. - -In the second form, the code within the BLOCK is parsed only once--at the -same time the code surrounding the eval itself was parsed--and executed -within the context of the current Perl program. This form is typically -used to trap exceptions more efficiently than the first (see below), while -also providing the benefit of checking the code within BLOCK at compile -time. - -The final semicolon, if any, may be omitted from the value of EXPR or within -the BLOCK. - -In both forms, the value returned is the value of the last expression -evaluated inside the mini-program; a return statement may be also used, just -as with subroutines. The expression providing the return value is evaluated -in void, scalar, or list context, depending on the context of the eval itself. -See L</wantarray> for more on how the evaluation context can be determined. - -If there is a syntax error or runtime error, or a C<die> statement is -executed, an undefined value is returned by C<eval>, and C<$@> is set to the -error message. If there was no error, C<$@> is guaranteed to be a null -string. Beware that using C<eval> neither silences perl from printing -warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. -To do either of those, you have to use the C<$SIG{__WARN__}> facility. See -L</warn> and L<perlvar>. - -Note that, because C<eval> traps otherwise-fatal errors, it is useful for -determining whether a particular feature (such as C<socket> or C<symlink>) -is implemented. It is also Perl's exception trapping mechanism, where -the die operator is used to raise exceptions. - -If the code to be executed doesn't vary, you may use the eval-BLOCK -form to trap run-time errors without incurring the penalty of -recompiling each time. The error, if any, is still returned in C<$@>. -Examples: - - # make divide-by-zero nonfatal - eval { $answer = $a / $b; }; warn $@ if $@; - - # same thing, but less efficient - eval '$answer = $a / $b'; warn $@ if $@; - - # a compile-time error - eval { $answer = }; # WRONG - - # a run-time error - eval '$answer ='; # sets $@ - -Due to the current arguably broken state of C<__DIE__> hooks, when using -the C<eval{}> form as an exception trap in libraries, you may wish not -to trigger any C<__DIE__> hooks that user code may have installed. -You can use the C<local $SIG{__DIE__}> construct for this purpose, -as shown in this example: - - # a very private exception trap for divide-by-zero - eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; - warn $@ if $@; - -This is especially significant, given that C<__DIE__> hooks can call -C<die> again, which has the effect of changing their error messages: - - # __DIE__ hooks may modify error messages - { - local $SIG{'__DIE__'} = - sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; - eval { die "foo lives here" }; - print $@ if $@; # prints "bar lives here" - } - -Because this promotes action at a distance, this counterintuitive behavior -may be fixed in a future release. - -With an C<eval>, you should be especially careful to remember what's -being looked at when: - - eval $x; # CASE 1 - eval "$x"; # CASE 2 - - eval '$x'; # CASE 3 - eval { $x }; # CASE 4 - - eval "\$$x++"; # CASE 5 - $$x++; # CASE 6 - -Cases 1 and 2 above behave identically: they run the code contained in -the variable $x. (Although case 2 has misleading double quotes making -the reader wonder what else might be happening (nothing is).) Cases 3 -and 4 likewise behave in the same way: they run the code C<'$x'>, which -does nothing but return the value of $x. (Case 4 is preferred for -purely visual reasons, but it also has the advantage of compiling at -compile-time instead of at run-time.) Case 5 is a place where -normally you I<would> like to use double quotes, except that in this -particular situation, you can just use symbolic references instead, as -in case 6. - -C<eval BLOCK> does I<not> count as a loop, so the loop control statements -C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. - -=item exec LIST - -=item exec PROGRAM LIST - -The C<exec> function executes a system command I<and never returns>-- -use C<system> instead of C<exec> if you want it to return. It fails and -returns false only if the command does not exist I<and> it is executed -directly instead of via your system's command shell (see below). - -Since it's a common mistake to use C<exec> instead of C<system>, Perl -warns you if there is a following statement which isn't C<die>, C<warn>, -or C<exit> (if C<-w> is set - but you always do that). If you -I<really> want to follow an C<exec> with some other statement, you -can use one of these styles to avoid the warning: - - exec ('foo') or print STDERR "couldn't exec foo: $!"; - { exec ('foo') }; print STDERR "couldn't exec foo: $!"; - -If there is more than one argument in LIST, or if LIST is an array -with more than one value, calls execvp(3) with the arguments in LIST. -If there is only one scalar argument or an array with one element in it, -the argument is checked for shell metacharacters, and if there are any, -the entire argument is passed to the system's command shell for parsing -(this is C</bin/sh -c> on Unix platforms, but varies on other platforms). -If there are no shell metacharacters in the argument, it is split into -words and passed directly to C<execvp>, which is more efficient. -Examples: - - exec '/bin/echo', 'Your arguments are: ', @ARGV; - exec "sort $outfile | uniq"; - -If you don't really want to execute the first argument, but want to lie -to the program you are executing about its own name, you can specify -the program you actually want to run as an "indirect object" (without a -comma) in front of the LIST. (This always forces interpretation of the -LIST as a multivalued list, even if there is only a single scalar in -the list.) Example: - - $shell = '/bin/csh'; - exec $shell '-sh'; # pretend it's a login shell - -or, more directly, - - exec {'/bin/csh'} '-sh'; # pretend it's a login shell - -When the arguments get executed via the system shell, results will -be subject to its quirks and capabilities. See L<perlop/"`STRING`"> -for details. - -Using an indirect object with C<exec> or C<system> is also more -secure. This usage (which also works fine with system()) forces -interpretation of the arguments as a multivalued list, even if the -list had just one argument. That way you're safe from the shell -expanding wildcards or splitting up words with whitespace in them. - - @args = ( "echo surprise" ); - - exec @args; # subject to shell escapes - # if @args == 1 - exec { $args[0] } @args; # safe even with one-arg list - -The first version, the one without the indirect object, ran the I<echo> -program, passing it C<"surprise"> an argument. The second version -didn't--it tried to run a program literally called I<"echo surprise">, -didn't find it, and set C<$?> to a non-zero value indicating failure. - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before the exec, but this may not be supported on some platforms -(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH -in English) or call the C<autoflush()> method of C<IO::Handle> on any -open handles in order to avoid lost output. - -Note that C<exec> will not call your C<END> blocks, nor will it call -any C<DESTROY> methods in your objects. - -=item exists EXPR - -Given an expression that specifies a hash element or array element, -returns true if the specified element in the hash or array has ever -been initialized, even if the corresponding value is undefined. The -element is not autovivified if it doesn't exist. - - print "Exists\n" if exists $hash{$key}; - print "Defined\n" if defined $hash{$key}; - print "True\n" if $hash{$key}; - - print "Exists\n" if exists $array[$index]; - print "Defined\n" if defined $array[$index]; - print "True\n" if $array[$index]; - -A hash or array element can be true only if it's defined, and defined if -it exists, but the reverse doesn't necessarily hold true. - -Given an expression that specifies the name of a subroutine, -returns true if the specified subroutine has ever been declared, even -if it is undefined. Mentioning a subroutine name for exists or defined -does not count as declaring it. Note that a subroutine which does not -exist may still be callable: its package may have an C<AUTOLOAD> -method that makes it spring into existence the first time that it is -called -- see L<perlsub>. - - print "Exists\n" if exists &subroutine; - print "Defined\n" if defined &subroutine; - -Note that the EXPR can be arbitrarily complicated as long as the final -operation is a hash or array key lookup or subroutine name: - - if (exists $ref->{A}->{B}->{$key}) { } - if (exists $hash{A}{B}{$key}) { } - - if (exists $ref->{A}->{B}->[$ix]) { } - if (exists $hash{A}{B}[$ix]) { } - - if (exists &{$ref->{A}{B}{$key}}) { } - -Although the deepest nested array or hash will not spring into existence -just because its existence was tested, any intervening ones will. -Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring -into existence due to the existence test for the $key element above. -This happens anywhere the arrow operator is used, including even: - - undef $ref; - if (exists $ref->{"Some key"}) { } - print $ref; # prints HASH(0x80d3d5c) - -This surprising autovivification in what does not at first--or even -second--glance appear to be an lvalue context may be fixed in a future -release. - -See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics -on how exists() acts when used on a pseudo-hash. - -Use of a subroutine call, rather than a subroutine name, as an argument -to exists() is an error. - - exists ⊂ # OK - exists &sub(); # Error - -=item exit EXPR - -Evaluates EXPR and exits immediately with that value. Example: - - $ans = <STDIN>; - exit 0 if $ans =~ /^[Xx]/; - -See also C<die>. If EXPR is omitted, exits with C<0> status. The only -universally recognized values for EXPR are C<0> for success and C<1> -for error; other values are subject to interpretation depending on the -environment in which the Perl program is running. For example, exiting -69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause -the mailer to return the item undelivered, but that's not true everywhere. - -Don't use C<exit> to abort a subroutine if there's any chance that -someone might want to trap whatever error happened. Use C<die> instead, -which can be trapped by an C<eval>. - -The exit() function does not always exit immediately. It calls any -defined C<END> routines first, but these C<END> routines may not -themselves abort the exit. Likewise any object destructors that need to -be called are called before the real exit. If this is a problem, you -can call C<POSIX:_exit($status)> to avoid END and destructor processing. -See L<perlmod> for details. - -=item exp EXPR - -=item exp - -Returns I<e> (the natural logarithm base) to the power of EXPR. -If EXPR is omitted, gives C<exp($_)>. - -=item fcntl FILEHANDLE,FUNCTION,SCALAR - -Implements the fcntl(2) function. You'll probably have to say - - use Fcntl; - -first to get the correct constant definitions. Argument processing and -value return works just like C<ioctl> below. -For example: - - use Fcntl; - fcntl($filehandle, F_GETFL, $packed_return_buffer) - or die "can't fcntl F_GETFL: $!"; - -You don't have to check for C<defined> on the return from C<fnctl>. -Like C<ioctl>, it maps a C<0> return from the system call into -C<"0 but true"> in Perl. This string is true in boolean context and C<0> -in numeric context. It is also exempt from the normal B<-w> warnings -on improper numeric conversions. - -Note that C<fcntl> will produce a fatal error if used on a machine that -doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) -manpage to learn what functions are available on your system. - -=item fileno FILEHANDLE - -Returns the file descriptor for a filehandle, or undefined if the -filehandle is not open. This is mainly useful for constructing -bitmaps for C<select> and low-level POSIX tty-handling operations. -If FILEHANDLE is an expression, the value is taken as an indirect -filehandle, generally its name. - -You can use this to find out whether two handles refer to the -same underlying descriptor: - - if (fileno(THIS) == fileno(THAT)) { - print "THIS and THAT are dups\n"; - } - -=item flock FILEHANDLE,OPERATION - -Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true -for success, false on failure. Produces a fatal error if used on a -machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). -C<flock> is Perl's portable file locking interface, although it locks -only entire files, not records. - -Two potentially non-obvious but traditional C<flock> semantics are -that it waits indefinitely until the lock is granted, and that its locks -B<merely advisory>. Such discretionary locks are more flexible, but offer -fewer guarantees. This means that files locked with C<flock> may be -modified by programs that do not also use C<flock>. See L<perlport>, -your port's specific documentation, or your system-specific local manpages -for details. It's best to assume traditional behavior if you're writing -portable programs. (But if you're not, you should as always feel perfectly -free to write for your own system's idiosyncrasies (sometimes called -"features"). Slavish adherence to portability concerns shouldn't get -in the way of your getting your job done.) - -OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with -LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but -you can use the symbolic names if you import them from the Fcntl module, -either individually, or as a group using the ':flock' tag. LOCK_SH -requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN -releases a previously requested lock. If LOCK_NB is bitwise-or'ed with -LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking -waiting for the lock (check the return status to see if you got it). - -To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE -before locking or unlocking it. - -Note that the emulation built with lockf(3) doesn't provide shared -locks, and it requires that FILEHANDLE be open with write intent. These -are the semantics that lockf(3) implements. Most if not all systems -implement lockf(3) in terms of fcntl(2) locking, though, so the -differing semantics shouldn't bite too many people. - -Note also that some versions of C<flock> cannot lock things over the -network; you would need to use the more system-specific C<fcntl> for -that. If you like you can force Perl to ignore your system's flock(2) -function, and so provide its own fcntl(2)-based emulation, by passing -the switch C<-Ud_flock> to the F<Configure> program when you configure -perl. - -Here's a mailbox appender for BSD systems. - - use Fcntl ':flock'; # import LOCK_* constants - - sub lock { - flock(MBOX,LOCK_EX); - # and, in case someone appended - # while we were waiting... - seek(MBOX, 0, 2); - } - - sub unlock { - flock(MBOX,LOCK_UN); - } - - open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") - or die "Can't open mailbox: $!"; - - lock(); - print MBOX $msg,"\n\n"; - unlock(); - -On systems that support a real flock(), locks are inherited across fork() -calls, whereas those that must resort to the more capricious fcntl() -function lose the locks, making it harder to write servers. - -See also L<DB_File> for other flock() examples. - -=item fork - -Does a fork(2) system call to create a new process running the -same program at the same point. It returns the child pid to the -parent process, C<0> to the child process, or C<undef> if the fork is -unsuccessful. File descriptors (and sometimes locks on those descriptors) -are shared, while everything else is copied. On most systems supporting -fork(), great care has gone into making it extremely efficient (for -example, using copy-on-write technology on data pages), making it the -dominant paradigm for multitasking over the last few decades. - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before forking the child process, but this may not be supported -on some platforms (see L<perlport>). To be safe, you may need to set -C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of -C<IO::Handle> on any open handles in order to avoid duplicate output. - -If you C<fork> without ever waiting on your children, you will -accumulate zombies. On some systems, you can avoid this by setting -C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of -forking and reaping moribund children. - -Note that if your forked child inherits system file descriptors like -STDIN and STDOUT that are actually connected by a pipe or socket, even -if you exit, then the remote server (such as, say, a CGI script or a -backgrounded job launched from a remote shell) won't think you're done. -You should reopen those to F</dev/null> if it's any issue. - -=item format - -Declare a picture format for use by the C<write> function. For -example: - - format Something = - Test: @<<<<<<<< @||||| @>>>>> - $str, $%, '$' . int($num) - . - - $str = "widget"; - $num = $cost/$quantity; - $~ = 'Something'; - write; - -See L<perlform> for many details and examples. - -=item formline PICTURE,LIST - -This is an internal function used by C<format>s, though you may call it, -too. It formats (see L<perlform>) a list of values according to the -contents of PICTURE, placing the output into the format output -accumulator, C<$^A> (or C<$ACCUMULATOR> in English). -Eventually, when a C<write> is done, the contents of -C<$^A> are written to some filehandle, but you could also read C<$^A> -yourself and then set C<$^A> back to C<"">. Note that a format typically -does one C<formline> per line of form, but the C<formline> function itself -doesn't care how many newlines are embedded in the PICTURE. This means -that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. -You may therefore need to use multiple formlines to implement a single -record format, just like the format compiler. - -Be careful if you put double quotes around the picture, because an C<@> -character may be taken to mean the beginning of an array name. -C<formline> always returns true. See L<perlform> for other examples. - -=item getc FILEHANDLE - -=item getc - -Returns the next character from the input file attached to FILEHANDLE, -or the undefined value at end of file, or if there was an error. -If FILEHANDLE is omitted, reads from STDIN. This is not particularly -efficient. However, it cannot be used by itself to fetch single -characters without waiting for the user to hit enter. For that, try -something more like: - - if ($BSD_STYLE) { - system "stty cbreak </dev/tty >/dev/tty 2>&1"; - } - else { - system "stty", '-icanon', 'eol', "\001"; - } - - $key = getc(STDIN); - - if ($BSD_STYLE) { - system "stty -cbreak </dev/tty >/dev/tty 2>&1"; - } - else { - system "stty", 'icanon', 'eol', '^@'; # ASCII null - } - print "\n"; - -Determination of whether $BSD_STYLE should be set -is left as an exercise to the reader. - -The C<POSIX::getattr> function can do this more portably on -systems purporting POSIX compliance. See also the C<Term::ReadKey> -module from your nearest CPAN site; details on CPAN can be found on -L<perlmodlib/CPAN>. - -=item getlogin - -Implements the C library function of the same name, which on most -systems returns the current login from F</etc/utmp>, if any. If null, -use C<getpwuid>. - - $login = getlogin || getpwuid($<) || "Kilroy"; - -Do not consider C<getlogin> for authentication: it is not as -secure as C<getpwuid>. - -=item getpeername SOCKET - -Returns the packed sockaddr address of other end of the SOCKET connection. - - use Socket; - $hersockaddr = getpeername(SOCK); - ($port, $iaddr) = sockaddr_in($hersockaddr); - $herhostname = gethostbyaddr($iaddr, AF_INET); - $herstraddr = inet_ntoa($iaddr); - -=item getpgrp PID - -Returns the current process group for the specified PID. Use -a PID of C<0> to get the current process group for the -current process. Will raise an exception if used on a machine that -doesn't implement getpgrp(2). If PID is omitted, returns process -group of current process. Note that the POSIX version of C<getpgrp> -does not accept a PID argument, so only C<PID==0> is truly portable. - -=item getppid - -Returns the process id of the parent process. - -=item getpriority WHICH,WHO - -Returns the current priority for a process, a process group, or a user. -(See L<getpriority(2)>.) Will raise a fatal exception if used on a -machine that doesn't implement getpriority(2). - -=item getpwnam NAME - -=item getgrnam NAME - -=item gethostbyname NAME - -=item getnetbyname NAME - -=item getprotobyname NAME - -=item getpwuid UID - -=item getgrgid GID - -=item getservbyname NAME,PROTO - -=item gethostbyaddr ADDR,ADDRTYPE - -=item getnetbyaddr ADDR,ADDRTYPE - -=item getprotobynumber NUMBER - -=item getservbyport PORT,PROTO - -=item getpwent - -=item getgrent - -=item gethostent - -=item getnetent - -=item getprotoent - -=item getservent - -=item setpwent - -=item setgrent - -=item sethostent STAYOPEN - -=item setnetent STAYOPEN - -=item setprotoent STAYOPEN - -=item setservent STAYOPEN - -=item endpwent - -=item endgrent - -=item endhostent - -=item endnetent - -=item endprotoent - -=item endservent - -These routines perform the same functions as their counterparts in the -system library. In list context, the return values from the -various get routines are as follows: - - ($name,$passwd,$uid,$gid, - $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* - ($name,$passwd,$gid,$members) = getgr* - ($name,$aliases,$addrtype,$length,@addrs) = gethost* - ($name,$aliases,$addrtype,$net) = getnet* - ($name,$aliases,$proto) = getproto* - ($name,$aliases,$port,$proto) = getserv* - -(If the entry doesn't exist you get a null list.) - -The exact meaning of the $gcos field varies but it usually contains -the real name of the user (as opposed to the login name) and other -information pertaining to the user. Beware, however, that in many -system users are able to change this information and therefore it -cannot be trusted and therefore the $gcos is tainted (see -L<perlsec>). The $passwd and $shell, user's encrypted password and -login shell, are also tainted, because of the same reason. - -In scalar context, you get the name, unless the function was a -lookup by name, in which case you get the other thing, whatever it is. -(If the entry doesn't exist you get the undefined value.) For example: - - $uid = getpwnam($name); - $name = getpwuid($num); - $name = getpwent(); - $gid = getgrnam($name); - $name = getgrgid($num; - $name = getgrent(); - #etc. - -In I<getpw*()> the fields $quota, $comment, and $expire are special -cases in the sense that in many systems they are unsupported. If the -$quota is unsupported, it is an empty scalar. If it is supported, it -usually encodes the disk quota. If the $comment field is unsupported, -it is an empty scalar. If it is supported it usually encodes some -administrative comment about the user. In some systems the $quota -field may be $change or $age, fields that have to do with password -aging. In some systems the $comment field may be $class. The $expire -field, if present, encodes the expiration period of the account or the -password. For the availability and the exact meaning of these fields -in your system, please consult your getpwnam(3) documentation and your -F<pwd.h> file. You can also find out from within Perl what your -$quota and $comment fields mean and whether you have the $expire field -by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, -C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password -files are only supported if your vendor has implemented them in the -intuitive fashion that calling the regular C library routines gets the -shadow versions if you're running under privilege or if there exists -the shadow(3) functions as found in System V ( this includes Solaris -and Linux.) Those systems which implement a proprietary shadow password -facility are unlikely to be supported. - -The $members value returned by I<getgr*()> is a space separated list of -the login names of the members of the group. - -For the I<gethost*()> functions, if the C<h_errno> variable is supported in -C, it will be returned to you via C<$?