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diff --git a/contrib/perl5/pod/perlfunc.pod b/contrib/perl5/pod/perlfunc.pod new file mode 100644 index 0000000..4eac093 --- /dev/null +++ b/contrib/perl5/pod/perlfunc.pod @@ -0,0 +1,4440 @@ +=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 and 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 list argument.) For instance, splice() has three scalar +arguments followed by a list. + +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. + +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 a 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 + +=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<package>, C<use> + +=item Miscellaneous functions + +C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, 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<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 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. + -s File has nonzero size (returns size). + + -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 a 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. + +The interpretation of the file permission operators C<-r>, C<-R>, C<-w>, +C<-W>, C<-x>, and C<-X> is 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 as AFS access control lists. Also note that, for the superuser, +C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return +C<1> if any execute bit is set in the mode. Scripts run by the superuser may +thus need to do a C<stat()> to determine the actual mode of the +file, or temporarily set the uid to something else. + +Example: + + while (<>) { + chop; + next unless -f $_; # ignore specials + #... + } + +Note that C<-s/a/b/> does not do a negated substitution. Saying +C<-exp($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 (E<gt>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<$_>. + +=item accept NEWSOCKET,GENERICSOCKET + +Accepts an incoming socket connect, just as the accept(2) system call +does. Returns the packed address if it succeeded, FALSE otherwise. +See example in L<perlipc/"Sockets: Client/Server Communication">. + +=item alarm SECONDS + +=item alarm + +Arranges to have a SIGALRM delivered to this process after the +specified number of seconds have elapsed. If SECONDS is not specified, +the value stored in C<$_> 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 +C<syscall()> interface to access setitimer(2) if your system supports it, +or else see L</select()>. It is usually a mistake to intermix C<alarm()> +and C<sleep()> calls. + +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 $@ eq "alarm\n"; # propagate unexpected errors + # timed out + } + else { + # didn't + } + +=item atan2 Y,X + +Returns the arctangent of Y/X in the range -PI to PI. + +For the tangent operation, you may use the C<POSIX::tan()> +function, or use the familiar relation: + + sub tan { sin($_[0]) / cos($_[0]) } + +=item bind SOCKET,NAME + +Binds a network address to a socket, just as the bind system call +does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a +packed address of the appropriate type for the socket. See the examples in +L<perlipc/"Sockets: Client/Server Communication">. + +=item binmode FILEHANDLE + +Arranges for the file to be read or written in "binary" mode in operating +systems that distinguish between binary and text files. Files that are +not in binary mode have CR LF sequences translated to LF on input and LF +translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS +and similarly archaic systems, it may be imperative--otherwise your +MS-DOS-damaged C library may mangle your file. The key distinction between +systems that need C<binmode()> and those that don't is their text file +formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single +character, and that encode that character in C as C<"\n">, do not need +C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value +is taken as the name of the filehandle. + +=item bless REF,CLASSNAME + +=item bless REF + +This function tells the thingy referenced by REF that it is now +an object in the CLASSNAME package--or the current package if no CLASSNAME +is specified, which is often the case. It returns the reference for +convenience, because a C<bless()> is often the last thing in a constructor. +Always use the two-argument version if the function doing the blessing +might be inherited by a derived class. See L<perltoot> and L<perlobj> +for more about the blessing (and blessings) of objects. + +=item caller EXPR + +=item caller + +Returns the context of the current subroutine call. In scalar context, +returns the caller's package name if there is a caller, that is, if +we're in a subroutine or C<eval()> or C<require()>, and the undefined value +otherwise. In list context, returns + + ($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, $is_require) = caller($i); + +Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine +call, but an C<eval()>. In such a case additional elements C<$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, C<$evaltext> contains the text of the +C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement, +C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that +each C<use> statement creates a C<require> frame inside an C<eval EXPR>) +frame. + +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 E<gt> 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 home directory. Returns TRUE upon success, FALSE +otherwise. See 