> if the function call fails. The -C<@addrs> value returned by a successful call is a list of the raw -addresses returned by the corresponding system library call. In the -Internet domain, each address is four bytes long and you can unpack it -by saying something like: - - ($a,$b,$c,$d) = unpack('C4',$addr[0]); - -The Socket library makes this slightly easier: - - use Socket; - $iaddr = inet_aton("127.1"); # or whatever address - $name = gethostbyaddr($iaddr, AF_INET); - - # or going the other way - $straddr = inet_ntoa($iaddr); - -If you get tired of remembering which element of the return list -contains which return value, by-name interfaces are provided -in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, -C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, -and C<User::grent>. These override the normal built-ins, supplying -versions that return objects with the appropriate names -for each field. For example: - - use File::stat; - use User::pwent; - $is_his = (stat($filename)->uid == pwent($whoever)->uid); - -Even though it looks like they're the same method calls (uid), -they aren't, because a C<File::stat> object is different from -a C<User::pwent> object. - -=item getsockname SOCKET - -Returns the packed sockaddr address of this end of the SOCKET connection, -in case you don't know the address because you have several different -IPs that the connection might have come in on. - - use Socket; - $mysockaddr = getsockname(SOCK); - ($port, $myaddr) = sockaddr_in($mysockaddr); - printf "Connect to %s [%s]\n", - scalar gethostbyaddr($myaddr, AF_INET), - inet_ntoa($myaddr); - -=item getsockopt SOCKET,LEVEL,OPTNAME - -Returns the socket option requested, or undef if there is an error. - -=item glob EXPR - -=item glob - -Returns the value of EXPR with filename expansions such as the -standard Unix shell F</bin/csh> would do. This is the internal function -implementing the C<< <*.c> >> operator, but you can use it directly. -If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is -discussed in more detail in L<perlop/"I/O Operators">. - -Beginning with v5.6.0, this operator is implemented using the standard -C<File::Glob> extension. See L<File::Glob> for details. - -=item gmtime EXPR - -Converts a time as returned by the time function to a 8-element list -with the time localized for the standard Greenwich time zone. -Typically used as follows: - - # 0 1 2 3 4 5 6 7 - ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = - gmtime(time); - -All list elements are numeric, and come straight out of the C `struct -tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the -specified time. $mday is the day of the month, and $mon is the month -itself, in the range C<0..11> with 0 indicating January and 11 -indicating December. $year is the number of years since 1900. That -is, $year is C<123> in year 2023. $wday is the day of the week, with -0 indicating Sunday and 3 indicating Wednesday. $yday is the day of -the year, in the range C<0..364> (or C<0..365> in leap years.) - -Note that the $year element is I<not> simply the last two digits of -the year. If you assume it is, then you create non-Y2K-compliant -programs--and you wouldn't want to do that, would you? - -The proper way to get a complete 4-digit year is simply: - - $year += 1900; - -And to get the last two digits of the year (e.g., '01' in 2001) do: - - $year = sprintf("%02d", $year % 100); - -If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>). - -In scalar context, C<gmtime()> returns the ctime(3) value: - - $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" - -Also see the C<timegm> function provided by the C<Time::Local> module, -and the strftime(3) function available via the POSIX module. - -This scalar value is B<not> locale dependent (see L<perllocale>), but -is instead a Perl builtin. Also see the C<Time::Local> module, and the -strftime(3) and mktime(3) functions available via the POSIX module. To -get somewhat similar but locale dependent date strings, set up your -locale environment variables appropriately (please see L<perllocale>) -and try for example: - - use POSIX qw(strftime); - $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; - -Note that the C<%a> and C<%b> escapes, which represent the short forms -of the day of the week and the month of the year, may not necessarily -be three characters wide in all locales. - -=item goto LABEL - -=item goto EXPR - -=item goto &NAME - -The C<goto-LABEL> form finds the statement labeled with LABEL and resumes -execution there. It may not be used to go into any construct that -requires initialization, such as a subroutine or a C<foreach> loop. It -also can't be used to go into a construct that is optimized away, -or to get out of a block or subroutine given to C<sort>. -It can be used to go almost anywhere else within the dynamic scope, -including out of subroutines, but it's usually better to use some other -construct such as C<last> or C<die>. The author of Perl has never felt the -need to use this form of C<goto> (in Perl, that is--C is another matter). - -The C<goto-EXPR> form expects a label name, whose scope will be resolved -dynamically. This allows for computed C<goto>s per FORTRAN, but isn't -necessarily recommended if you're optimizing for maintainability: - - goto ("FOO", "BAR", "GLARCH")[$i]; - -The C<goto-&NAME> form is quite different from the other forms of C<goto>. -In fact, it isn't a goto in the normal sense at all, and doesn't have -the stigma associated with other gotos. Instead, it -substitutes a call to the named subroutine for the currently running -subroutine. This is used by C<AUTOLOAD> subroutines that wish to load -another subroutine and then pretend that the other subroutine had been -called in the first place (except that any modifications to C<@_> -in the current subroutine are propagated to the other subroutine.) -After the C<goto>, not even C<caller> will be able to tell that this -routine was called first. - -NAME needn't be the name of a subroutine; it can be a scalar variable -containing a code reference, or a block which evaluates to a code -reference. - -=item grep BLOCK LIST - -=item grep EXPR,LIST - -This is similar in spirit to, but not the same as, grep(1) and its -relatives. In particular, it is not limited to using regular expressions. - -Evaluates the BLOCK or EXPR for each element of LIST (locally setting -C<$_> to each element) and returns the list value consisting of those -elements for which the expression evaluated to true. In scalar -context, returns the number of times the expression was true. - - @foo = grep(!/^#/, @bar); # weed out comments - -or equivalently, - - @foo = grep {!/^#/} @bar; # weed out comments - -Note that C<$_> is an alias to the list value, so it can be used to -modify the elements of the LIST. While this is useful and supported, -it can cause bizarre results if the elements of LIST are not variables. -Similarly, grep returns aliases into the original list, much as a for -loop's index variable aliases the list elements. That is, modifying an -element of a list returned by grep (for example, in a C<foreach>, C<map> -or another C<grep>) actually modifies the element in the original list. -This is usually something to be avoided when writing clear code. - -See also L</map> for a list composed of the results of the BLOCK or EXPR. - -=item hex EXPR - -=item hex - -Interprets EXPR as a hex string and returns the corresponding value. -(To convert strings that might start with either 0, 0x, or 0b, see -L</oct>.) If EXPR is omitted, uses C<$_>. - - print hex '0xAf'; # prints '175' - print hex 'aF'; # same - -Hex strings may only represent integers. Strings that would cause -integer overflow trigger a warning. - -=item import - -There is no builtin C<import> function. It is just an ordinary -method (subroutine) defined (or inherited) by modules that wish to export -names to another module. The C<use> function calls the C<import> method -for the package used. See also L</use>, L<perlmod>, and L<Exporter>. - -=item index STR,SUBSTR,POSITION - -=item index STR,SUBSTR - -The index function searches for one string within another, but without -the wildcard-like behavior of a full regular-expression pattern match. -It returns the position of the first occurrence of SUBSTR in STR at -or after POSITION. If POSITION is omitted, starts searching from the -beginning of the string. The return value is based at C<0> (or whatever -you've set the C<$[> variable to--but don't do that). If the substring -is not found, returns one less than the base, ordinarily C<-1>. - -=item int EXPR - -=item int - -Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. -You should not use this function for rounding: one because it truncates -towards C<0>, and two because machine representations of floating point -numbers can sometimes produce counterintuitive results. For example, -C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's -because it's really more like -268.99999999999994315658 instead. Usually, -the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> -functions will serve you better than will int(). - -=item ioctl FILEHANDLE,FUNCTION,SCALAR - -Implements the ioctl(2) function. You'll probably first have to say - - require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph - -to get the correct function definitions. If F<ioctl.ph> doesn't -exist or doesn't have the correct definitions you'll have to roll your -own, based on your C header files such as F<< <sys/ioctl.h> >>. -(There is a Perl script called B<h2ph> that comes with the Perl kit that -may help you in this, but it's nontrivial.) SCALAR will be read and/or -written depending on the FUNCTION--a pointer to the string value of SCALAR -will be passed as the third argument of the actual C<ioctl> call. (If SCALAR -has no string value but does have a numeric value, that value will be -passed rather than a pointer to the string value. To guarantee this to be -true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> -functions may be needed to manipulate the values of structures used by -C<ioctl>. - -The return value of C<ioctl> (and C<fcntl>) is as follows: - - if OS returns: then Perl returns: - -1 undefined value - 0 string "0 but true" - anything else that number - -Thus Perl returns true on success and false on failure, yet you can -still easily determine the actual value returned by the operating -system: - - $retval = ioctl(...) || -1; - printf "System returned %d\n", $retval; - -The special string "C<0> but true" is exempt from B<-w> complaints -about improper numeric conversions. - -Here's an example of setting a filehandle named C<REMOTE> to be -non-blocking at the system level. You'll have to negotiate C<$|> -on your own, though. - - use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); - - $flags = fcntl(REMOTE, F_GETFL, 0) - or die "Can't get flags for the socket: $!\n"; - - $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) - or die "Can't set flags for the socket: $!\n"; - -=item join EXPR,LIST - -Joins the separate strings of LIST into a single string with fields -separated by the value of EXPR, and returns that new string. Example: - - $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); - -Beware that unlike C<split>, C<join> doesn't take a pattern as its -first argument. Compare L</split>. - -=item keys HASH - -Returns a list consisting of all the keys of the named hash. (In -scalar context, returns the number of keys.) The keys are returned in -an apparently random order. The actual random order is subject to -change in future versions of perl, but it is guaranteed to be the same -order as either the C<values> or C<each> function produces (given -that the hash has not been modified). As a side effect, it resets -HASH's iterator. - -Here is yet another way to print your environment: - - @keys = keys %ENV; - @values = values %ENV; - while (@keys) { - print pop(@keys), '=', pop(@values), "\n"; - } - -or how about sorted by key: - - foreach $key (sort(keys %ENV)) { - print $key, '=', $ENV{$key}, "\n"; - } - -The returned values are copies of the original keys in the hash, so -modifying them will not affect the original hash. Compare L</values>. - -To sort a hash by value, you'll need to use a C<sort> function. -Here's a descending numeric sort of a hash by its values: - - foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { - printf "%4d %s\n", $hash{$key}, $key; - } - -As an lvalue C<keys> allows you to increase the number of hash buckets -allocated for the given hash. This can gain you a measure of efficiency if -you know the hash is going to get big. (This is similar to pre-extending -an array by assigning a larger number to $#array.) If you say - - keys %hash = 200; - -then C<%hash> will have at least 200 buckets allocated for it--256 of them, -in fact, since it rounds up to the next power of two. These -buckets will be retained even if you do C<%hash = ()>, use C<undef -%hash> if you want to free the storage while C<%hash> is still in scope. -You can't shrink the number of buckets allocated for the hash using -C<keys> in this way (but you needn't worry about doing this by accident, -as trying has no effect). - -See also C<each>, C<values> and C<sort>. - -=item kill SIGNAL, LIST - -Sends a signal to a list of processes. Returns the number of -processes successfully signaled (which is not necessarily the -same as the number actually killed). - - $cnt = kill 1, $child1, $child2; - kill 9, @goners; - -If SIGNAL is zero, no signal is sent to the process. This is a -useful way to check that the process is alive and hasn't changed -its UID. See L<perlport> for notes on the portability of this -construct. - -Unlike in the shell, if SIGNAL is negative, it kills -process groups instead of processes. (On System V, a negative I<PROCESS> -number will also kill process groups, but that's not portable.) That -means you usually want to use positive not negative signals. You may also -use a signal name in quotes. See L<perlipc/"Signals"> for details. - -=item last LABEL - -=item last - -The C<last> command is like the C<break> statement in C (as used in -loops); it immediately exits the loop in question. If the LABEL is -omitted, the command refers to the innermost enclosing loop. The -C<continue> block, if any, is not executed: - - LINE: while (<STDIN>) { - last LINE if /^$/; # exit when done with header - #... - } - -C<last> cannot be used to exit a block which returns a value such as -C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit -a grep() or map() operation. - -Note that a block by itself is semantically identical to a loop -that executes once. Thus C<last> can be used to effect an early -exit out of such a block. - -See also L</continue> for an illustration of how C<last>, C<next>, and -C<redo> work. - -=item lc EXPR - -=item lc - -Returns an lowercased version of EXPR. This is the internal function -implementing the C<\L> escape in double-quoted strings. -Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> -and L<utf8>. - -If EXPR is omitted, uses C<$_>. - -=item lcfirst EXPR - -=item lcfirst - -Returns the value of EXPR with the first character lowercased. This is -the internal function implementing the C<\l> escape in double-quoted strings. -Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. - -If EXPR is omitted, uses C<$_>. - -=item length EXPR - -=item length - -Returns the length in characters of the value of EXPR. If EXPR is -omitted, returns length of C<$_>. Note that this cannot be used on -an entire array or hash to find out how many elements these have. -For that, use C<scalar @array> and C<scalar keys %hash> respectively. - -=item link OLDFILE,NEWFILE - -Creates a new filename linked to the old filename. Returns true for -success, false otherwise. - -=item listen SOCKET,QUEUESIZE - -Does the same thing that the listen system call does. Returns true if -it succeeded, false otherwise. See the example in -L<perlipc/"Sockets: Client/Server Communication">. - -=item local EXPR - -You really probably want to be using C<my> instead, because C<local> isn't -what most people think of as "local". See -L<perlsub/"Private Variables via my()"> for details. - -A local modifies the listed variables to be local to the enclosing -block, file, or eval. If more than one value is listed, the list must -be placed in parentheses. See L<perlsub/"Temporary Values via local()"> -for details, including issues with tied arrays and hashes. - -=item localtime EXPR - -Converts a time as returned by the time function to a 9-element list -with the time analyzed for the local time zone. Typically used as -follows: - - # 0 1 2 3 4 5 6 7 8 - ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = - localtime(time); - -All list elements are numeric, and come straight out of the C `struct -tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the -specified time. $mday is the day of the month, and $mon is the month -itself, in the range C<0..11> with 0 indicating January and 11 -indicating December. $year is the number of years since 1900. That -is, $year is C<123> in year 2023. $wday is the day of the week, with -0 indicating Sunday and 3 indicating Wednesday. $yday is the day of -the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst -is true if the specified time occurs during daylight savings time, -false otherwise. - -Note that the $year element is I<not> simply the last two digits of -the year. If you assume it is, then you create non-Y2K-compliant -programs--and you wouldn't want to do that, would you? - -The proper way to get a complete 4-digit year is simply: - - $year += 1900; - -And to get the last two digits of the year (e.g., '01' in 2001) do: - - $year = sprintf("%02d", $year % 100); - -If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). - -In scalar context, C<localtime()> returns the ctime(3) value: - - $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" - -This scalar value is B<not> locale dependent, see L<perllocale>, but -instead a Perl builtin. Also see the C<Time::Local> module -(to convert the second, minutes, hours, ... back to seconds since the -stroke of midnight the 1st of January 1970, the value returned by -time()), and the strftime(3) and mktime(3) functions available via the -POSIX module. To get somewhat similar but locale dependent date -strings, set up your locale environment variables appropriately -(please see L<perllocale>) and try for example: - - use POSIX qw(strftime); - $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; - -Note that the C<%a> and C<%b>, the short forms of the day of the week -and the month of the year, may not necessarily be three characters wide. - -=item lock - - lock I<THING> - -This function places an advisory lock on a variable, subroutine, -or referenced object contained in I<THING> until the lock goes out -of scope. This is a built-in function only if your version of Perl -was built with threading enabled, and if you've said C<use Threads>. -Otherwise a user-defined function by this name will be called. See -L<Thread>. - -=item log EXPR - -=item log - -Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, -returns log of C<$_>. To get the log of another base, use basic algebra: -The base-N log of a number is equal to the natural log of that number -divided by the natural log of N. For example: - - sub log10 { - my $n = shift; - return log($n)/log(10); - } - -See also L</exp> for the inverse operation. - -=item lstat FILEHANDLE - -=item lstat EXPR - -=item lstat - -Does the same thing as the C<stat> function (including setting the -special C<_> filehandle) but stats a symbolic link instead of the file -the symbolic link points to. If symbolic links are unimplemented on -your system, a normal C<stat> is done. - -If EXPR is omitted, stats C<$_>. - -=item m// - -The match operator. See L<perlop>. - -=item map BLOCK LIST - -=item map EXPR,LIST - -Evaluates the BLOCK or EXPR for each element of LIST (locally setting -C<$_> to each element) and returns the list value composed of the -results of each such evaluation. In scalar context, returns the -total number of elements so generated. Evaluates BLOCK or EXPR in -list context, so each element of LIST may produce zero, one, or -more elements in the returned value. - - @chars = map(chr, @nums); - -translates a list of numbers to the corresponding characters. And - - %hash = map { getkey($_) => $_ } @array; - -is just a funny way to write - - %hash = (); - foreach $_ (@array) { - $hash{getkey($_)} = $_; - } - -Note that C<$_> is an alias to the list value, so it can be used to -modify the elements of the LIST. While this is useful and supported, -it can cause bizarre results if the elements of LIST are not variables. -Using a regular C<foreach> loop for this purpose would be clearer in -most cases. See also L</grep> for an array composed of those items of -the original list for which the BLOCK or EXPR evaluates to true. - -C<{> starts both hash references and blocks, so C<map { ...> could be either -the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look -ahead for the closing C<}> it has to take a guess at which its dealing with -based what it finds just after the C<{>. Usually it gets it right, but if it -doesn't it won't realize something is wrong until it gets to the C<}> and -encounters the missing (or unexpected) comma. The syntax error will be -reported close to the C<}> but you'll need to change something near the C<{> -such as using a unary C<+> to give perl some help: - - %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong - %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right - %hash = map { ("\L$_", 1) } @array # this also works - %hash = map { lc($_), 1 } @array # as does this. - %hash = map +( lc($_), 1 ), @array # this is EXPR and works! - - %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) - -or to force an anon hash constructor use C<+{> - - @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end - -and you get list of anonymous hashes each with only 1 entry. - -=item mkdir FILENAME,MASK - -=item mkdir FILENAME - -Creates the directory specified by FILENAME, with permissions -specified by MASK (as modified by C<umask>). If it succeeds it -returns true, otherwise it returns false and sets C<$!> (errno). -If omitted, MASK defaults to 0777. - -In general, it is better to create directories with permissive MASK, -and let the user modify that with their C<umask>, than it is to supply -a restrictive MASK and give the user no way to be more permissive. -The exceptions to this rule are when the file or directory should be -kept private (mail files, for instance). The perlfunc(1) entry on -C<umask> discusses the choice of MASK in more detail. - -=item msgctl ID,CMD,ARG - -Calls the System V IPC function msgctl(2). You'll probably have to say - - use IPC::SysV; - -first to get the correct constant definitions. If CMD is C<IPC_STAT>, -then ARG must be a variable which will hold the returned C<msqid_ds> -structure. Returns like C<ioctl>: the undefined value for error, -C<"0 but true"> for zero, or the actual return value otherwise. See also -L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. - -=item msgget KEY,FLAGS - -Calls the System V IPC function msgget(2). Returns the message queue -id, or the undefined value if there is an error. See also -L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. - -=item msgrcv ID,VAR,SIZE,TYPE,FLAGS - -Calls the System V IPC function msgrcv to receive a message from -message queue ID into variable VAR with a maximum message size of -SIZE. Note that when a message is received, the message type as a -native long integer will be the first thing in VAR, followed by the -actual message. This packing may be opened with C<unpack("l! a*")>. -Taints the variable. Returns true if successful, or false if there is -an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and -C<IPC::SysV::Msg> documentation. - -=item msgsnd ID,MSG,FLAGS - -Calls the System V IPC function msgsnd to send the message MSG to the -message queue ID. MSG must begin with the native long integer message -type, and be followed by the length of the actual message, and finally -the message itself. This kind of packing can be achieved with -C<pack("l! a*", $type, $message)>. Returns true if successful, -or false if there is an error. See also C<IPC::SysV> -and C<IPC::SysV::Msg> documentation. - -=item my EXPR - -=item my EXPR : ATTRIBUTES - -A C<my> declares the listed variables to be local (lexically) to the -enclosing block, file, or C<eval>. If -more than one value is listed, the list must be placed in parentheses. See -L<perlsub/"Private Variables via my()"> for details. - -=item next LABEL - -=item next - -The C<next> command is like the C<continue> statement in C; it starts -the next iteration of the loop: - - LINE: while (<STDIN>) { - next LINE if /^#/; # discard comments - #... - } - -Note that if there were a C<continue> block on the above, it would get -executed even on discarded lines. If the LABEL is omitted, the command -refers to the innermost enclosing loop. - -C<next> cannot be used to exit a block which returns a value such as -C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit -a grep() or map() operation. - -Note that a block by itself is semantically identical to a loop -that executes once. Thus C<next> will exit such a block early. - -See also L</continue> for an illustration of how C<last>, C<next>, and -C<redo> work. - -=item no Module LIST - -See the L</use> function, which C<no> is the opposite of. - -=item oct EXPR - -=item oct - -Interprets EXPR as an octal string and returns the corresponding -value. (If EXPR happens to start off with C<0x>, interprets it as a -hex string. If EXPR starts off with C<0b>, it is interpreted as a -binary string.) The following will handle decimal, binary, octal, and -hex in the standard Perl or C notation: - - $val = oct($val) if $val =~ /^0/; - -If EXPR is omitted, uses C<$_>. To go the other way (produce a number -in octal), use sprintf() or printf(): - - $perms = (stat("filename"))[2] & 07777; - $oct_perms = sprintf "%lo", $perms; - -The oct() function is commonly used when a string such as C<644> needs -to be converted into a file mode, for example. (Although perl will -automatically convert strings into numbers as needed, this automatic -conversion assumes base 10.) - -=item open FILEHANDLE,MODE,LIST - -=item open FILEHANDLE,EXPR - -=item open FILEHANDLE - -Opens the file whose filename is given by EXPR, and associates it with -FILEHANDLE. If FILEHANDLE is an expression, its value is used as the -name of the real filehandle wanted. (This is considered a symbolic -reference, so C<use strict 'refs'> should I<not> be in effect.) - -If EXPR is omitted, the scalar -variable of the same name as the FILEHANDLE contains the filename. -(Note that lexical variables--those declared with C<my>--will not work -for this purpose; so if you're using C<my>, specify EXPR in your call -to open.) See L<perlopentut> for a kinder, gentler explanation of opening -files. - -If MODE is C<< '<' >> or nothing, the file is opened for input. -If MODE is C<< '>' >>, the file is truncated and opened for -output, being created if necessary. If MODE is C<<< '>>' >>>, -the file is opened for appending, again being created if necessary. -You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that -you want both read and write access to the file; thus C<< '+<' >> is almost -always preferred for read/write updates--the C<< '+>' >> mode would clobber the -file first. You can't usually use either read-write mode for updating -textfiles, since they have variable length records. See the B<-i> -switch in L<perlrun> for a better approach. The file is created with -permissions of C<0666> modified by the process' C<umask> value. - -These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, -C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. - -In the 2-arguments (and 1-argument) form of the call the mode and -filename should be concatenated (in this order), possibly separated by -spaces. It is possible to omit the mode if the mode is C<< '<' >>. - -If the filename begins with C<'|'>, the filename is interpreted as a -command to which output is to be piped, and if the filename ends with a -C<'|'>, the filename is interpreted as a command which pipes output to -us. See L<perlipc/"Using open() for IPC"> -for more examples of this. (You are not allowed to C<open> to a command -that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, -and L<perlipc/"Bidirectional Communication with Another Process"> -for alternatives.) - -If MODE is C<'|-'>, the filename is interpreted as a -command to which output is to be piped, and if MODE is -C<'-|'>, the filename is interpreted as a command which pipes output to -us. In the 2-arguments (and 1-argument) form one should replace dash -(C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> -for more examples of this. (You are not allowed to C<open> to a command -that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, -and L<perlipc/"Bidirectional Communication"> for alternatives.) - -In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN -and opening C<< '>-' >> opens STDOUT. - -Open returns -nonzero upon success, the undefined value otherwise. If the C<open> -involved a pipe, the return value happens to be the pid of the -subprocess. - -If you're unfortunate enough to be running Perl on a system that -distinguishes between text files and binary files (modern operating -systems don't care), then you should check out L</binmode> for tips for -dealing with this. The key distinction between systems that need C<binmode> -and those that don't is their text file formats. Systems like Unix, MacOS, and -Plan9, which delimit lines with a single character, and which encode that -character in C as C<"\n">, do not need C<binmode>. The rest need it. - -When opening a file, it's usually a bad idea to continue normal execution -if the request failed, so C<open> is frequently used in connection with -C<die>. Even if C<die> won't do what you want (say, in a CGI script, -where you want to make a nicely formatted error message (but there are -modules that can help with that problem)) you should always check -the return value from opening a file. The infrequent exception is when -working with an unopened filehandle is actually what you want to do. - -Examples: - - $ARTICLE = 100; - open ARTICLE or die "Can't find article $ARTICLE: $!\n"; - while (<ARTICLE>) {... - - open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) - # if the open fails, output is discarded - - open(DBASE, '+<', 'dbase.mine') # open for update - or die "Can't open 'dbase.mine' for update: $!"; - - open(DBASE, '+<dbase.mine') # ditto - or die "Can't open 'dbase.mine' for update: $!"; - - open(ARTICLE, '-|', "caesar <$article") # decrypt article - or die "Can't start caesar: $!"; - - open(ARTICLE, "caesar <$article |") # ditto - or die "Can't start caesar: $!"; - - open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id - or die "Can't start sort: $!"; - - # process argument list of files along with any includes - - foreach $file (@ARGV) { - process($file, 'fh00'); - } - - sub process { - my($filename, $input) = @_; - $input++; # this is a string increment - unless (open($input, $filename)) { - print STDERR "Can't open $filename: $!\n"; - return; - } - - local $_; - while (<$input>) { # note use of indirection - if (/^#include "(.*)"/) { - process($1, $input); - next; - } - #... # whatever - } - } - -You may also, in the Bourne shell tradition, specify an EXPR beginning -with C<< '>&' >>, in which case the rest of the string is interpreted as the -name of a filehandle (or file descriptor, if numeric) to be -duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, -C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The -mode you specify should match the mode of the original filehandle. -(Duping a filehandle does not take into account any existing contents of -stdio buffers.) Duping file handles is not yet supported for 3-argument -open(). - -Here is a script that saves, redirects, and restores STDOUT and -STDERR: - - #!/usr/bin/perl - open(OLDOUT, ">&STDOUT"); - open(OLDERR, ">&STDERR"); - - open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; - open(STDERR, ">&STDOUT") || die "Can't dup stdout"; - - select(STDERR); $| = 1; # make unbuffered - select(STDOUT); $| = 1; # make unbuffered - - print STDOUT "stdout 1\n"; # this works for - print STDERR "stderr 1\n"; # subprocesses too - - close(STDOUT); - close(STDERR); - - open(STDOUT, ">&OLDOUT"); - open(STDERR, ">&OLDERR"); - - print STDOUT "stdout 2\n"; - print STDERR "stderr 2\n"; - -If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an -equivalent of C's C<fdopen> of that file descriptor; this is more -parsimonious of file descriptors. For example: - - open(FILEHANDLE, "<&=$fd") - -Note that this feature depends on the fdopen() C library function. -On many UNIX systems, fdopen() is known to fail when file descriptors -exceed a certain value, typically 255. If you need more file -descriptors than that, consider rebuilding Perl to use the C<sfio> -library. - -If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> -with 2-arguments (or 1-argument) form of open(), then -there is an implicit fork done, and the return value of open is the pid -of the child within the parent process, and C<0> within the child -process. (Use C<defined($pid)> to determine whether the open was successful.) -The filehandle behaves normally for the parent, but i/o to that -filehandle is piped from/to the STDOUT/STDIN of the child process. -In the child process the filehandle isn't opened--i/o happens from/to -the new STDOUT or STDIN. Typically this is used like the normal -piped open when you want to exercise more control over just how the -pipe command gets executed, such as when you are running setuid, and -don't want to have to scan shell commands for metacharacters. -The following triples are more or less equivalent: - - open(FOO, "|tr '[a-z]' '[A-Z]'"); - open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); - open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; - - open(FOO, "cat -n '$file'|"); - open(FOO, '-|', "cat -n '$file'"); - open(FOO, '-|') || exec 'cat', '-n', $file; - -See L<perlipc/"Safe Pipe Opens"> for more examples of this. - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before any operation that may do a fork, but this may not be -supported on some platforms (see L<perlport>). To be safe, you may need -to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method -of C<IO::Handle> on any open handles. - -On systems that support a -close-on-exec flag on files, the flag will be set for the newly opened -file descriptor as determined by the value of $^F. See L<perlvar/$^F>. - -Closing any piped filehandle causes the parent process to wait for the -child to finish, and returns the status value in C<$?>. - -The filename passed to 2-argument (or 1-argument) form of open() -will have leading and trailing -whitespace deleted, and the normal redirection characters -honored. This property, known as "magic open", -can often be used to good effect. A user could specify a filename of -F<"rsh cat file |">, or you could change certain filenames as needed: - - $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; - open(FH, $filename) or die "Can't open $filename: $!"; - -Use 3-argument form to open a file with arbitrary weird characters in it, - - open(FOO, '<', $file); - -otherwise it's necessary to protect any leading and trailing whitespace: - - $file =~ s#^(\s)#./$1#; - open(FOO, "< $file\0"); - -(this may not work on some bizarre filesystems). One should -conscientiously choose between the I<magic> and 3-arguments form -of open(): - - open IN, $ARGV[0]; - -will allow the user to specify an argument of the form C<"rsh cat file |">, -but will not work on a filename which happens to have a trailing space, while - - open IN, '<', $ARGV[0]; - -will have exactly the opposite restrictions. - -If you want a "real" C C<open> (see L<open(2)> on your system), then you -should use the C<sysopen> function, which involves no such magic (but -may use subtly different filemodes than Perl open(), which is mapped -to C fopen()). This is -another way to protect your filenames from interpretation. For example: - - use IO::Handle; - sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) - or die "sysopen $path: $!"; - $oldfh = select(HANDLE); $| = 1; select($oldfh); - print HANDLE "stuff $$\n"; - seek(HANDLE, 0, 0); - print "File contains: ", <HANDLE>; - -Using the constructor from the C<IO::Handle> package (or one of its -subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous -filehandles that have the scope of whatever variables hold references to -them, and automatically close whenever and however you leave that scope: - - use IO::File; - #... - sub read_myfile_munged { - my $ALL = shift; - my $handle = new IO::File; - open($handle, "myfile") or die "myfile: $!"; - $first = <$handle> - or return (); # Automatically closed here. - mung $first or die "mung failed"; # Or here. - return $first, <$handle> if $ALL; # Or here. - $first; # Or here. - } - -See L</seek> for some details about mixing reading and writing. - -=item opendir DIRHANDLE,EXPR - -Opens a directory named EXPR for processing by C<readdir>, C<telldir>, -C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. -DIRHANDLEs have their own namespace separate from FILEHANDLEs. - -=item ord EXPR - -=item ord - -Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If -EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. -See L<utf8> for more about Unicode. - -=item our EXPR - -An C<our> declares the listed variables to be valid globals within -the enclosing block, file, or C<eval>. That is, it has the same -scoping rules as a "my" declaration, but does not create a local -variable. If more than one value is listed, the list must be placed -in parentheses. The C<our> declaration has no semantic effect unless -"use strict vars" is in effect, in which case it lets you use the -declared global variable without qualifying it with a package name. -(But only within the lexical scope of the C<our> declaration. In this -it differs from "use vars", which is package scoped.) - -An C<our> declaration declares a global variable that will be visible -across its entire lexical scope, even across package boundaries. The -package in which the variable is entered is determined at the point -of the declaration, not at the point of use. This means the following -behavior holds: - - package Foo; - our $bar; # declares $Foo::bar for rest of lexical scope - $bar = 20; - - package Bar; - print $bar; # prints 20 - -Multiple C<our> declarations in the same lexical scope are allowed -if they are in different packages. If they happened to be in the same -package, Perl will emit warnings if you have asked for them. - - use warnings; - package Foo; - our $bar; # declares $Foo::bar for rest of lexical scope - $bar = 20; - - package Bar; - our $bar = 30; # declares $Bar::bar for rest of lexical scope - print $bar; # prints 30 - - our $bar; # emits warning - -=item pack TEMPLATE,LIST - -Takes a LIST of values and converts it into a string using the rules -given by the TEMPLATE. The resulting string is the concatenation of -the converted values. Typically, each converted value looks -like its machine-level representation. For example, on 32-bit machines -a converted integer may be represented by a sequence of 4 bytes. - -The TEMPLATE is a -sequence of characters that give the order and type of values, as -follows: - - a A string with arbitrary binary data, will be null padded. - A An ASCII string, will be space padded. - Z A null terminated (asciz) string, will be null padded. - - b A bit string (ascending bit order inside each byte, like vec()). - B A bit string (descending bit order inside each byte). - h A hex string (low nybble first). - H A hex string (high nybble first). - - c A signed char value. - C An unsigned char value. Only does bytes. See U for Unicode. - - s A signed short value. - S An unsigned short value. - (This 'short' is _exactly_ 16 bits, which may differ from - what a local C compiler calls 'short'. If you want - native-length shorts, use the '!' suffix.) - - i A signed integer value. - I An unsigned integer value. - (This 'integer' is _at_least_ 32 bits wide. Its exact - size depends on what a local C compiler calls 'int', - and may even be larger than the 'long' described in - the next item.) - - l A signed long value. - L An unsigned long value. - (This 'long' is _exactly_ 32 bits, which may differ from - what a local C compiler calls 'long'. If you want - native-length longs, use the '!' suffix.) - - n An unsigned short in "network" (big-endian) order. - N An unsigned long in "network" (big-endian) order. - v An unsigned short in "VAX" (little-endian) order. - V An unsigned long in "VAX" (little-endian) order. - (These 'shorts' and 'longs' are _exactly_ 16 bits and - _exactly_ 32 bits, respectively.) - - q A signed quad (64-bit) value. - Q An unsigned quad value. - (Quads are available only if your system supports 64-bit - integer values _and_ if Perl has been