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 $mode, 'foo'; # this is best + +=item chomp VARIABLE + +=item chomp LIST + +=item chomp + +This is a slightly safer version of L</chop>. It removes any +line ending that corresponds to the current value of C<$/> (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. If +VARIABLE is omitted, it chomps C<$_>. Example: + + while (<>) { + chomp; # avoid \n on last field + @array = split(/:/); + # ... + } + +You can actually chomp anything that's an lvalue, including an assignment: + + chomp($cwd = `pwd`); + chomp($answer = <STDIN>); + +If you chomp a list, each element is chomped, and the total number of +characters removed is returned. + +=item chop VARIABLE + +=item chop LIST + +=item chop + +Chops off the last character of a string and returns the character +chopped. It's used primarily to remove the newline from the end of an +input record, but is much more efficient than C<s/\n//> because it neither +scans nor copies the string. If VARIABLE is omitted, chops C<$_>. +Example: + + while (<>) { + chop; # avoid \n on last field + @array = split(/:/); + #... + } + +You can actually chop anything that's an lvalue, including an assignment: + + chop($cwd = `pwd`); + chop($answer = <STDIN>); + +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<NUMERICAL> uid and gid, in that order. +Returns the number of files successfully changed. + + $cnt = chown $uid, $gid, 'foo', 'bar'; + chown $uid, $gid, @filenames; + +Here's an example that looks up nonnumeric uids in the passwd file: + + print "User: "; + chop($user = <STDIN>); + print "Files: "; + chop($pattern = <STDIN>); + + ($login,$pass,$uid,$gid) = getpwnam($user) + or die "$user not in passwd file"; + + @ary = glob($pattern); # expand filenames + chown $uid, $gid, @ary; + +On most systems, you are not allowed to change the ownership of the +file unless you're the superuser, although you should be able to change +the group to any of your secondary groups. On insecure systems, these +restrictions may be relaxed, but this is not a portable assumption. + +=item chr NUMBER + +=item chr + +Returns the character represented by that NUMBER in the character set. +For example, C<chr(65)> is C<"A"> in ASCII. For the reverse, use L</ord>. + +If NUMBER is omitted, uses C<$_>. + +=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<$!> will be set to C<0>.) Also, closing a pipe +waits for the process executing on the pipe to complete, in case you +want to look at the output of the pipe afterwards. Closing a pipe +explicitly also puts the exit status value of the command into C<$?>. + +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: $!"; + +FILEHANDLE may be an expression whose value can be used as an indirect +filehandle, usually the real filehandle name. + +=item closedir DIRHANDLE + +Closes a directory opened by C<opendir()> and returns the success of that +system call. + +DIRHANDLE may be an expression whose value can be used as an indirect +dirhandle, usually the real dirhandle name. + +=item connect SOCKET,NAME + +Attempts to connect to a remote socket, just as the connect system call +does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a +packed address of the appropriate type for the socket. See the examples in +L<perlipc/"Sockets: Client/Server Communication">. + +=item continue BLOCK + +Actually a flow control statement rather than a function. If there is a +C<continue> BLOCK attached to a BLOCK (typically in a C<while> or +C<foreach>), it is always executed just before the conditional is about to +be evaluated again, just like the third part of a C<for> loop in C. Thus +it can be used to increment a loop variable, even when the loop has been +continued via the C<next> statement (which is similar to the C C<continue> +statement). + +C<last>, C<next>, or C<redo> may appear within a C<continue> +block. C<last> and C<redo> will behave as if they had been executed within +the main block. So will C<next>, but since it will execute a C<continue> +block, it may be more entertaining. + + while (EXPR) { + ### redo always comes here + do_something; + } continue { + ### next always comes here + do_something_else; + # then back the top to re-check EXPR + } + ### last always comes here + +Omitting the C<continue> section is semantically equivalent to using an +empty one, logically enough. In that case, C<next> goes directly back +to check the condition at the top of the loop. + +=item cos EXPR + +Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, +takes cosine of C<$_>. + +For the inverse cosine operation, you may use the C<POSIX::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.) + +Here's an example that makes sure that whoever runs this program knows +their own password: + + $pwd = (getpwuid($<))[1]; + $salt = substr($pwd, 0, 2); + + system "stty -echo"; + print "Password: "; + chop($word = <STDIN>); + print "\n"; + system "stty echo"; + + if (crypt($word, $salt) ne $pwd) { + die "Sorry...