compiled to support those. - Causes a fatal error otherwise.) - - f A single-precision float in the native format. - d A double-precision float in the native format. - - p A pointer to a null-terminated string. - P A pointer to a structure (fixed-length string). - - u A uuencoded string. - U A Unicode character number. Encodes to UTF-8 internally. - Works even if C<use utf8> is not in effect. - - w A BER compressed integer. Its bytes represent an unsigned - integer in base 128, most significant digit first, with as - few digits as possible. Bit eight (the high bit) is set - on each byte except the last. - - x A null byte. - X Back up a byte. - @ Null fill to absolute position. - -The following rules apply: - -=over 8 - -=item * - -Each letter may optionally be followed by a number giving a repeat -count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, -C<H>, and C<P> the pack function will gobble up that many values from -the LIST. A C<*> for the repeat count means to use however many items are -left, except for C<@>, C<x>, C<X>, where it is equivalent -to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the -same). - -When used with C<Z>, C<*> results in the addition of a trailing null -byte (so the packed result will be one longer than the byte C<length> -of the item). - -The repeat count for C<u> is interpreted as the maximal number of bytes -to encode per line of output, with 0 and 1 replaced by 45. - -=item * - -The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a -string of length count, padding with nulls or spaces as necessary. When -unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything -after the first null, and C<a> returns data verbatim. When packing, -C<a>, and C<Z> are equivalent. - -If the value-to-pack is too long, it is truncated. If too long and an -explicit count is provided, C<Z> packs only C<$count-1> bytes, followed -by a null byte. Thus C<Z> always packs a trailing null byte under -all circumstances. - -=item * - -Likewise, the C<b> and C<B> fields pack a string that many bits long. -Each byte of the input field of pack() generates 1 bit of the result. -Each result bit is based on the least-significant bit of the corresponding -input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and -C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. - -Starting from the beginning of the input string of pack(), each 8-tuple -of bytes is converted to 1 byte of output. With format C<b> -the first byte of the 8-tuple determines the least-significant bit of a -byte, and with format C<B> it determines the most-significant bit of -a byte. - -If the length of the input string is not exactly divisible by 8, the -remainder is packed as if the input string were padded by null bytes -at the end. Similarly, during unpack()ing the "extra" bits are ignored. - -If the input string of pack() is longer than needed, extra bytes are ignored. -A C<*> for the repeat count of pack() means to use all the bytes of -the input field. On unpack()ing the bits are converted to a string -of C<"0">s and C<"1">s. - -=item * - -The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, -representable as hexadecimal digits, 0-9a-f) long. - -Each byte of the input field of pack() generates 4 bits of the result. -For non-alphabetical bytes the result is based on the 4 least-significant -bits of the input byte, i.e., on C<ord($byte)%16>. In particular, -bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes -C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result -is compatible with the usual hexadecimal digits, so that C<"a"> and -C<"A"> both generate the nybble C<0xa==10>. The result for bytes -C<"g".."z"> and C<"G".."Z"> is not well-defined. - -Starting from the beginning of the input string of pack(), each pair -of bytes is converted to 1 byte of output. With format C<h> the -first byte of the pair determines the least-significant nybble of the -output byte, and with format C<H> it determines the most-significant -nybble. - -If the length of the input string is not even, it behaves as if padded -by a null byte at the end. Similarly, during unpack()ing the "extra" -nybbles are ignored. - -If the input string of pack() is longer than needed, extra bytes are ignored. -A C<*> for the repeat count of pack() means to use all the bytes of -the input field. On unpack()ing the bits are converted to a string -of hexadecimal digits. - -=item * - -The C<p> type packs a pointer to a null-terminated string. You are -responsible for ensuring the string is not a temporary value (which can -potentially get deallocated before you get around to using the packed result). -The C<P> type packs a pointer to a structure of the size indicated by the -length. A NULL pointer is created if the corresponding value for C<p> or -C<P> is C<undef>, similarly for unpack(). - -=item * - -The C</> template character allows packing and unpacking of strings where -the packed structure contains a byte count followed by the string itself. -You write I<length-item>C</>I<string-item>. - -The I<length-item> can be any C<pack> template letter, -and describes how the length value is packed. -The ones likely to be of most use are integer-packing ones like -C<n> (for Java strings), C<w> (for ASN.1 or SNMP) -and C<N> (for Sun XDR). - -The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. -For C<unpack> the length of the string is obtained from the I<length-item>, -but if you put in the '*' it will be ignored. - - unpack 'C/a', "\04Gurusamy"; gives 'Guru' - unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') - pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" - -The I<length-item> is not returned explicitly from C<unpack>. - -Adding a count to the I<length-item> letter is unlikely to do anything -useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a -I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, -which Perl does not regard as legal in numeric strings. - -=item * - -The integer types C<s>, C<S>, C<l>, and C<L> may be -immediately followed by a C<!> suffix to signify native shorts or -longs--as you can see from above for example a bare C<l> does mean -exactly 32 bits, the native C<long> (as seen by the local C compiler) -may be larger. This is an issue mainly in 64-bit platforms. You can -see whether using C<!> makes any difference by - - print length(pack("s")), " ", length(pack("s!")), "\n"; - print length(pack("l")), " ", length(pack("l!")), "\n"; - -C<i!> and C<I!> also work but only because of completeness; -they are identical to C<i> and C<I>. - -The actual sizes (in bytes) of native shorts, ints, longs, and long -longs on the platform where Perl was built are also available via -L<Config>: - - use Config; - print $Config{shortsize}, "\n"; - print $Config{intsize}, "\n"; - print $Config{longsize}, "\n"; - print $Config{longlongsize}, "\n"; - -(The C<$Config{longlongsize}> will be undefine if your system does -not support long longs.) - -=item * - -The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> -are inherently non-portable between processors and operating systems -because they obey the native byteorder and endianness. For example a -4-byte integer 0x12345678 (305419896 decimal) be ordered natively -(arranged in and handled by the CPU registers) into bytes as - - 0x12 0x34 0x56 0x78 # big-endian - 0x78 0x56 0x34 0x12 # little-endian - -Basically, the Intel and VAX CPUs are little-endian, while everybody -else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and -Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq -used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode. - -The names `big-endian' and `little-endian' are comic references to -the classic "Gulliver's Travels" (via the paper "On Holy Wars and a -Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and -the egg-eating habits of the Lilliputians. - -Some systems may have even weirder byte orders such as - - 0x56 0x78 0x12 0x34 - 0x34 0x12 0x78 0x56 - -You can see your system's preference with - - print join(" ", map { sprintf "%#02x", $_ } - unpack("C*",pack("L",0x12345678))), "\n"; - -The byteorder on the platform where Perl was built is also available -via L<Config>: - - use Config; - print $Config{byteorder}, "\n"; - -Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> -and C<'87654321'> are big-endian. - -If you want portable packed integers use the formats C<n>, C<N>, -C<v>, and C<V>, their byte endianness and size is known. -See also L<perlport>. - -=item * - -Real numbers (floats and doubles) are in the native machine format only; -due to the multiplicity of floating formats around, and the lack of a -standard "network" representation, no facility for interchange has been -made. This means that packed floating point data written on one machine -may not be readable on another - even if both use IEEE floating point -arithmetic (as the endian-ness of the memory representation is not part -of the IEEE spec). See also L<perlport>. - -Note that Perl uses doubles internally for all numeric calculation, and -converting from double into float and thence back to double again will -lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general -equal $foo). - -=item * - -If the pattern begins with a C<U>, the resulting string will be treated -as Unicode-encoded. You can force UTF8 encoding on in a string with an -initial C<U0>, and the bytes that follow will be interpreted as Unicode -characters. If you don't want this to happen, you can begin your pattern -with C<C0> (or anything else) to force Perl not to UTF8 encode your -string, and then follow this with a C<U*> somewhere in your pattern. - -=item * - -You must yourself do any alignment or padding by inserting for example -enough C<'x'>es while packing. There is no way to pack() and unpack() -could know where the bytes are going to or coming from. Therefore -C<pack> (and C<unpack>) handle their output and input as flat -sequences of bytes. - -=item * - -A comment in a TEMPLATE starts with C<#> and goes to the end of line. - -=item * - -If TEMPLATE requires more arguments to pack() than actually given, pack() -assumes additional C<""> arguments. If TEMPLATE requires less arguments -to pack() than actually given, extra arguments are ignored. - -=back - -Examples: - - $foo = pack("CCCC",65,66,67,68); - # foo eq "ABCD" - $foo = pack("C4",65,66,67,68); - # same thing - $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); - # same thing with Unicode circled letters - - $foo = pack("ccxxcc",65,66,67,68); - # foo eq "AB\0\0CD" - - # note: the above examples featuring "C" and "c" are true - # only on ASCII and ASCII-derived systems such as ISO Latin 1 - # and UTF-8. In EBCDIC the first example would be - # $foo = pack("CCCC",193,194,195,196); - - $foo = pack("s2",1,2); - # "\1\0\2\0" on little-endian - # "\0\1\0\2" on big-endian - - $foo = pack("a4","abcd","x","y","z"); - # "abcd" - - $foo = pack("aaaa","abcd","x","y","z"); - # "axyz" - - $foo = pack("a14","abcdefg"); - # "abcdefg\0\0\0\0\0\0\0" - - $foo = pack("i9pl", gmtime); - # a real struct tm (on my system anyway) - - $utmp_template = "Z8 Z8 Z16 L"; - $utmp = pack($utmp_template, @utmp1); - # a struct utmp (BSDish) - - @utmp2 = unpack($utmp_template, $utmp); - # "@utmp1" eq "@utmp2" - - sub bintodec { - unpack("N", pack("B32", substr("0" x 32 . shift, -32))); - } - - $foo = pack('sx2l', 12, 34); - # short 12, two zero bytes padding, long 34 - $bar = pack('s@4l', 12, 34); - # short 12, zero fill to position 4, long 34 - # $foo eq $bar - -The same template may generally also be used in unpack(). - -=item package NAMESPACE - -=item package - -Declares the compilation unit as being in the given namespace. The scope -of the package declaration is from the declaration itself through the end -of the enclosing block, file, or eval (the same as the C<my> operator). -All further unqualified dynamic identifiers will be in this namespace. -A package statement affects only dynamic variables--including those -you've used C<local> on--but I<not> lexical variables, which are created -with C<my>. Typically it would be the first declaration in a file to -be included by the C<require> or C<use> operator. You can switch into a -package in more than one place; it merely influences which symbol table -is used by the compiler for the rest of that block. You can refer to -variables and filehandles in other packages by prefixing the identifier -with the package name and a double colon: C<$Package::Variable>. -If the package name is null, the C<main> package as assumed. That is, -C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, -still seen in older code). - -If NAMESPACE is omitted, then there is no current package, and all -identifiers must be fully qualified or lexicals. This is stricter -than C<use strict>, since it also extends to function names. - -See L<perlmod/"Packages"> for more information about packages, modules, -and classes. See L<perlsub> for other scoping issues. - -=item pipe READHANDLE,WRITEHANDLE - -Opens a pair of connected pipes like the corresponding system call. -Note that if you set up a loop of piped processes, deadlock can occur -unless you are very careful. In addition, note that Perl's pipes use -stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE -after each command, depending on the application. - -See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> -for examples of such things. - -On systems that support a close-on-exec flag on files, the flag will be set -for the newly opened file descriptors as determined by the value of $^F. -See L<perlvar/$^F>. - -=item pop ARRAY - -=item pop - -Pops and returns the last value of the array, shortening the array by -one element. Has an effect similar to - - $ARRAY[$#ARRAY--] - -If there are no elements in the array, returns the undefined value -(although this may happen at other times as well). If ARRAY is -omitted, pops the C<@ARGV> array in the main program, and the C<@_> -array in subroutines, just like C<shift>. - -=item pos SCALAR - -=item pos - -Returns the offset of where the last C<m//g> search left off for the variable -in question (C<$_> is used when the variable is not specified). May be -modified to change that offset. Such modification will also influence -the C<\G> zero-width assertion in regular expressions. See L<perlre> and -L<perlop>. - -=item print FILEHANDLE LIST - -=item print LIST - -=item print - -Prints a string or a list of strings. Returns true if successful. -FILEHANDLE may be a scalar variable name, in which case the variable -contains the name of or a reference to the filehandle, thus introducing -one level of indirection. (NOTE: If FILEHANDLE is a variable and -the next token is a term, it may be misinterpreted as an operator -unless you interpose a C<+> or put parentheses around the arguments.) -If FILEHANDLE is omitted, prints by default to standard output (or -to the last selected output channel--see L</select>). If LIST is -also omitted, prints C<$_> to the currently selected output channel. -To set the default output channel to something other than STDOUT -use the select operation. The current value of C<$,> (if any) is -printed between each LIST item. The current value of C<$\> (if -any) is printed after the entire LIST has been printed. Because -print takes a LIST, anything in the LIST is evaluated in list -context, and any subroutine that you call will have one or more of -its expressions evaluated in list context. Also be careful not to -follow the print keyword with a left parenthesis unless you want -the corresponding right parenthesis to terminate the arguments to -the print--interpose a C<+> or put parentheses around all the -arguments. - -Note that if you're storing FILEHANDLES in an array or other expression, -you will have to use a block returning its value instead: - - print { $files[$i] } "stuff\n"; - print { $OK ? STDOUT : STDERR } "stuff\n"; - -=item printf FILEHANDLE FORMAT, LIST - -=item printf FORMAT, LIST - -Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> -(the output record separator) is not appended. The first argument -of the list will be interpreted as the C<printf> format. If C<use locale> is -in effect, the character used for the decimal point in formatted real numbers -is affected by the LC_NUMERIC locale. See L<perllocale>. - -Don't fall into the trap of using a C<printf> when a simple -C<print> would do. The C<print> is more efficient and less -error prone. - -=item prototype FUNCTION - -Returns the prototype of a function as a string (or C<undef> if the -function has no prototype). FUNCTION is a reference to, or the name of, -the function whose prototype you want to retrieve. - -If FUNCTION is a string starting with C<CORE::>, the rest is taken as a -name for Perl builtin. If the builtin is not I<overridable> (such as -C<qw//>) or its arguments cannot be expressed by a prototype (such as -C<system>) returns C<undef> because the builtin does not really behave -like a Perl function. Otherwise, the string describing the equivalent -prototype is returned. - -=item push ARRAY,LIST - -Treats ARRAY as a stack, and pushes the values of LIST -onto the end of ARRAY. The length of ARRAY increases by the length of -LIST. Has the same effect as - - for $value (LIST) { - $ARRAY[++$#ARRAY] = $value; - } - -but is more efficient. Returns the new number of elements in the array. - -=item q/STRING/ - -=item qq/STRING/ - -=item qr/STRING/ - -=item qx/STRING/ - -=item qw/STRING/ - -Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. - -=item quotemeta EXPR - -=item quotemeta - -Returns the value of EXPR with all non-"word" -characters backslashed. (That is, all characters not matching -C</[A-Za-z_0-9]/> will be preceded by a backslash in the -returned string, regardless of any locale settings.) -This is the internal function implementing -the C<\Q> escape in double-quoted strings. - -If EXPR is omitted, uses C<$_>. - -=item rand EXPR - -=item rand - -Returns a random fractional number greater than or equal to C<0> and less -than the value of EXPR. (EXPR should be positive.) If EXPR is -omitted, the value C<1> is used. Automatically calls C<srand> unless -C<srand> has already been called. See also C<srand>. - -(Note: If your rand function consistently returns numbers that are too -large or too small, then your version of Perl was probably compiled -with the wrong number of RANDBITS.) - -=item read FILEHANDLE,SCALAR,LENGTH,OFFSET - -=item read FILEHANDLE,SCALAR,LENGTH - -Attempts to read LENGTH bytes of data into variable SCALAR from the -specified FILEHANDLE. Returns the number of bytes actually read, C<0> -at end of file, or undef if there was an error. SCALAR will be grown -or shrunk to the length actually read. If SCALAR needs growing, the -new bytes will be zero bytes. An OFFSET may be specified to place -the read data into some other place in SCALAR than the beginning. -The call is actually implemented in terms of stdio's fread(3) call. -To get a true read(2) system call, see C<sysread>. - -=item readdir DIRHANDLE - -Returns the next directory entry for a directory opened by C<opendir>. -If used in list context, returns all the rest of the entries in the -directory. If there are no more entries, returns an undefined value in -scalar context or a null list in list context. - -If you're planning to filetest the return values out of a C<readdir>, you'd -better prepend the directory in question. Otherwise, because we didn't -C<chdir> there, it would have been testing the wrong file. - - opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; - @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); - closedir DIR; - -=item readline EXPR - -Reads from the filehandle whose typeglob is contained in EXPR. In scalar -context, each call reads and returns the next line, until end-of-file is -reached, whereupon the subsequent call returns undef. In list context, -reads until end-of-file is reached and returns a list of lines. Note that -the notion of "line" used here is however you may have defined it -with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. - -When C<$/> is set to C<undef>, when readline() is in scalar -context (i.e. file slurp mode), and when an empty file is read, it -returns C<''> the first time, followed by C<undef> subsequently. - -This is the internal function implementing the C<< <EXPR> >> -operator, but you can use it directly. The C<< <EXPR> >> -operator is discussed in more detail in L<perlop/"I/O Operators">. - - $line = <STDIN>; - $line = readline(*STDIN); # same thing - -=item readlink EXPR - -=item readlink - -Returns the value of a symbolic link, if symbolic links are -implemented. If not, gives a fatal error. If there is some system -error, returns the undefined value and sets C<$!> (errno). If EXPR is -omitted, uses C<$_>. - -=item readpipe EXPR - -EXPR is executed as a system command. -The collected standard output of the command is returned. -In scalar context, it comes back as a single (potentially -multi-line) string. In list context, returns a list of lines -(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). -This is the internal function implementing the C<qx/EXPR/> -operator, but you can use it directly. The C<qx/EXPR/> -operator is discussed in more detail in L<perlop/"I/O Operators">. - -=item recv SOCKET,SCALAR,LENGTH,FLAGS - -Receives a message on a socket. Attempts to receive LENGTH bytes of -data into variable SCALAR from the specified SOCKET filehandle. SCALAR -will be grown or shrunk to the length actually read. Takes the same -flags as the system call of the same name. Returns the address of the -sender if SOCKET's protocol supports this; returns an empty string -otherwise. If there's an error, returns the undefined value. This call -is actually implemented in terms of recvfrom(2) system call. See -L<perlipc/"UDP: Message Passing"> for examples. - -=item redo LABEL - -=item redo - -The C<redo> command restarts the loop block without evaluating the -conditional again. The C<continue> block, if any, is not executed. If -the LABEL is omitted, the command refers to the innermost enclosing -loop. This command is normally used by programs that want to lie to -themselves about what was just input: - - # a simpleminded Pascal comment stripper - # (warning: assumes no { or } in strings) - LINE: while (<STDIN>) { - while (s|({.