\n"; + } else { + print "ok\n"; + } + +Of course, typing in your own password to whoever asks you +for it is unwise. + +=item dbmclose HASH + +[This function has been superseded by the C<untie()> function.] + +Breaks the binding between a DBM file and a hash. + +=item dbmopen HASH,DBNAME,MODE + +[This function has been superseded by the C<tie()> function.] + +This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a +hash. HASH is the name of the hash. (Unlike normal C<open()>, the first +argument is I<NOT> a filehandle, even though it looks like one). DBNAME +is the name of the database (without the F<.dir> or F<.pag> extension if +any). If the database does not exist, it is created with protection +specified by MODE (as modified by the C<umask()>). If your system supports +only the older DBM functions, you may perform only one C<dbmopen()> in your +program. In older versions of Perl, if your system had neither DBM nor +ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to +sdbm(3). + +If you don't have write access to the DBM file, you can only read hash +variables, not set them. If you want to test whether you can write, +either use file tests or try setting a dummy hash entry inside an C<eval()>, +which will trap the error. + +Note that functions such as C<keys()> and C<values()> may return huge lists +when used on large DBM files. You may prefer to use the C<each()> +function to iterate over large DBM files. Example: + + # print out history file offsets + dbmopen(%HIST,'/usr/lib/news/history',0666); + while (($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. + +=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()> to check whether a subroutine exists, by +saying C<defined &func> without parentheses. On the other hand, use +of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to +produce intuitive results, and should probably be avoided. + +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<$1> is defined, despite the fact that it +matched "nothing". But it didn't really match nothing--rather, it +matched something that happened to be C<0> characters long. This is all +very above-board and honest. When a function returns an undefined value, +it's an admission that it couldn't give you an honest answer. So you +should use C<defined()> only when you're questioning the integrity of what +you're trying to do. At other times, a simple comparison to C<0> or C<""> is +what you want. + +Currently, using C<defined()> on an entire array or hash reports whether +memory for that aggregate has ever been allocated. So an array you set +to the empty list appears undefined initially, and one that once was full +and that you then set to the empty list still appears defined. You +should instead use a simple test for size: + + if (@an_array) { print "has array elements\n" } + if (%a_hash) { print "has hash members\n" } + +Using C<undef()> on these, however, does clear their memory and then report +them as not defined anymore, but you shouldn't do that unless you don't +plan to use them again, because it saves time when you load them up +again to have memory already ready to be filled. The normal way to +free up space used by an aggregate is to assign the empty list. + +This counterintuitive behavior of C<defined()> on aggregates may be +changed, fixed, or broken in a future release of Perl. + +See also L</undef>, L</exists>, L</ref>. + +=item delete EXPR + +Deletes the specified key(s) and their associated values from a hash. +For each key, returns the deleted value associated with that key, or +the undefined value if there was no such key. Deleting from C<$ENV{}> +modifies the environment. Deleting from a hash tied to a DBM file +deletes the entry from the DBM file. (But deleting from a C<tie()>d hash +doesn't necessarily return anything.) + +The following deletes all the values of a hash: + + foreach $key (keys %HASH) { + delete $HASH{$key}; + } + +And so does this: + + delete @HASH{keys %HASH} + +(But both of these are slower than just assigning the empty list, or +using C<undef()>.) Note that the EXPR can be arbitrarily complicated as +long as the final operation is a hash element lookup or hash slice: + + delete $ref->[$x][$y]{$key}; + delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; + +=item die LIST + +Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with +the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of +C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> +is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into +C<$@> 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: $!\n" + +If the value of EXPR does not end in a newline, the current script line +number and input line number (if any) are also printed, and a newline +is supplied. Hint: sometimes appending C<", stopped"> to your message +will cause it to make better sense when the string C<"at foo line 123"> is +appended. Suppose you are running script "canasta". + + die "/etc/games is no good"; + die "/etc/games is no good, stopped"; + +produce, respectively + + /etc/games is no good at canasta line 123. + /etc/games is no good, stopped at canasta line 123. + +See also C<exit()> and C<warn()>. + +If LIST is empty and C<$@> 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. + +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. + +Note that the C<$SIG{__DIE__}> hook is 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>). + +=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.) + +=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, and searches all the B<-I> +libraries if the file isn't in the current directory (see also the @INC +array in L<perlvar/Predefined Names>). It is also different in how +code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing +scope like 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 + +This causes an immediate core dump. Primarily this is so that you can +use the B<undump> program 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. WARNING: Any files +opened at the time of the dump will NOT be open any more when the +program is reincarnated, with possible resulting confusion on the part +of Perl. See also B<-u> option in L<perlrun>. + +Example: + + #!/usr/bin/perl + require 'getopt.pl'; + require 'stat.pl'; + %days = ( + 'Sun' => 1, + 'Mon' => 2, + 'Tue' => 3, + 'Wed' => 4, + 'Thu' => 5, + 'Fri' => 6, + 'Sat' => 7, + ); + + dump QUICKSTART if $ARGV[0] eq '-d'; + + QUICKSTART: + Getopt('f'); + +This operator is largely obsolete, partly because it's very hard to +convert a core file into an executable, and because the real perl-to-C +compiler has superseded it. + +=item each HASH + +When called in list context, returns a 2-element list consisting of the +key and value for the next element of a hash, so that you can iterate over +it. When called in scalar context, returns the key for only the "next" +element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically +false; you may wish to avoid constructs like C<while ($k = each %foo) {}> +for this reason.) + +Entries are returned in an apparently random order. 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. + +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()> and C<values()>. + +=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. Filetypes such +as terminals may lose the end-of-file condition if you do. + +An C<eof> without an argument uses the last file read as argument. +Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of +the files listed on the command line, i.e., C<eof()> is reasonable to +use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the +last file. Use C<eof(ARGV)> or eof without the parentheses to test +I<EACH> file in a while (E<lt>E<gt>) loop. 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 break; 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 return false values 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 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 $@ + +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" + } + +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 C<$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 C<$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. + +=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. Note: +C<exec()> and C<system()> do not flush your output buffer, so you may need to +set C<$|> to avoid lost output. 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 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" ); + + system @args; # subject to shell escapes + # if @args == 1 + system { $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. + +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 + +Returns TRUE if the specified hash key exists in its hash array, even +if the corresponding value is undefined. + + print "Exists\n" if exists $array{$key}; + print "Defined\n" if defined $array{$key}; + print "True\n" if $array{$key}; + +A hash element can be TRUE only if it's defined, and defined if +it exists, but the reverse doesn't necessarily hold true. + +Note that the EXPR can be arbitrarily complicated as long as the final +operation is a hash key lookup: + + if (exists $ref->{"A"}{"B"}{$key}) { ... } + +Although the last element will not spring into existence just because its +existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}> +C<$ref-E<gt>{"B"}> will spring into existence due to the existence +test for a $key element. This autovivification may be fixed in a later +release. + +=item exit EXPR + +Evaluates EXPR and exits immediately with that value. (Actually, it +calls any defined C<END> routines first, but the C<END> routines may not +abort the exit. Likewise any object destructors that need to be called +are called before exit.) Example: + + $ans = <STDIN>; + exit 0 if $ans =~ /^[Xx]/; + +See also C<die()>. If EXPR is omitted, exits with C<0> status. The only +universally portable values for EXPR are C<0> for success and C<1> for error; +all other values are subject to unpredictable interpretation depending +on the environment in which the Perl program is running. + +You shouldn'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()>. + +All C<END{}> blocks are run at exit time. See L<perlsub> 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). + +=item fileno FILEHANDLE + +Returns the file descriptor for a filehandle. This is 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. + +On many platforms (including most versions or clones of Unix), locks +established by C<flock()> are 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()>. Windows NT and OS/2 are among the platforms which +enforce mandatory locking. See your local documentation for details. + +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 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 added to 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 mis-coordination, Perl flushes FILEHANDLE +before (un)locking 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 (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(); + +See also L<DB_File> for other flock() examples. + +=item fork + +Does a fork(2) system call. Returns the child pid to the parent process, +C<0> to the child process, or C<undef> if the fork is unsuccessful. + +Note: unflushed buffers remain unflushed in both processes, which means +you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> +method of C<IO::Handle> to avoid duplicate output. + +If you C<fork()> without ever waiting on your children, you will accumulate +zombies: + + $SIG{CHLD} = sub { wait }; + +There's also the double-fork trick (error checking on +C<fork()> returns omitted); + + unless ($pid = fork) { + unless (fork) { + exec "what you really wanna do"; + die "no exec"; + # ... or ... + ## (some_perl_code_here) + exit 0; + } + exit 0; + } + waitpid($pid,0); + +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, httpd or rsh) 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. It cannot be used to get unbuffered single-characters, +however. 