*}.*){.*}|$1 |) {} - s|{.*}| |; - if (s|{.*| |) { - $front = $_; - while (<STDIN>) { - if (/}/) { # end of comment? - s|^|$front\{|; - redo LINE; - } - } - } - print; - } - -C<redo> cannot be used to retry a block which returns a value such as -C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit -a grep() or map() operation. - -Note that a block by itself is semantically identical to a loop -that executes once. Thus C<redo> inside such a block will effectively -turn it into a looping construct. - -See also L</continue> for an illustration of how C<last>, C<next>, and -C<redo> work. - -=item ref EXPR - -=item ref - -Returns a true value if EXPR is a reference, false otherwise. If EXPR -is not specified, C<$_> will be used. The value returned depends on the -type of thing the reference is a reference to. -Builtin types include: - - SCALAR - ARRAY - HASH - CODE - REF - GLOB - LVALUE - -If the referenced object has been blessed into a package, then that package -name is returned instead. You can think of C<ref> as a C<typeof> operator. - - if (ref($r) eq "HASH") { - print "r is a reference to a hash.\n"; - } - unless (ref($r)) { - print "r is not a reference at all.\n"; - } - if (UNIVERSAL::isa($r, "HASH")) { # for subclassing - print "r is a reference to something that isa hash.\n"; - } - -See also L<perlref>. - -=item rename OLDNAME,NEWNAME - -Changes the name of a file; an existing file NEWNAME will be -clobbered. Returns true for success, false otherwise. - -Behavior of this function varies wildly depending on your system -implementation. For example, it will usually not work across file system -boundaries, even though the system I<mv> command sometimes compensates -for this. Other restrictions include whether it works on directories, -open files, or pre-existing files. Check L<perlport> and either the -rename(2) manpage or equivalent system documentation for details. - -=item require VERSION - -=item require EXPR - -=item require - -Demands some semantics specified by EXPR, or by C<$_> if EXPR is not -supplied. - -If a VERSION is specified as a literal of the form v5.6.1, -demands that the current version of Perl (C<$^V> or $PERL_VERSION) be -at least as recent as that version, at run time. (For compatibility -with older versions of Perl, a numeric argument will also be interpreted -as VERSION.) Compare with L</use>, which can do a similar check at -compile time. - - require v5.6.1; # run time version check - require 5.6.1; # ditto - require 5.005_03; # float version allowed for compatibility - -Otherwise, demands that a library file be included if it hasn't already -been included. The file is included via the do-FILE mechanism, which is -essentially just a variety of C<eval>. Has semantics similar to the following -subroutine: - - sub require { - my($filename) = @_; - return 1 if $INC{$filename}; - my($realfilename,$result); - ITER: { - foreach $prefix (@INC) { - $realfilename = "$prefix/$filename"; - if (-f $realfilename) { - $INC{$filename} = $realfilename; - $result = do $realfilename; - last ITER; - } - } - die "Can't find $filename in \@INC"; - } - delete $INC{$filename} if $@ || !$result; - die $@ if $@; - die "$filename did not return true value" unless $result; - return $result; - } - -Note that the file will not be included twice under the same specified -name. The file must return true as the last statement to indicate -successful execution of any initialization code, so it's customary to -end such a file with C<1;> unless you're sure it'll return true -otherwise. But it's better just to put the C<1;>, in case you add more -statements. - -If EXPR is a bareword, the require assumes a "F<.pm>" extension and -replaces "F<::>" with "F</>" in the filename for you, -to make it easy to load standard modules. This form of loading of -modules does not risk altering your namespace. - -In other words, if you try this: - - require Foo::Bar; # a splendid bareword - -The require function will actually look for the "F<Foo/Bar.pm>" file in the -directories specified in the C<@INC> array. - -But if you try this: - - $class = 'Foo::Bar'; - require $class; # $class is not a bareword - #or - require "Foo::Bar"; # not a bareword because of the "" - -The require function will look for the "F<Foo::Bar>" file in the @INC array and -will complain about not finding "F<Foo::Bar>" there. In this case you can do: - - eval "require $class"; - -For a yet-more-powerful import facility, see L</use> and L<perlmod>. - -=item reset EXPR - -=item reset - -Generally used in a C<continue> block at the end of a loop to clear -variables and reset C<??> searches so that they work again. The -expression is interpreted as a list of single characters (hyphens -allowed for ranges). All variables and arrays beginning with one of -those letters are reset to their pristine state. If the expression is -omitted, one-match searches (C<?pattern?>) are reset to match again. Resets -only variables or searches in the current package. Always returns -1. Examples: - - reset 'X'; # reset all X variables - reset 'a-z'; # reset lower case variables - reset; # just reset ?one-time? searches - -Resetting C<"A-Z"> is not recommended because you'll wipe out your -C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package -variables--lexical variables are unaffected, but they clean themselves -up on scope exit anyway, so you'll probably want to use them instead. -See L</my>. - -=item return EXPR - -=item return - -Returns from a subroutine, C<eval>, or C<do FILE> with the value -given in EXPR. Evaluation of EXPR may be in list, scalar, or void -context, depending on how the return value will be used, and the context -may vary from one execution to the next (see C<wantarray>). If no EXPR -is given, returns an empty list in list context, the undefined value in -scalar context, and (of course) nothing at all in a void context. - -(Note that in the absence of a explicit C<return>, a subroutine, eval, -or do FILE will automatically return the value of the last expression -evaluated.) - -=item reverse LIST - -In list context, returns a list value consisting of the elements -of LIST in the opposite order. In scalar context, concatenates the -elements of LIST and returns a string value with all characters -in the opposite order. - - print reverse <>; # line tac, last line first - - undef $/; # for efficiency of <> - print scalar reverse <>; # character tac, last line tsrif - -This operator is also handy for inverting a hash, although there are some -caveats. If a value is duplicated in the original hash, only one of those -can be represented as a key in the inverted hash. Also, this has to -unwind one hash and build a whole new one, which may take some time -on a large hash, such as from a DBM file. - - %by_name = reverse %by_address; # Invert the hash - -=item rewinddir DIRHANDLE - -Sets the current position to the beginning of the directory for the -C<readdir> routine on DIRHANDLE. - -=item rindex STR,SUBSTR,POSITION - -=item rindex STR,SUBSTR - -Works just like index() except that it returns the position of the LAST -occurrence of SUBSTR in STR. If POSITION is specified, returns the -last occurrence at or before that position. - -=item rmdir FILENAME - -=item rmdir - -Deletes the directory specified by FILENAME if that directory is empty. If it -succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If -FILENAME is omitted, uses C<$_>. - -=item s/// - -The substitution operator. See L<perlop>. - -=item scalar EXPR - -Forces EXPR to be interpreted in scalar context and returns the value -of EXPR. - - @counts = ( scalar @a, scalar @b, scalar @c ); - -There is no equivalent operator to force an expression to -be interpolated in list context because in practice, this is never -needed. If you really wanted to do so, however, you could use -the construction C<@{[ (some expression) ]}>, but usually a simple -C<(some expression)> suffices. - -Because C<scalar> is unary operator, if you accidentally use for EXPR a -parenthesized list, this behaves as a scalar comma expression, evaluating -all but the last element in void context and returning the final element -evaluated in scalar context. This is seldom what you want. - -The following single statement: - - print uc(scalar(&foo,$bar)),$baz; - -is the moral equivalent of these two: - - &foo; - print(uc($bar),$baz); - -See L<perlop> for more details on unary operators and the comma operator. - -=item seek FILEHANDLE,POSITION,WHENCE - -Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. -FILEHANDLE may be an expression whose value gives the name of the -filehandle. The values for WHENCE are C<0> to set the new position to -POSITION, C<1> to set it to the current position plus POSITION, and -C<2> to set it to EOF plus POSITION (typically negative). For WHENCE -you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> -(start of the file, current position, end of the file) from the Fcntl -module. Returns C<1> upon success, C<0> otherwise. - -If you want to position file for C<sysread> or C<syswrite>, don't use -C<seek>--buffering makes its effect on the file's system position -unpredictable and non-portable. Use C<sysseek> instead. - -Due to the rules and rigors of ANSI C, on some systems you have to do a -seek whenever you switch between reading and writing. Amongst other -things, this may have the effect of calling stdio's clearerr(3). -A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: - - seek(TEST,0,1); - -This is also useful for applications emulating C<tail -f>. Once you hit -EOF on your read, and then sleep for a while, you might have to stick in a -seek() to reset things. The C<seek> doesn't change the current position, -but it I<does> clear the end-of-file condition on the handle, so that the -next C<< <FILE> >> makes Perl try again to read something. We hope. - -If that doesn't work (some stdios are particularly cantankerous), then -you may need something more like this: - - for (;;) { - for ($curpos = tell(FILE); $_ = <FILE>; - $curpos = tell(FILE)) { - # search for some stuff and put it into files - } - sleep($for_a_while); - seek(FILE, $curpos, 0); - } - -=item seekdir DIRHANDLE,POS - -Sets the current position for the C<readdir> routine on DIRHANDLE. POS -must be a value returned by C<telldir>. Has the same caveats about -possible directory compaction as the corresponding system library -routine. - -=item select FILEHANDLE - -=item select - -Returns the currently selected filehandle. Sets the current default -filehandle for output, if FILEHANDLE is supplied. This has two -effects: first, a C<write> or a C<print> without a filehandle will -default to this FILEHANDLE. Second, references to variables related to -output will refer to this output channel. For example, if you have to -set the top of form format for more than one output channel, you might -do the following: - - select(REPORT1); - $^ = 'report1_top'; - select(REPORT2); - $^ = 'report2_top'; - -FILEHANDLE may be an expression whose value gives the name of the -actual filehandle. Thus: - - $oldfh = select(STDERR); $| = 1; select($oldfh); - -Some programmers may prefer to think of filehandles as objects with -methods, preferring to write the last example as: - - use IO::Handle; - STDERR->autoflush(1); - -=item select RBITS,WBITS,EBITS,TIMEOUT - -This calls the select(2) system call with the bit masks specified, which -can be constructed using C<fileno> and C<vec>, along these lines: - - $rin = $win = $ein = ''; - vec($rin,fileno(STDIN),1) = 1; - vec($win,fileno(STDOUT),1) = 1; - $ein = $rin | $win; - -If you want to select on many filehandles you might wish to write a -subroutine: - - sub fhbits { - my(@fhlist) = split(' ',$_[0]); - my($bits); - for (@fhlist) { - vec($bits,fileno($_),1) = 1; - } - $bits; - } - $rin = fhbits('STDIN TTY SOCK'); - -The usual idiom is: - - ($nfound,$timeleft) = - select($rout=$rin, $wout=$win, $eout=$ein, $timeout); - -or to block until something becomes ready just do this - - $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); - -Most systems do not bother to return anything useful in $timeleft, so -calling select() in scalar context just returns $nfound. - -Any of the bit masks can also be undef. The timeout, if specified, is -in seconds, which may be fractional. Note: not all implementations are -capable of returning the$timeleft. If not, they always return -$timeleft equal to the supplied $timeout. - -You can effect a sleep of 250 milliseconds this way: - - select(undef, undef, undef, 0.25); - -B<WARNING>: One should not attempt to mix buffered I/O (like C<read> -or <FH>) with C<select>, except as permitted by POSIX, and even -then only on POSIX systems. You have to use C<sysread> instead. - -=item semctl ID,SEMNUM,CMD,ARG - -Calls the System V IPC function C<semctl>. You'll probably have to say - - use IPC::SysV; - -first to get the correct constant definitions. If CMD is IPC_STAT or -GETALL, then ARG must be a variable which will hold the returned -semid_ds structure or semaphore value array. Returns like C<ioctl>: -the undefined value for error, "C<0 but true>" for zero, or the actual -return value otherwise. The ARG must consist of a vector of native -short integers, which may be created with C<pack("s!",(0)x$nsem)>. -See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> -documentation. - -=item semget KEY,NSEMS,FLAGS - -Calls the System V IPC function semget. Returns the semaphore id, or -the undefined value if there is an error. See also -L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> -documentation. - -=item semop KEY,OPSTRING - -Calls the System V IPC function semop to perform semaphore operations -such as signaling and waiting. OPSTRING must be a packed array of -semop structures. Each semop structure can be generated with -C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore -operations is implied by the length of OPSTRING. Returns true if -successful, or false if there is an error. As an example, the -following code waits on semaphore $semnum of semaphore id $semid: - - $semop = pack("sss", $semnum, -1, 0); - die "Semaphore trouble: $!\n" unless semop($semid, $semop); - -To signal the semaphore, replace C<-1> with C<1>. See also -L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> -documentation. - -=item send SOCKET,MSG,FLAGS,TO - -=item send SOCKET,MSG,FLAGS - -Sends a message on a socket. Takes the same flags as the system call -of the same name. On unconnected sockets you must specify a -destination to send TO, in which case it does a C C<sendto>. Returns -the number of characters sent, or the undefined value if there is an -error. The C system call sendmsg(2) is currently unimplemented. -See L<perlipc/"UDP: Message Passing"> for examples. - -=item setpgrp PID,PGRP - -Sets the current process group for the specified PID, C<0> for the current -process. Will produce a fatal error if used on a machine that doesn't -implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, -it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not -accept any arguments, so only C<setpgrp(0,0)> is portable. See also -C<POSIX::setsid()>. - -=item setpriority WHICH,WHO,PRIORITY - -Sets the current priority for a process, a process group, or a user. -(See setpriority(2).) Will produce a fatal error if used on a machine -that doesn't implement setpriority(2). - -=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL - -Sets the socket option requested. Returns undefined if there is an -error. OPTVAL may be specified as C<undef> if you don't want to pass an -argument. - -=item shift ARRAY - -=item shift - -Shifts the first value of the array off and returns it, shortening the -array by 1 and moving everything down. If there are no elements in the -array, returns the undefined value. If ARRAY is omitted, shifts the -C<@_> array within the lexical scope of subroutines and formats, and the -C<@ARGV> array at file scopes or within the lexical scopes established by -the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> -constructs. - -See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the -same thing to the left end of an array that C<pop> and C<push> do to the -right end. - -=item shmctl ID,CMD,ARG - -Calls the System V IPC function shmctl. You'll probably have to say - - use IPC::SysV; - -first to get the correct constant definitions. If CMD is C<IPC_STAT>, -then ARG must be a variable which will hold the returned C<shmid_ds> -structure. Returns like ioctl: the undefined value for error, "C<0> but -true" for zero, or the actual return value otherwise. -See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. - -=item shmget KEY,SIZE,FLAGS - -Calls the System V IPC function shmget. Returns the shared memory -segment id, or the undefined value if there is an error. -See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. - -=item shmread ID,VAR,POS,SIZE - -=item shmwrite ID,STRING,POS,SIZE - -Reads or writes the System V shared memory segment ID starting at -position POS for size SIZE by attaching to it, copying in/out, and -detaching from it. When reading, VAR must be a variable that will -hold the data read. When writing, if STRING is too long, only SIZE -bytes are used; if STRING is too short, nulls are written to fill out -SIZE bytes. Return true if successful, or false if there is an error. -shmread() taints the variable. See also L<perlipc/"SysV IPC">, -C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. - -=item shutdown SOCKET,HOW - -Shuts down a socket connection in the manner indicated by HOW, which -has the same interpretation as in the system call of the same name. - - shutdown(SOCKET, 0); # I/we have stopped reading data - shutdown(SOCKET, 1); # I/we have stopped writing data - shutdown(SOCKET, 2); # I/we have stopped using this socket - -This is useful with sockets when you want to tell the other -side you're done writing but not done reading, or vice versa. -It's also a more insistent form of close because it also -disables the file descriptor in any forked copies in other -processes. - -=item sin EXPR - -=item sin - -Returns the sine of EXPR (expressed in radians). If EXPR is omitted, -returns sine of C<$_>. - -For the inverse sine operation, you may use the C<Math::Trig::asin> -function, or use this relation: - - sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } - -=item sleep EXPR - -=item sleep - -Causes the script to sleep for EXPR seconds, or forever if no EXPR. -May be interrupted if the process receives a signal such as C<SIGALRM>. -Returns the number of seconds actually slept. You probably cannot -mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented -using C<alarm>. - -On some older systems, it may sleep up to a full second less than what -you requested, depending on how it counts seconds. Most modern systems -always sleep the full amount. They may appear to sleep longer than that, -however, because your process might not be scheduled right away in a -busy multitasking system. - -For delays of finer granularity than one second, you may use Perl's -C<syscall> interface to access setitimer(2) if your system supports -it, or else see L</select> above. The Time::HiRes module from CPAN -may also help. - -See also the POSIX module's C<pause> function. - -=item socket SOCKET,DOMAIN,TYPE,PROTOCOL - -Opens a socket of the specified kind and attaches it to filehandle -SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for -the system call of the same name. You should C<use Socket> first -to get the proper definitions imported. See the examples in -L<perlipc/"Sockets: Client/Server Communication">. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptor, as determined by the -value of $^F. See L<perlvar/$^F>. - -=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL - -Creates an unnamed pair of sockets in the specified domain, of the -specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as -for the system call of the same name. If unimplemented, yields a fatal -error. Returns true if successful. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptors, as determined by the value -of $^F. See L<perlvar/$^F>. - -Some systems defined C<pipe> in terms of C<socketpair>, in which a call -to C<pipe(Rdr, Wtr)> is essentially: - - use Socket; - socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); - shutdown(Rdr, 1); # no more writing for reader - shutdown(Wtr, 0); # no more reading for writer - -See L<perlipc> for an example of socketpair use. - -=item sort SUBNAME LIST - -=item sort BLOCK LIST - -=item sort LIST - -Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK -is omitted, C<sort>s in standard string comparison order. If SUBNAME is -specified, it gives the name of a subroutine that returns an integer -less than, equal to, or greater than C<0>, depending on how the elements -of the list are to be ordered. (The C<< <=> >> and C<cmp> -operators are extremely useful in such routines.) SUBNAME may be a -scalar variable name (unsubscripted), in which case the value provides -the name of (or a reference to) the actual subroutine to use. In place -of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort -subroutine. - -If the subroutine's prototype is C<($$)>, the elements to be compared -are passed by reference in C<@_>, as for a normal subroutine. This is -slower than unprototyped subroutines, where the elements to be -compared are passed into the subroutine -as the package global variables $a and $b (see example below). Note that -in the latter case, it is usually counter-productive to declare $a and -$b as lexicals. - -In either case, the subroutine may not be recursive. The values to be -compared are always passed by reference, so don't modify them. - -You also cannot exit out of the sort block or subroutine using any of the -loop control operators described in L<perlsyn> or with C<goto>. - -When C<use locale> is in effect, C<sort LIST> sorts LIST according to the -current collation locale. See L<perllocale>. - -Examples: - - # sort lexically - @articles = sort @files; - - # same thing, but with explicit sort routine - @articles = sort {$a cmp $b} @files; - - # now case-insensitively - @articles = sort {uc($a) cmp uc($b)} @files; - - # same thing in reversed order - @articles = sort {$b cmp $a} @files; - - # sort numerically ascending - @articles = sort {$a <=> $b} @files; - - # sort numerically descending - @articles = sort {$b <=> $a} @files; - - # this sorts the %age hash by value instead of key - # using an in-line function - @eldest = sort { $age{$b} <=> $age{$a} } keys %age; - - # sort using explicit subroutine name - sub byage { - $age{$a} <=> $age{$b}; # presuming numeric - } - @sortedclass = sort byage @class; - - sub backwards { $b cmp $a } - @harry = qw(dog cat x Cain Abel); - @george = qw(gone chased yz Punished Axed); - print sort @harry; - # prints AbelCaincatdogx - print sort backwards @harry; - # prints xdogcatCainAbel - print sort @george, 'to', @harry; - # prints AbelAxedCainPunishedcatchaseddoggonetoxyz - - # inefficiently sort by descending numeric compare using - # the first integer after the first = sign, or the - # whole record case-insensitively otherwise - - @new = sort { - ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] - || - uc($a) cmp uc($b) - } @old; - - # same thing, but much more efficiently; - # we'll build auxiliary indices instead - # for speed - @nums = @caps = (); - for (@old) { - push @nums, /=(\d+)/; - push @caps, uc($_); - } - - @new = @old[ sort { - $nums[$b] <=> $nums[$a] - || - $caps[$a] cmp $caps[$b] - } 0..$#old - ]; - - # same thing, but without any temps - @new = map { $_->[0] } - sort { $b->[1] <=> $a->[1] - || - $a->[2] cmp $b->[2] - } map { [$_, /=(\d+)/, uc($_)] } @old; - - # using a prototype allows you to use any comparison subroutine - # as a sort subroutine (including other package's subroutines) - package other; - sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here - - package main; - @new = sort other::backwards @old; - -If you're using strict, you I<must not> declare $a -and $b as lexicals. They are package globals. That means -if you're in the C<main> package and type - - @articles = sort {$b <=> $a} @files; - -then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), -but if you're in the C<FooPack> package, it's the same as typing - - @articles = sort {$FooPack::b <=> $FooPack::a} @files; - -The comparison function is required to behave. If it returns -inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and -sometimes saying the opposite, for example) the results are not -well-defined. - -=item splice ARRAY,OFFSET,LENGTH,LIST - -=item splice ARRAY,OFFSET,LENGTH - -=item splice ARRAY,OFFSET - -=item splice ARRAY - -Removes the elements designated by OFFSET and LENGTH from an array, and -replaces them with the elements of LIST, if any. In list context, -returns the elements removed from the array. In scalar context, -returns the last element removed, or C<undef> if no elements are -removed. The array grows or shrinks as necessary. -If OFFSET is negative then it starts that far from the end of the array. -If LENGTH is omitted, removes everything from OFFSET onward. -If LENGTH is negative, leaves that many elements off the end of the array. -If both OFFSET and LENGTH are omitted, removes everything. - -The following equivalences hold (assuming C<$[ == 0>): - - push(@a,$x,$y) splice(@a,@a,0,$x,$y) - pop(@a) splice(@a,-1) - shift(@a) splice(@a,0,1) - unshift(@a,$x,$y) splice(@a,0,0,$x,$y) - $a[$x] = $y splice(@a,$x,1,$y) - -Example, assuming array lengths are passed before arrays: - - sub aeq { # compare two list values - my(@a) = splice(@_,0,shift); - my(@b) = splice(@_,0,shift); - return 0 unless @a == @b; # same len? - while (@a) { - return 0 if pop(@a) ne pop(@b); - } - return 1; - } - if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } - -=item split /PATTERN/,EXPR,LIMIT - -=item split /PATTERN/,EXPR - -=item split /PATTERN/ - -=item split - -Splits a string into a list of strings and returns that list. By default, -empty leading fields are preserved, and empty trailing ones are deleted. - -In scalar context, returns the number of fields found and splits into -the C<@_> array. Use of split in scalar context is deprecated, however, -because it clobbers your subroutine arguments. - -If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, -splits on whitespace (after skipping any leading whitespace). Anything -matching PATTERN is taken to be a delimiter separating the fields. (Note -that the delimiter may be longer than one character.) - -If LIMIT is specified and positive, splits into no more than that -many fields (though it may split into fewer). If LIMIT is unspecified -or zero, trailing null fields are stripped (which potential users -of C<pop> would do well to remember). If LIMIT is negative, it is -treated as if an arbitrarily large LIMIT had been specified. - -A pattern matching the null string (not to be confused with -a null pattern C<//>, which is just one member of the set of patterns -matching a null string) will split the value of EXPR into separate -characters at each point it matches that way. For example: - - print join(':', split(/ */, 'hi there')); - -produces the output 'h:i:t:h:e:r:e'. - -Empty leading (or trailing) fields are produced when there positive width -matches at the beginning (or end) of the string; a zero-width match at the -beginning (or end) of the string does not produce an empty field. For -example: - - print join(':', split(/(?=\w)/, 'hi there!')); - -produces the output 'h:i :t:h:e:r:e!'. - -The LIMIT parameter can be used to split a line partially - - ($login, $passwd, $remainder) = split(/:/, $_, 3); - -When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT -one larger than the number of variables in the list, to avoid -unnecessary work. For the list above LIMIT would have been 4 by -default. In time critical applications it behooves you not to split -into more fields than you really need. - -If the PATTERN contains parentheses, additional list elements are -created from each matching substring in the delimiter. - - split(/([,-])/, "1-10,20", 3); - -produces the list value - - (1, '-', 10, ',', 20) - -If you had the entire header of a normal Unix email message in $header, -you could split it up into fields and their values this way: - - $header =~ s/\n\s+/ /g; # fix continuation lines - %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); - -The pattern C</PATTERN/> may be replaced with an expression to specify -patterns that vary at runtime. (To do runtime compilation only once, -use C</$variable/o>.) - -As a special case, specifying a PATTERN of space (C<' '>) will split on -white space just as C<split> with no arguments does. Thus, C<split(' ')> can -be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> -will give you as many null initial fields as there are leading spaces. -A C<split> on C</\s+/> is like a C<split(' ')> except that any leading -whitespace produces a null first field. A C<split> with no arguments -really does a C<split(' ', $_)> internally. - -A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't -much use otherwise. - -Example: - - open(PASSWD, '/etc/passwd'); - while (<PASSWD>) { - chomp; - ($login, $passwd, $uid, $gid, - $gcos, $home, $shell) = split(/:/); - #... - } - - -=item sprintf FORMAT, LIST - -Returns a string formatted by the usual C<printf> conventions of the C -library function C<sprintf>. See below for more details -and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of -the general principles. - -For example: - - # Format number with up to 8 leading zeroes - $result = sprintf("%08d", $number); - - # Round number to 3 digits after decimal point - $rounded = sprintf("%.3f", $number); - -Perl does its own C<sprintf> formatting--it emulates the C -function C<sprintf>, but it doesn't use it (except for floating-point -numbers, and even then only the standard modifiers are allowed). As a -result, any non-standard extensions in your local C<sprintf> are not -available from Perl. - -Unlike C<printf>, C<sprintf> does not do what you probably mean when you -pass it an array as your first argument. The array is given scalar context, -and instead of using the 0th element of the array as the format, Perl will -use the count of elements in the array as the format, which is almost never -useful. - -Perl's C<sprintf> permits the following universally-known conversions: - - %% a percent sign - %c a character with the given number - %s a string - %d a signed integer, in decimal - %u an unsigned integer, in decimal - %o an unsigned integer, in octal - %x an unsigned integer, in hexadecimal - %e a floating-point number, in scientific notation - %f a floating-point number, in fixed decimal notation - %g a floating-point number, in %e or %f notation - -In addition, Perl permits the following widely-supported conversions: - - %X like %x, but using upper-case letters - %E like %e, but using an upper-case "E" - %G like %g, but with an upper-case "E" (if applicable) - %b an unsigned integer, in binary - %p a pointer (outputs the Perl value's address in hexadecimal) - %n special: *stores* the number of characters output so far - into the next variable in the parameter list - -Finally, for backward (and we do mean "backward") compatibility, Perl -permits these unnecessary but widely-supported conversions: - - %i a synonym for %d - %D a synonym for %ld - %U a synonym for %lu - %O a synonym for %lo - %F a synonym for %f - -Note that the number of exponent digits in the scientific notation by -C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the -exponent less than 100 is system-dependent: it may be three or less -(zero-padded as necessary). In other words, 1.23 times ten to the -99th may be either "1.23e99" or "1.23e099". - -Perl permits the following universally-known flags between the C<%> -and the conversion letter: - - space prefix positive number with a space - + prefix positive number with a plus sign - - left-justify within the field - 0 use zeros, not spaces, to right-justify - # prefix non-zero octal with "0", non-zero hex with "0x" - number minimum field width - .number "precision": digits after decimal point for - floating-point, max length for string, minimum length - for integer - l interpret integer as C type "long" or "unsigned long" - h interpret integer as C type "short" or "unsigned short" - If no flags, interpret integer as C type "int" or "unsigned" - -There are also two Perl-specific flags: - - V interpret integer as Perl's standard integer type - v interpret string as a vector of integers, output as - numbers separated either by dots, or by an arbitrary - string received from the argument list when the flag - is preceded by C<*> - -Where a number would appear in the flags, an asterisk (C<*>) may be -used instead, in which case Perl uses the next item in the parameter -list as the given number (that is, as the field width or precision). -If a field width obtained through C<*> is negative, it has the same -effect as the C<-> flag: left-justification. - -The C<v> flag is useful for displaying ordinal values of characters -in arbitrary strings: - - printf "version is v%vd\n", $^V; # Perl's version - printf "address is %*vX\n", ":", $addr; # IPv6 address - printf "bits are %*vb\n", " ", $bits; # random bitstring - -If C<use locale> is in effect, the character used for the decimal -point in formatted real numbers is affected by the LC_NUMERIC locale. -See L<perllocale>. - -If Perl understands "quads" (64-bit integers) (this requires -either that the platform natively support quads or that Perl -be specifically compiled to support quads), the characters - - d u o x X b i D U O - -print quads, and they may optionally be preceded by - - ll L q - -For example - - %lld %16LX %qo - -You can find out whether your Perl supports quads via L<Config>: - - use Config; - ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) && - print "quads\n"; - -If Perl understands "long doubles" (this requires that the platform -support long doubles), the flags - - e f g E F G - -may optionally be preceded by - - ll L - -For example - - %llf %Lg - -You can find out whether your Perl supports long doubles via L<Config>: - - use Config; - $Config{d_longdbl} eq 'define' && print "long doubles\n"; - -=item sqrt EXPR - -=item sqrt - -Return the square root of EXPR. If EXPR is omitted, returns square -root of C<$_>. Only works on non-negative operands, unless you've -loaded the standard Math::Complex module. - - use Math::Complex; - print sqrt(-2); # prints 1.4142135623731i - -=item srand EXPR - -=item srand - -Sets the random number seed for the C<rand> operator. If EXPR is -omitted, uses a semi-random value supplied by the kernel (if it supports -the F</dev/urandom> device) or based on the current time and process -ID, among other things. In versions of Perl prior to 5.004 the default -seed was just the current C<time>. This isn't a particularly good seed, -so many old programs supply their own seed value (often C<time ^ $$> or -C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more. - -In fact, it's usually not necessary to call C<srand> at all, because if -it is not called explicitly, it is called implicitly at the first use of -the C<rand> operator. However, this was not the case in version of Perl -before 5.004, so if your script will run under older Perl versions, it -should call C<srand>. - -Note that you need something much more random than the default seed for -cryptographic purposes. Checksumming the compressed output of one or more -rapidly changing operating system status programs is the usual method. For -example: - - srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); - -If you're particularly concerned with this, see the C<Math::TrulyRandom> -module in CPAN. - -Do I<not> call C<srand> multiple times in your program unless you know -exactly what you're doing and why you're doing it. The point of the -function is to "seed" the C<rand> function so that C<rand> can produce -a different sequence each time you run your program. Just do it once at the -top of your program, or you I<won't> get random numbers out of C<rand>! - -Frequently called programs (like CGI scripts) that simply use - - time ^ $$ - -for a seed can fall prey to the mathematical property that - - a^b == (a+1)^(b+1) - -one-third of the time. So don't do that. - -=item stat FILEHANDLE - -=item stat EXPR - -=item stat - -Returns a 13-element list giving the status info for a file, either -the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, -it stats C<$_>. Returns a null list if the stat fails. Typically used -as follows: - - ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, - $atime,$mtime,$ctime,$blksize,$blocks) - = stat($filename); - -Not all fields are supported on all filesystem types. Here are the -meaning of the fields: - - 0 dev device number of filesystem - 1 ino inode number - 2 mode file mode (type and permissions) - 3 nlink number of (hard) links to the file - 4 uid numeric user ID of file's owner - 5 gid numeric group ID of file's owner - 6 rdev the device identifier (special files only) - 7 size total size of file, in bytes - 8 atime last access time in seconds since the epoch - 9 mtime last modify time in seconds since the epoch - 10 ctime inode change time (NOT creation time!) in seconds since the epoch - 11 blksize preferred block size for file system I/O - 12 blocks actual number of blocks allocated - -(The epoch was at 00:00 January 1, 1970 GMT.) - -If stat is passed the special filehandle consisting of an underline, no -stat is done, but the current contents of the stat structure from the -last stat or filetest are returned. Example: - - if (-x $file && (($d) = stat(_)) && $d < 0) { - print "$file is executable NFS file\n"; - } - -(This works on machines only for which the device number is negative -under NFS.) - -Because the mode contains both the file type and its permissions, you -should mask off the file type portion and (s)printf using a C<"%o"> -if you want to see the real permissions. - - $mode = (stat($filename))[2]; - printf "Permissions are %04o\n", $mode & 07777; - -In scalar context, C<stat> returns a boolean value indicating success -or failure, and, if successful, sets the information associated with -the special filehandle C<_>. - -The File::stat module provides a convenient, by-name access mechanism: - - use File::stat; - $sb = stat($filename); - printf "File is %s, size is %s, perm %04o, mtime %s\n", - $filename, $sb->size, $sb->mode & 07777, - scalar localtime $sb->mtime; - -You can import symbolic mode constants (C<S_IF*>) and functions -(C<S_IS*>) from the Fcntl module: - - use Fcntl ':mode'; - - $mode = (stat($filename))[2]; - - $user_rwx = ($mode & S_IRWXU) >> 6; - $group_read = ($mode & S_IRGRP) >> 3; - $other_execute = $mode & S_IXOTH; - - printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; - - $is_setuid = $mode & S_ISUID; - $is_setgid = S_ISDIR($mode); - -You could write the last two using the C<-u> and C<-d> operators. -The commonly available S_IF* constants are - - # Permissions: read, write, execute, for user, group, others. - - S_IRWXU S_IRUSR S_IWUSR S_IXUSR - S_IRWXG S_IRGRP S_IWGRP S_IXGRP - S_IRWXO S_IROTH S_IWOTH S_IXOTH - - # Setuid/Setgid/Stickiness. - - S_ISUID S_ISGID S_ISVTX S_ISTXT - - # File types. Not necessarily all are available on your system. - - S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT - - # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. - - S_IREAD S_IWRITE S_IEXEC - -and the S_IF* functions are - - S_IFMODE($mode) the part of $mode containing the permission bits - and the setuid/setgid/sticky bits - - S_IFMT($mode) the part of $mode containing the file type - which can be bit-anded with e.g. S_IFREG - or with the following functions - - # The operators -f, -d, -l, -b, -c, -p, and -s. - - S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) - S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) - - # No direct -X operator counterpart, but for the first one - # the -g operator is often equivalent. The ENFMT stands for - # record flocking enforcement, a platform-dependent feature. - - S_ISENFMT($mode) S_ISWHT($mode) - -See your native chmod(2) and stat(2) documentation for more details -about the S_* constants. - -=item study SCALAR - -=item study - -Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of -doing many pattern matches on the string before it is next modified. -This may or may not save time, depending on the nature and number of -patterns you are searching on, and on the distribution of character -frequencies in the string to be searched--you probably want to compare -run times with and without it to see which runs faster. Those loops -which scan for many short constant strings (including the constant -parts of more complex patterns) will benefit most. You may have only -one C<study> active at a time--if you study a different scalar the first -is "unstudied". (The way C<study> works is this: a linked list of every -character in the string to be searched is made, so we know, for -example, where all the C<'k'> characters are. From each search string, -the rarest character is selected, based on some static frequency tables -constructed from some C programs and English text. Only those places -that contain this "rarest" character are examined.) - -For example, here is a loop that inserts index producing entries -before any line containing a certain pattern: - - while (<>) { - study; - print ".IX foo\n" if /\bfoo\b/; - print ".IX bar\n" if /\bbar\b/; - print ".IX blurfl\n" if /\bblurfl\b/; - # ... - print; - } - -In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> -will be looked at, because C<f> is rarer than C<o>. In general, this is -a big win except in pathological cases. The only question is whether -it saves you more time than it took to build the linked list in the -first place. - -Note that if you have to look for strings that you don't know till -runtime, you can build an entire loop as a string and C<eval> that to -avoid recompiling all your patterns all the time. Together with -undefining C<$/> to input entire files as one record, this can be very -fast, often faster than specialized programs like fgrep(1). The following -scans a list of files (C<@files>) for a list of words (C<@words>), and prints -out the names of those files that contain a match: - - $search = 'while (<>) { study;'; - foreach $word (@words) { - $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; - } - $search .