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<perlmod/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) = unpack_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.) + +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 C<$quota>, C<$comment>, and C<$expire> are special +cases in the sense that in many systems they are unsupported. If the +C<$quota> is unsupported, it is an empty scalar. If it is supported, it +usually encodes the disk quota. If the C<$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 C<$change> or C<$age>, fields that have to do with password +aging. In some systems the C<$comment> field may be C<$class>. The C<$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 which meaning +your C<$quota> and C<$comment> fields have and whether you have the C<$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>. + +The C<$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]); + +If you get tired of remembering which element of the return list contains +which return value, by-name interfaces are also provided in 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-in, replacing them with 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. + + use Socket; + $mysockaddr = getsockname(SOCK); + ($port, $myaddr) = unpack_sockaddr_in($mysockaddr); + +=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/sh> would +do. This is the internal function implementing the C<E<lt>*.cE<gt>> +operator, but you can use it directly. If EXPR is omitted, C<$_> is used. +The C<E<lt>*.cE<gt>> operator is discussed in more detail in +L<perlop/"I/O Operators">. + +=item gmtime EXPR + +Converts a time as returned by the time function to a 9-element array +with the time localized for the standard Greenwich time zone. +Typically used as follows: + + # 0 1 2 3 4 5 6 7 8 + ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = + gmtime(time); + +All array elements are numeric, and come straight out of a struct tm. +In particular this means that C<$mon> has the range C<0..11> and C<$wday> has +the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of +years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year. + +If EXPR is omitted, does C<gmtime(time())>. + +In scalar context, 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 +instead a Perl builtin. Also see the C<Time::Local> module, and the +strftime(3) and mktime(3) function 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>, the short forms of the day of the week +and the month of the year, may not necessarily be three characters wide. + +=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 highly magical, and 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. + +=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 a 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, because C<$_> is a reference into the list value, it can be used +to modify the elements of the array. While this is useful and +supported, it can cause bizarre results if the LIST is not a named +array. Similarly, grep returns aliases into the original list, +much like the way that 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. + +See also L</map> for an array 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 or 0x +see L</oct>.) If EXPR is omitted, uses C<$_>. + + print hex '0xAf'; # prints '175' + print hex 'aF'; # same + +=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 + +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 for rounding, because it truncates +towards C<0>, and because machine representations of floating point +numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>, +or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better. + +=item ioctl FILEHANDLE,FUNCTION,SCALAR + +Implements the ioctl(2) function. You'll probably have to say + + require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph + +first 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<E<lt>sys/ioctl.hE<gt>>. +(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 are useful for manipulating the values of structures used by +C<ioctl()>. The following example sets the erase character to DEL. + + require 'ioctl.ph'; + $getp = &TIOCGETP; + die "NO TIOCGETP" if $@ || !$getp; + $sgttyb_t = "ccccs"; # 4 chars and a short + if (ioctl(STDIN,$getp,$sgttyb)) { + @ary = unpack($sgttyb_t,$sgttyb); + $ary[2] = 127; + $sgttyb = pack($sgttyb_t,@ary); + ioctl(STDIN,&TIOCSETP,$sgttyb) + || die "Can't 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(...)) || ($retval = -1); + printf "System returned %d\n", $retval; + +The special string "C<0> but true" is excempt from B<-w> complaints +about improper numeric conversions. + +=item join EXPR,LIST + +Joins the separate strings of LIST into a single string with +fields separated by the value of EXPR, and returns the string. +Example: + + $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); + +See L</split>. + +=item keys HASH + +Returns a list consisting of all the keys of the named hash. (In a +scalar context, returns the number of keys.) The keys are returned in +an apparently random order, but it is 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 >= 0) { + print pop(@keys), '=', pop(@values), "\n"; + } + +or how about sorted by key: + + foreach $key (sort(keys %ENV)) { + print $key, '=', $ENV{$key}, "\n"; + } + +To sort an array 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). + +=item kill LIST + +Sends a signal to a list of processes. The first element of +the list must be the signal to send. Returns the number of +processes successfully signaled. + + $cnt = kill 1, $child1, $child2; + kill 9, @goners; + +Unlike in the shell, in Perl if the I<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 + #... + } + +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 C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>. + +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 C<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 bytes of the value of EXPR. If EXPR is +omitted, returns length of C<$_>. + +=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 example in L<perlipc/"Sockets: Client/Server Communication">. + +=item local EXPR + +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. + +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. + +=item localtime EXPR + +Converts a time as returned by the time function to a 9-element array +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 array elements are numeric, and come straight out of a struct tm. +In particular this means that C<$mon> has the range C<0..11> and C<$wday> has +the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of +years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year. + +If EXPR is omitted, uses the current time (C<localtime(time)>). + +In scalar context, 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, and the +strftime(3) and mktime(3) function 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 log EXPR + +=item log + +Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log +of C<$_>. + +=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. Evaluates BLOCK or EXPR in a 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, because C<$_> is a reference into the list value, it can be used +to modify the elements of the array. While this is useful and +supported, it can cause bizarre results if the LIST is not a named +array. See also L</grep> for an array composed of those items of the +original list for which the BLOCK or EXPR evaluates to true. + +=item mkdir FILENAME,MODE + +Creates the directory specified by FILENAME, with permissions specified +by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise +it returns FALSE and sets C<$!> (errno). + +=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 +C<IPC::SysV> and C<IPC::Semaphore::Msg> 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 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 long integer message type, +which may be created with C<pack("l", $type)>. Returns TRUE if +successful, or FALSE if there is an error. See also C<IPC::SysV> +and C<IPC::SysV::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 if a message is received, the message type will be +the first thing in VAR, and the maximum length of VAR is SIZE plus the +size of the message type. 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 + +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. + +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 instead.) The following will handle decimal, octal, and +hex in the standard Perl or C notation: + + $val = oct($val) if $val =~ /^0/; + +If EXPR is omitted, uses C<$_>. This 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,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. 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.) + +If the filename begins with C<'E<lt>'> or nothing, the file is opened for input. +If the filename begins with C<'E<gt>'>, the file is truncated and opened for +output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>, +the file is opened for appending, again being created if necessary. +You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that +you want both read and write access to the file; thus C<'+E<lt>'> is almost +always preferred for read/write updates--the C<'+E<gt>'> 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 prefix and the filename may be separated with spaces. +These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>, +C<'w+'>, C<'a'>, and C<'a+'>. + +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 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.) + +Opening C<'-'> opens STDIN and opening C<'E<gt>-'> 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(ARTICLE, "caesar <$article |") # decrypt article + 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<'E<gt>&'>, 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<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>, +C<+E<gt>E<gt>>, and C<+E<lt>>. 