= "}"; - @ARGV = @files; - undef $/; - eval $search; # this screams - $/ = "\n"; # put back to normal input delimiter - foreach $file (sort keys(%seen)) { - print $file, "\n"; - } - -=item sub BLOCK - -=item sub NAME - -=item sub NAME BLOCK - -This is subroutine definition, not a real function I<per se>. With just a -NAME (and possibly prototypes or attributes), it's just a forward declaration. -Without a NAME, it's an anonymous function declaration, and does actually -return a value: the CODE ref of the closure you just created. See L<perlsub> -and L<perlref> for details. - -=item substr EXPR,OFFSET,LENGTH,REPLACEMENT - -=item substr EXPR,OFFSET,LENGTH - -=item substr EXPR,OFFSET - -Extracts a substring out of EXPR and returns it. First character is at -offset C<0>, or whatever you've set C<$[> to (but don't do that). -If OFFSET is negative (or more precisely, less than C<$[>), starts -that far from the end of the string. If LENGTH is omitted, returns -everything to the end of the string. If LENGTH is negative, leaves that -many characters off the end of the string. - -You can use the substr() function as an lvalue, in which case EXPR -must itself be an lvalue. If you assign something shorter than LENGTH, -the string will shrink, and if you assign something longer than LENGTH, -the string will grow to accommodate it. To keep the string the same -length you may need to pad or chop your value using C<sprintf>. - -If OFFSET and LENGTH specify a substring that is partly outside the -string, only the part within the string is returned. If the substring -is beyond either end of the string, substr() returns the undefined -value and produces a warning. When used as an lvalue, specifying a -substring that is entirely outside the string is a fatal error. -Here's an example showing the behavior for boundary cases: - - my $name = 'fred'; - substr($name, 4) = 'dy'; # $name is now 'freddy' - my $null = substr $name, 6, 2; # returns '' (no warning) - my $oops = substr $name, 7; # returns undef, with warning - substr($name, 7) = 'gap'; # fatal error - -An alternative to using substr() as an lvalue is to specify the -replacement string as the 4th argument. This allows you to replace -parts of the EXPR and return what was there before in one operation, -just as you can with splice(). - -=item symlink OLDFILE,NEWFILE - -Creates a new filename symbolically linked to the old filename. -Returns C<1> for success, C<0> otherwise. On systems that don't support -symbolic links, produces a fatal error at run time. To check for that, -use eval: - - $symlink_exists = eval { symlink("",""); 1 }; - -=item syscall LIST - -Calls the system call specified as the first element of the list, -passing the remaining elements as arguments to the system call. If -unimplemented, produces a fatal error. The arguments are interpreted -as follows: if a given argument is numeric, the argument is passed as -an int. If not, the pointer to the string value is passed. You are -responsible to make sure a string is pre-extended long enough to -receive any result that might be written into a string. You can't use a -string literal (or other read-only string) as an argument to C<syscall> -because Perl has to assume that any string pointer might be written -through. If your -integer arguments are not literals and have never been interpreted in a -numeric context, you may need to add C<0> to them to force them to look -like numbers. This emulates the C<syswrite> function (or vice versa): - - require 'syscall.ph'; # may need to run h2ph - $s = "hi there\n"; - syscall(&SYS_write, fileno(STDOUT), $s, length $s); - -Note that Perl supports passing of up to only 14 arguments to your system call, -which in practice should usually suffice. - -Syscall returns whatever value returned by the system call it calls. -If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). -Note that some system calls can legitimately return C<-1>. The proper -way to handle such calls is to assign C<$!=0;> before the call and -check the value of C<$!> if syscall returns C<-1>. - -There's a problem with C<syscall(&SYS_pipe)>: it returns the file -number of the read end of the pipe it creates. There is no way -to retrieve the file number of the other end. You can avoid this -problem by using C<pipe> instead. - -=item sysopen FILEHANDLE,FILENAME,MODE - -=item sysopen FILEHANDLE,FILENAME,MODE,PERMS - -Opens the file whose filename is given by FILENAME, and associates it -with FILEHANDLE. If FILEHANDLE is an expression, its value is used as -the name of the real filehandle wanted. This function calls the -underlying operating system's C<open> function with the parameters -FILENAME, MODE, PERMS. - -The possible values and flag bits of the MODE parameter are -system-dependent; they are available via the standard module C<Fcntl>. -See the documentation of your operating system's C<open> to see which -values and flag bits are available. You may combine several flags -using the C<|>-operator. - -Some of the most common values are C<O_RDONLY> for opening the file in -read-only mode, C<O_WRONLY> for opening the file in write-only mode, -and C<O_RDWR> for opening the file in read-write mode, and. - -For historical reasons, some values work on almost every system -supported by perl: zero means read-only, one means write-only, and two -means read/write. We know that these values do I<not> work under -OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to -use them in new code. - -If the file named by FILENAME does not exist and the C<open> call creates -it (typically because MODE includes the C<O_CREAT> flag), then the value of -PERMS specifies the permissions of the newly created file. If you omit -the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. -These permission values need to be in octal, and are modified by your -process's current C<umask>. - -In many systems the C<O_EXCL> flag is available for opening files in -exclusive mode. This is B<not> locking: exclusiveness means here that -if the file already exists, sysopen() fails. The C<O_EXCL> wins -C<O_TRUNC>. - -Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. - -You should seldom if ever use C<0644> as argument to C<sysopen>, because -that takes away the user's option to have a more permissive umask. -Better to omit it. See the perlfunc(1) entry on C<umask> for more -on this. - -Note that C<sysopen> depends on the fdopen() C library function. -On many UNIX systems, fdopen() is known to fail when file descriptors -exceed a certain value, typically 255. If you need more file -descriptors than that, consider rebuilding Perl to use the C<sfio> -library, or perhaps using the POSIX::open() function. - -See L<perlopentut> for a kinder, gentler explanation of opening files. - -=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET - -=item sysread FILEHANDLE,SCALAR,LENGTH - -Attempts to read LENGTH bytes of data into variable SCALAR from the -specified FILEHANDLE, using the system call read(2). It bypasses stdio, -so mixing this with other kinds of reads, C<print>, C<write>, -C<seek>, C<tell>, or C<eof> can cause confusion because stdio -usually buffers data. Returns the number of bytes actually read, C<0> -at end of file, or undef if there was an error. SCALAR will be grown or -shrunk so that the last byte actually read is the last byte of the -scalar after the read. - -An OFFSET may be specified to place the read data at some place in the -string other than the beginning. A negative OFFSET specifies -placement at that many bytes counting backwards from the end of the -string. A positive OFFSET greater than the length of SCALAR results -in the string being padded to the required size with C<"\0"> bytes before -the result of the read is appended. - -There is no syseof() function, which is ok, since eof() doesn't work -very well on device files (like ttys) anyway. Use sysread() and check -for a return value for 0 to decide whether you're done. - -=item sysseek FILEHANDLE,POSITION,WHENCE - -Sets FILEHANDLE's system position using the system call lseek(2). It -bypasses stdio, so mixing this with reads (other than C<sysread>), -C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion. -FILEHANDLE may be an expression whose value gives the name of the -filehandle. The values for WHENCE are C<0> to set the new position to -POSITION, C<1> to set the it to the current position plus POSITION, -and C<2> to set it to EOF plus POSITION (typically negative). For -WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and -C<SEEK_END> (start of the file, current position, end of the file) -from the Fcntl module. - -Returns the new position, or the undefined value on failure. A position -of zero is returned as the string C<"0 but true">; thus C<sysseek> returns -true on success and false on failure, yet you can still easily determine -the new position. - -=item system LIST - -=item system PROGRAM LIST - -Does exactly the same thing as C<exec LIST>, except that a fork is -done first, and the parent process waits for the child process to -complete. Note that argument processing varies depending on the -number of arguments. If there is more than one argument in LIST, -or if LIST is an array with more than one value, starts the program -given by the first element of the list with arguments given by the -rest of the list. If there is only one scalar argument, the argument -is checked for shell metacharacters, and if there are any, the -entire argument is passed to the system's command shell for parsing -(this is C</bin/sh -c> on Unix platforms, but varies on other -platforms). If there are no shell metacharacters in the argument, -it is split into words and passed directly to C<execvp>, which is -more efficient. - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before any operation that may do a fork, but this may not be -supported on some platforms (see L<perlport>). To be safe, you may need -to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method -of C<IO::Handle> on any open handles. - -The return value is the exit status of the program as -returned by the C<wait> call. To get the actual exit value divide by -256. See also L</exec>. This is I<not> what you want to use to capture -the output from a command, for that you should use merely backticks or -C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 -indicates a failure to start the program (inspect $! for the reason). - -Like C<exec>, C<system> allows you to lie to a program about its name if -you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. - -Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the -program they're running doesn't actually interrupt your program. - - @args = ("command", "arg1", "arg2"); - system(@args) == 0 - or die "system @args failed: $?" - -You can check all the failure possibilities by inspecting -C<$?> like this: - - $exit_value = $? >> 8; - $signal_num = $? & 127; - $dumped_core = $? & 128; - -When the arguments get executed via the system shell, results -and return codes will be subject to its quirks and capabilities. -See L<perlop/"`STRING`"> and L</exec> for details. - -=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET - -=item syswrite FILEHANDLE,SCALAR,LENGTH - -=item syswrite FILEHANDLE,SCALAR - -Attempts to write LENGTH bytes of data from variable SCALAR to the -specified FILEHANDLE, using the system call write(2). If LENGTH -is not specified, writes whole SCALAR. It bypasses stdio, so mixing -this with reads (other than C<sysread())>, C<print>, C<write>, -C<seek>, C<tell>, or C<eof> may cause confusion because stdio -usually buffers data. Returns the number of bytes actually written, -or C<undef> if there was an error. If the LENGTH is greater than -the available data in the SCALAR after the OFFSET, only as much -data as is available will be written. - -An OFFSET may be specified to write the data from some part of the -string other than the beginning. A negative OFFSET specifies writing -that many bytes counting backwards from the end of the string. In the -case the SCALAR is empty you can use OFFSET but only zero offset. - -=item tell FILEHANDLE - -=item tell - -Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE -may be an expression whose value gives the name of the actual filehandle. -If FILEHANDLE is omitted, assumes the file last read. - -The return value of tell() for the standard streams like the STDIN -depends on the operating system: it may return -1 or something else. -tell() on pipes, fifos, and sockets usually returns -1. - -There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. - -=item telldir DIRHANDLE - -Returns the current position of the C<readdir> routines on DIRHANDLE. -Value may be given to C<seekdir> to access a particular location in a -directory. Has the same caveats about possible directory compaction as -the corresponding system library routine. - -=item tie VARIABLE,CLASSNAME,LIST - -This function binds a variable to a package class that will provide the -implementation for the variable. VARIABLE is the name of the variable -to be enchanted. CLASSNAME is the name of a class implementing objects -of correct type. Any additional arguments are passed to the C<new> -method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, -or C<TIEHASH>). Typically these are arguments such as might be passed -to the C<dbm_open()> function of C. The object returned by the C<new> -method is also returned by the C<tie> function, which would be useful -if you want to access other methods in CLASSNAME. - -Note that functions such as C<keys> and C<values> may return huge lists -when used on large objects, like DBM files. You may prefer to use the -C<each> function to iterate over such. Example: - - # print out history file offsets - use NDBM_File; - tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); - while (($key,$val) = each %HIST) { - print $key, ' = ', unpack('L',$val), "\n"; - } - untie(%HIST); - -A class implementing a hash should have the following methods: - - TIEHASH classname, LIST - FETCH this, key - STORE this, key, value - DELETE this, key - CLEAR this - EXISTS this, key - FIRSTKEY this - NEXTKEY this, lastkey - DESTROY this - UNTIE this - -A class implementing an ordinary array should have the following methods: - - TIEARRAY classname, LIST - FETCH this, key - STORE this, key, value - FETCHSIZE this - STORESIZE this, count - CLEAR this - PUSH this, LIST - POP this - SHIFT this - UNSHIFT this, LIST - SPLICE this, offset, length, LIST - EXTEND this, count - DESTROY this - UNTIE this - -A class implementing a file handle should have the following methods: - - TIEHANDLE classname, LIST - READ this, scalar, length, offset - READLINE this - GETC this - WRITE this, scalar, length, offset - PRINT this, LIST - PRINTF this, format, LIST - BINMODE this - EOF this - FILENO this - SEEK this, position, whence - TELL this - OPEN this, mode, LIST - CLOSE this - DESTROY this - UNTIE this - -A class implementing a scalar should have the following methods: - - TIESCALAR classname, LIST - FETCH this, - STORE this, value - DESTROY this - UNTIE this - -Not all methods indicated above need be implemented. See L<perltie>, -L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. - -Unlike C<dbmopen>, the C<tie> function will not use or require a module -for you--you need to do that explicitly yourself. See L<DB_File> -or the F<Config> module for interesting C<tie> implementations. - -For further details see L<perltie>, L<"tied VARIABLE">. - -=item tied VARIABLE - -Returns a reference to the object underlying VARIABLE (the same value -that was originally returned by the C<tie> call that bound the variable -to a package.) Returns the undefined value if VARIABLE isn't tied to a -package. - -=item time - -Returns the number of non-leap seconds since whatever time the system -considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, -and 00:00:00 UTC, January 1, 1970 for most other systems). -Suitable for feeding to C<gmtime> and C<localtime>. - -For measuring time in better granularity than one second, -you may use either the Time::HiRes module from CPAN, or -if you have gettimeofday(2), you may be able to use the -C<syscall> interface of Perl, see L<perlfaq8> for details. - -=item times - -Returns a four-element list giving the user and system times, in -seconds, for this process and the children of this process. - - ($user,$system,$cuser,$csystem) = times; - -=item tr/// - -The transliteration operator. Same as C<y///>. See L<perlop>. - -=item truncate FILEHANDLE,LENGTH - -=item truncate EXPR,LENGTH - -Truncates the file opened on FILEHANDLE, or named by EXPR, to the -specified length. Produces a fatal error if truncate isn't implemented -on your system. Returns true if successful, the undefined value -otherwise. - -=item uc EXPR - -=item uc - -Returns an uppercased version of EXPR. This is the internal function -implementing the C<\U> escape in double-quoted strings. -Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. -Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It -does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.) - -If EXPR is omitted, uses C<$_>. - -=item ucfirst EXPR - -=item ucfirst - -Returns the value of EXPR with the first character -in uppercase (titlecase in Unicode). This is -the internal function implementing the C<\u> escape in double-quoted strings. -Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> -and L<utf8>. - -If EXPR is omitted, uses C<$_>. - -=item umask EXPR - -=item umask - -Sets the umask for the process to EXPR and returns the previous value. -If EXPR is omitted, merely returns the current umask. - -The Unix permission C<rwxr-x---> is represented as three sets of three -bits, or three octal digits: C<0750> (the leading 0 indicates octal -and isn't one of the digits). The C<umask> value is such a number -representing disabled permissions bits. The permission (or "mode") -values you pass C<mkdir> or C<sysopen> are modified by your umask, so -even if you tell C<sysopen> to create a file with permissions C<0777>, -if your umask is C<0022> then the file will actually be created with -permissions C<0755>. If your C<umask> were C<0027> (group can't -write; others can't read, write, or execute), then passing -C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ -027> is C<0640>). - -Here's some advice: supply a creation mode of C<0666> for regular -files (in C<sysopen>) and one of C<0777> for directories (in -C<mkdir>) and executable files. This gives users the freedom of -choice: if they want protected files, they might choose process umasks -of C<022>, C<027>, or even the particularly antisocial mask of C<077>. -Programs should rarely if ever make policy decisions better left to -the user. The exception to this is when writing files that should be -kept private: mail files, web browser cookies, I<.rhosts> files, and -so on. - -If umask(2) is not implemented on your system and you are trying to -restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a -fatal error at run time. If umask(2) is not implemented and you are -not trying to restrict access for yourself, returns C<undef>. - -Remember that a umask is a number, usually given in octal; it is I<not> a -string of octal digits. See also L</oct>, if all you have is a string. - -=item undef EXPR - -=item undef - -Undefines the value of EXPR, which must be an lvalue. Use only on a -scalar value, an array (using C<@>), a hash (using C<%>), a subroutine -(using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> -will probably not do what you expect on most predefined variables or -DBM list values, so don't do that; see L<delete>.) Always returns the -undefined value. You can omit the EXPR, in which case nothing is -undefined, but you still get an undefined value that you could, for -instance, return from a subroutine, assign to a variable or pass as a -parameter. Examples: - - undef $foo; - undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; - undef @ary; - undef %hash; - undef &mysub; - undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. - return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; - select undef, undef, undef, 0.25; - ($a, $b, undef, $c) = &foo; # Ignore third value returned - -Note that this is a unary operator, not a list operator. - -=item unlink LIST - -=item unlink - -Deletes a list of files. Returns the number of files successfully -deleted. - - $cnt = unlink 'a', 'b', 'c'; - unlink @goners; - unlink <*.bak>; - -Note: C<unlink> will not delete directories unless you are superuser and -the B<-U> flag is supplied to Perl. Even if these conditions are -met, be warned that unlinking a directory can inflict damage on your -filesystem. Use C<rmdir> instead. - -If LIST is omitted, uses C<$_>. - -=item unpack TEMPLATE,EXPR - -C<unpack> does the reverse of C<pack>: it takes a string -and expands it out into a list of values. -(In scalar context, it returns merely the first value produced.) - -The string is broken into chunks described by the TEMPLATE. Each chunk -is converted separately to a value. Typically, either the string is a result -of C<pack>, or the bytes of the string represent a C structure of some -kind. - -The TEMPLATE has the same format as in the C<pack> function. -Here's a subroutine that does substring: - - sub substr { - my($what,$where,$howmuch) = @_; - unpack("x$where a$howmuch", $what); - } - -and then there's - - sub ordinal { unpack("c",$_[0]); } # same as ord() - -In addition to fields allowed in pack(), you may prefix a field with -a %<number> to indicate that -you want a <number>-bit checksum of the items instead of the items -themselves. Default is a 16-bit checksum. Checksum is calculated by -summing numeric values of expanded values (for string fields the sum of -C<ord($char)> is taken, for bit fields the sum of zeroes and ones). - -For example, the following -computes the same number as the System V sum program: - - $checksum = do { - local $/; # slurp! - unpack("%32C*",<>) % 65535; - }; - -The following efficiently counts the number of set bits in a bit vector: - - $setbits = unpack("%32b*", $selectmask); - -The C<p> and C<P> formats should be used with care. Since Perl -has no way of checking whether the value passed to C<unpack()> -corresponds to a valid memory location, passing a pointer value that's -not known to be valid is likely to have disastrous consequences. - -If the repeat count of a field is larger than what the remainder of -the input string allows, repeat count is decreased. If the input string -is longer than one described by the TEMPLATE, the rest is ignored. - -See L</pack> for more examples and notes. - -=item untie VARIABLE - -Breaks the binding between a variable and a package. (See C<tie>.) - -=item unshift ARRAY,LIST - -Does the opposite of a C<shift>. Or the opposite of a C<push>, -depending on how you look at it. Prepends list to the front of the -array, and returns the new number of elements in the array. - - unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; - -Note the LIST is prepended whole, not one element at a time, so the -prepended elements stay in the same order. Use C<reverse> to do the -reverse. - -=item use Module VERSION LIST - -=item use Module VERSION - -=item use Module LIST - -=item use Module - -=item use VERSION - -Imports some semantics into the current package from the named module, -generally by aliasing certain subroutine or variable names into your -package. It is exactly equivalent to - - BEGIN { require Module; import Module LIST; } - -except that Module I<must> be a bareword. - -VERSION, which can be specified as a literal of the form v5.6.1, demands -that the current version of Perl (C<$^V> or $PERL_VERSION) be at least -as recent as that version. (For compatibility with older versions of Perl, -a numeric literal will also be interpreted as VERSION.) If the version -of the running Perl interpreter is less than VERSION, then an error -message is printed and Perl exits immediately without attempting to -parse the rest of the file. Compare with L</require>, which can do a -similar check at run time. - - use v5.6.1; # compile time version check - use 5.6.1; # ditto - use 5.005_03; # float version allowed for compatibility - -This is often useful if you need to check the current Perl version before -C<use>ing library modules that have changed in incompatible ways from -older versions of Perl. (We try not to do this more than we have to.) - -The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The -C<require> makes sure the module is loaded into memory if it hasn't been -yet. The C<import> is not a builtin--it's just an ordinary static method -call into the C<Module> package to tell the module to import the list of -features back into the current package. The module can implement its -C<import> method any way it likes, though most modules just choose to -derive their C<import> method via inheritance from the C<Exporter> class that -is defined in the C<Exporter> module. See L<Exporter>. If no C<import> -method can be found then the call is skipped. - -If you do not want to call the package's C<import> method (for instance, -to stop your namespace from being altered), explicitly supply the empty list: - - use Module (); - -That is exactly equivalent to - - BEGIN { require Module } - -If the VERSION argument is present between Module and LIST, then the -C<use> will call the VERSION method in class Module with the given -version as an argument. The default VERSION method, inherited from -the UNIVERSAL class, croaks if the given version is larger than the -value of the variable C<$Module::VERSION>. - -Again, there is a distinction between omitting LIST (C<import> called -with no arguments) and an explicit empty LIST C<()> (C<import> not -called). Note that there is no comma after VERSION! - -Because this is a wide-open interface, pragmas (compiler directives) -are also implemented this way. Currently implemented pragmas are: - - use constant; - use diagnostics; - use integer; - use sigtrap qw(SEGV BUS); - use strict qw(subs vars refs); - use subs qw(afunc blurfl); - use warnings qw(all); - -Some of these pseudo-modules import semantics into the current -block scope (like C<strict> or C<integer>, unlike ordinary modules, -which import symbols into the current package (which are effective -through the end of the file). - -There's a corresponding C<no> command that unimports meanings imported -by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. - - no integer; - no strict 'refs'; - no warnings; - -If no C<unimport> method can be found the call fails with a fatal error. - -See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> -for the C<-M> and C<-m> command-line options to perl that give C<use> -functionality from the command-line. - -=item utime LIST - -Changes the access and modification times on each file of a list of -files. The first two elements of the list must be the NUMERICAL access -and modification times, in that order. Returns the number of files -successfully changed. The inode change time of each file is set -to the current time. This code has the same effect as the C<touch> -command if the files already exist: - - #!/usr/bin/perl - $now = time; - utime $now, $now, @ARGV; - -=item values HASH - -Returns a list consisting of all the values of the named hash. (In a -scalar context, returns the number of values.) The values are -returned in an apparently random order. The actual random order is -subject to change in future versions of perl, but it is guaranteed to -be the same order as either the C<keys> or C<each> function would -produce on the same (unmodified) hash. - -Note that the values are not copied, which means modifying them will -modify the contents of the hash: - - for (values %hash) { s/foo/bar/g } # modifies %hash values - for (@hash{keys %hash}) { s/foo/bar/g } # same - -As a side effect, calling values() resets the HASH's internal iterator. -See also C<keys>, C<each>, and C<sort>. - -=item vec EXPR,OFFSET,BITS - -Treats the string in EXPR as a bit vector made up of elements of -width BITS, and returns the value of the element specified by OFFSET -as an unsigned integer. BITS therefore specifies the number of bits -that are reserved for each element in the bit vector. This must -be a power of two from 1 to 32 (or 64, if your platform supports -that). - -If BITS is 8, "elements" coincide with bytes of the input string. - -If BITS is 16 or more, bytes of the input string are grouped into chunks -of size BITS/8, and each group is converted to a number as with -pack()/unpack() with big-endian formats C<n>/C<N> (and analogously -for BITS==64). See L<"pack"> for details. - -If bits is 4 or less, the string is broken into bytes, then the bits -of each byte are broken into 8/BITS groups. Bits of a byte are -numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, -C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, -breaking the single input byte C<chr(0x36)> into two groups gives a list -C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. - -C<vec> may also be assigned to, in which case parentheses are needed -to give the expression the correct precedence as in - - vec($image, $max_x * $x + $y, 8) = 3; - -If the selected element is outside the string, the value 0 is returned. -If an element off the end of the string is written to, Perl will first -extend the string with sufficiently many zero bytes. It is an error -to try to write off the beginning of the string (i.e. negative OFFSET). - -The string should not contain any character with the value > 255 (which -can only happen if you're using UTF8 encoding). If it does, it will be -treated as something which is not UTF8 encoded. When the C<vec> was -assigned to, other parts of your program will also no longer consider the -string to be UTF8 encoded. In other words, if you do have such characters -in your string, vec() will operate on the actual byte string, and not the -conceptual character string. - -Strings created with C<vec> can also be manipulated with the logical -operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit -vector operation is desired when both operands are strings. -See L<perlop/"Bitwise String Operators">. - -The following code will build up an ASCII string saying C<'PerlPerlPerl'>. -The comments show the string after each step. Note that this code works -in the same way on big-endian or little-endian machines. - - my $foo = ''; - vec($foo, 0, 32) = 0x5065726C; # 'Perl' - - # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits - print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') - - vec($foo, 2, 16) = 0x5065; # 'PerlPe' - vec($foo, 3, 16) = 0x726C; # 'PerlPerl' - vec($foo, 8, 8) = 0x50; # 'PerlPerlP' - vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' - vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" - vec($foo, 21, 4) = 7; # 'PerlPerlPer' - # 'r' is "\x72" - vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" - vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" - vec($foo, 94, 1) = 1; # 'PerlPerlPerl' - # 'l' is "\x6c" - -To transform a bit vector into a string or list of 0's and 1's, use these: - - $bits = unpack("b*", $vector); - @bits = split(//, unpack("b*", $vector)); - -If you know the exact length in bits, it can be used in place of the C<*>. - -Here is an example to illustrate how the bits actually fall in place: - - #!/usr/bin/perl -wl - - print <<'EOT'; - 0 1 2 3 - unpack("V",$_) 01234567890123456789012345678901 - ------------------------------------------------------------------ - EOT - - for $w (0..3) { - $width = 2**$w; - for ($shift=0; $shift < $width; ++$shift) { - for ($off=0; $off < 32/$width; ++$off) { - $str = pack("B*", "0"x32); - $bits = (1<<$shift); - vec($str, $off, $width) = $bits; - $res = unpack("b*",$str); - $val = unpack("V", $str); - write; - } - } - } - - format STDOUT = - vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> - $off, $width, $bits, $val, $res - . - __END__ - -Regardless of the machine architecture on which it is run, the above -example should print the following table: - - 0 1 2 3 - unpack("V",$_) 01234567890123456789012345678901 - ------------------------------------------------------------------ - vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 - vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 - vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 - vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 - vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 - vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 - vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 - vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 - vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 - vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 - vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 - vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 - vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 - vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 - vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 - vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 - vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 - vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 - vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 - vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 - vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 - vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 - vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 - vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 - vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 - vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 - vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 - vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 - vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 - vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 - vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 - vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 - vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 - vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 - vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 - vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 - vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 - vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 - vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 - vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 - vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 - vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 - vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 - vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 - vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 - vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 - vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 - vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 - vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 - vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 - vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 - vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 - vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 - vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 - vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 - vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 - vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 - vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 - vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 - vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 - vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 - vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 - vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 - vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 - vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 - vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 - vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 - vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 - vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 - vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 - vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 - vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 - vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 - vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 - vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 - vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 - vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 - vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 - vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 - vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 - vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 - vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 - vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 - vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 - vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 - vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 - vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 - vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 - vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 - vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 - vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 - vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 - vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 - vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 - vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 - vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 - vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 - vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 - vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 - vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 - vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 - vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 - vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 - vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 - vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 - vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 - vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 - vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 - vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 - vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 - vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 - vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 - vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 - vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 - vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 - vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 - vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 - vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 - vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 - vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 - vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 - vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 - vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 - vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 - -=item wait - -Behaves like the wait(2) system call on your system: it waits for a child -process to terminate and returns the pid of the deceased process, or -C<-1> if there are no child processes. The status is returned in C<$?>. -Note that a return value of C<-1> could mean that child processes are -being automatically reaped, as described in L<perlipc>. - -=item waitpid PID,FLAGS - -Waits for a particular child process to terminate and returns the pid of -the deceased process, or C<-1> if there is no such child process. On some -systems, a value of 0 indicates that there are processes still running. -The status is returned in C<$?>. If you say - - use POSIX ":sys_wait_h"; - #... - do { - $kid = waitpid(-1,&WNOHANG); - } until $kid == -1; - -then you can do a non-blocking wait for all pending zombie processes. -Non-blocking wait is available on machines supporting either the -waitpid(2) or wait4(2) system calls. However, waiting for a particular -pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the -system call by remembering the status values of processes that have -exited but have not been harvested by the Perl script yet.) - -Note that on some systems, a return value of C<-1> could mean that child -processes are being automatically reaped. See L<perlipc> for details, -and for other examples. - -=item wantarray - -Returns true if the context of the currently executing subroutine is -looking for a list value. Returns false if the context is looking -for a scalar. Returns the undefined value if the context is looking -for no value (void context). - - return unless defined wantarray; # don't bother doing more - my @a = complex_calculation(); - return wantarray ? @a : "@a"; - -This function should have been named wantlist() instead. - -=item warn LIST - -Produces a message on STDERR just like C<die>, but doesn't exit or throw -an exception. - -If LIST is empty and C<$@> already contains a value (typically from a -previous eval) that value is used after appending C<"\t...caught"> -to C<$@>. This is useful for staying almost, but not entirely similar to -C<die>. - -If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. - -No message is printed if there is a C<$SIG{__WARN__}> handler -installed. It is the handler's responsibility to deal with the message -as it sees fit (like, for instance, converting it into a C<die>). Most -handlers must therefore make arrangements to actually display the -warnings that they are not prepared to deal with, by calling C<warn> -again in the handler. Note that this is quite safe and will not -produce an endless loop, since C<__WARN__> hooks are not called from -inside one. - -You will find this behavior is slightly different from that of -C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can -instead call C<die> again to change it). - -Using a C<__WARN__> handler provides a powerful way to silence all -warnings (even the so-called mandatory ones). An example: - - # wipe out *all* compile-time warnings - BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } - my $foo = 10; - my $foo = 20; # no warning about duplicate my $foo, - # but hey, you asked for it! - # no compile-time or run-time warnings before here - $DOWARN = 1; - - # run-time warnings enabled after here - warn "\$foo is alive and $foo!"; # does show up - -See L<perlvar> for details on setting C<%SIG> entries, and for more -examples. See the Carp module for other kinds of warnings using its -carp() and cluck() functions. - -=item write FILEHANDLE - -=item write EXPR - -=item write - -Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, -using the format associated with that file. By default the format for -a file is the one having the same name as the filehandle, but the -format for the current output channel (see the C<select> function) may be set -explicitly by assigning the name of the format to the C<$~> variable. - -Top of form processing is handled automatically: if there is -insufficient room on the current page for the formatted record, the -page is advanced by writing a form feed, a special top-of-page format -is used to format the new page header, and then the record is written. -By default the top-of-page format is the name of the filehandle with -"_TOP" appended, but it may be dynamically set to the format of your -choice by assigning the name to the C<$^> variable while the filehandle is -selected. The number of lines remaining on the current page is in -variable C<$->, which can be set to C<0> to force a new page. - -If FILEHANDLE is unspecified, output goes to the current default output -channel, which starts out as STDOUT but may be changed by the -C<select> operator. If the FILEHANDLE is an EXPR, then the expression -is evaluated and the resulting string is used to look up the name of -the FILEHANDLE at run time. For more on formats, see L<perlform>. - -Note that write is I<not> the opposite of C<read>. Unfortunately. - -=item y/// - -The transliteration operator. Same as C<tr///>. See L<perlop>. - -=back |