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.) +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<'E<lt>&=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") + +If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, 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 pairs are more or less equivalent: + + open(FOO, "|tr '[a-z]' '[A-Z]'"); + open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; + + open(FOO, "cat -n '$file'|"); + open(FOO, "-|") || exec 'cat', '-n', $file; + +See L<perlipc/"Safe Pipe Opens"> for more examples of this. + +NOTE: On any operation that may do a fork, any unflushed buffers remain +unflushed in both processes, which means you may need to set C<$|> to +avoid duplicate output. + +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 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: $!"; + +However, to open a file with arbitrary weird characters in it, it's +necessary to protect any leading and trailing whitespace: + + $file =~ s#^(\s)#./$1#; + open(FOO, "< $file\0"); + +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. 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. + $first; # Or here. + } + +See L</seek()> for some details about mixing reading and writing. + +=item opendir DIRHANDLE,EXPR + +Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>, +C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful. +DIRHANDLEs have their own namespace separate from FILEHANDLEs. + +=item ord EXPR + +=item ord + +Returns the numeric ascii value of the first character of EXPR. If +EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. + +=item pack TEMPLATE,LIST + +Takes an array or list of values and packs it into a binary structure, +returning the string containing the structure. The TEMPLATE is a +sequence of characters that give the order and type of values, as +follows: + + A An ascii string, will be space padded. + a An ascii string, will be null padded. + b A bit string (ascending bit order, like vec()). + B A bit string (descending bit order). + h A hex string (low nybble first). + H A hex string (high nybble first). + + c A signed char value. + C An unsigned char value. + + 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'.) + + 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'.) + + n A short in "network" (big-endian) order. + N A long in "network" (big-endian) order. + v A short in "VAX" (little-endian) order. + V A long in "VAX" (little-endian) order. + (These 'shorts' and 'longs' are _exactly_ 16 bits and + _exactly_ 32 bits, respectively.) + + 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. + + 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. + +Each letter may optionally be followed by a number giving a repeat +count. With all types except C<"a">, C<"A">, 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. The C<"a"> and C<"A"> +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, but C<"a"> does not.) Likewise, the C<"b"> and C<"B"> +fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a +string that many nybbles long. 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"> 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>. +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). 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", $foo)>) will not in general equal C<$foo>). + +Examples: + + $foo = pack("cccc",65,66,67,68); + # foo eq "ABCD" + $foo = pack("c4",65,66,67,68); + # same thing + + $foo = pack("ccxxcc",65,66,67,68); + # foo eq "AB\0\0CD" + + $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" + + $foo = pack("i9pl", gmtime); + # a real struct tm (on my system anyway) + + sub bintodec { + unpack("N", pack("B32", substr("0" x 32 . shift, -32))); + } + +The same template may generally also be used in the unpack function. + +=item package + +=item package NAMESPACE + +Declares the compilation unit as being in the given namespace. The scope +of the package declaration is from the declaration itself through the end of +the enclosing block (the same scope as the C<local()> operator). All further +unqualified dynamic identifiers will be in this namespace. A package +statement affects only dynamic variables--including those you've used +C<local()> on--but I<not> lexical variables created with C<my()>. Typically it +would be the first declaration in a file to be included by the C<require> +or C<use> operator. You can switch into a package in more than one place; +it merely influences which symbol table is used by the compiler for the +rest of that block. You can refer to variables and filehandles in other +packages by prefixing the identifier with the package name and a double +colon: C<$Package::Variable>. If the package name is null, the C<main> +package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>. + +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. + +=item pop ARRAY + +=item pop + +Pops and returns the last value of the array, shortening the array by +1. Has a similar effect to + + $tmp = $ARRAY[$#ARRAY--]; + +If there are no elements in the array, returns the undefined value. +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 +is in question (C<$_> is used when the variable is not specified). May be +modified to change that offset. Such modification will also influence +the C<\G> zero-width assertion in regular expressions. See L<perlre> and +L<perlop>. + +=item print FILEHANDLE LIST + +=item print LIST + +=item print + +Prints a string or a comma-separated list of strings. Returns TRUE +if successful. FILEHANDLE may be a scalar variable name, in which case +the variable contains the name of or a reference to the filehandle, thus introducing one +level of indirection. (NOTE: If FILEHANDLE is a variable and the next +token is a term, it may be misinterpreted as an operator unless you +interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is +omitted, prints by default to standard output (or to the last selected +output channel--see L</select>). If LIST is also omitted, prints C<$_> to +the currently selected output channel. To set the default output channel to something other than +STDOUT use the select operation. Note that, 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 builtin is not I<overridable> (such as +C<qw//>) or its arguments cannot be expressed by a prototype (such as +C<system()>) - in other words, the builtin does not behave like a Perl +function - returns C<undef>. 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] = $value; + } + +but is more efficient. Returns the new number of elements in the array. + +=item q/STRING/ + +=item qq/STRING/ + +=item qr/STRING/ + +=item qx/STRING/ + +=item qw/STRING/ + +Generalized quotes. See L<perlop>. + +=item quotemeta EXPR + +=item quotemeta + +Returns the value of EXPR with all non-alphanumeric +characters backslashed. (That is, all characters not matching +C</[A-Za-z_0-9]/> will be preceded by a backslash in the +returned string, regardless of any locale settings.) +This is the internal function implementing +the C<\Q> escape in double-quoted strings. + +If EXPR is omitted, uses C<$_>. + +=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. An OFFSET may be specified to +place the read data at some other place than the beginning of the +string. This 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/$_" } readdir(DIR); + closedir DIR; + +=item readline EXPR + +Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line +is read and returned. In list context, reads until end-of-file is +reached and returns a list of lines (however you've defined lines +with C<$/> or C<$INPUT_RECORD_SEPARATOR>). +This is the internal function implementing the C<E<lt>EXPRE<gt>> +operator, but you can use it directly. The C<E<lt>EXPRE<gt>> +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,LEN,FLAGS + +Receives a message on a socket. Attempts to receive LENGTH bytes of +data into variable SCALAR from the specified SOCKET filehandle. +Actually does a C C<recvfrom()>, so that it can return the address of the +sender. Returns the undefined value if there's an error. SCALAR will +be grown or shrunk to the length actually read. Takes the same flags +as the system call of the same name. +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; + } + +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<$_> will be used. The value returned depends on the +type of thing the reference is a reference to. +Builtin types include: + + REF + SCALAR + ARRAY + HASH + CODE + GLOB + +If the referenced object has been blessed into a package, then that package +name is returned instead. You can think of C<ref()> as a C<typeof()> operator. + + if (ref($r) eq "HASH") { + print "r is a reference to a hash.\n"; + } + if (!ref($r)) { + print "r is not a reference at all.\n"; + } + +See also L<perlref>. + +=item rename OLDNAME,NEWNAME + +Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will +not work across file system boundaries. + +=item require EXPR + +=item require + +Demands some semantics specified by EXPR, or by C<$_> if EXPR is not +supplied. If EXPR is numeric, demands that the current version of Perl +(C<$]> or $PERL_VERSION) be equal or greater than EXPR. + +Otherwise, demands that a library file be included if it hasn't already +been included. The file is included via the do-FILE mechanism, which is +essentially just a variety of C<eval()>. Has semantics similar to the following +subroutine: + + sub require { + my($filename) = @_; + return 1 if $INC{$filename}; + my($realfilename,$result); + ITER: { + foreach $prefix (@INC) { + $realfilename = "$prefix/$filename"; + if (-f $realfilename) { + $result = do $realfilename; + last ITER; + } + } + die "Can't find $filename in \@INC"; + } + die $@ if $@; + die "$filename did not return true value" unless $result; + $INC{$filename} = $realfilename; + 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 ?? 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, an undefined value in +scalar context, or nothing in a void context. + +(Note that in the absence of a 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 consisting of those bytes, +but in the opposite order. + + print reverse <>; # line tac, last line first + + undef $/; # for efficiency of <> + print scalar reverse <>; # byte tac, last line tsrif + +This operator is also handy for inverting a hash, although there are some +caveats. If a value is duplicated in the original hash, only one of those +can be represented as a key in the inverted hash. Also, this has to +unwind one hash and build a whole new one, which may take some time +on a large hash. + + %by_name = reverse %by_address; # Invert the hash + +=item rewinddir DIRHANDLE + +Sets the current position to the beginning of the directory for the +C<readdir()> routine on DIRHANDLE. + +=item rindex STR,SUBSTR,POSITION + +=item rindex STR,SUBSTR + +Works just like index except that it returns the position of the LAST +occurrence of SUBSTR in STR. If POSITION is specified, returns the +last occurrence at or before that position. + +=item rmdir FILENAME + +=item rmdir + +Deletes the directory specified by FILENAME if that directory is empty. If it +succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (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 it's in practice 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. + +=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> from either the +C<IO::Seekable> or the POSIX 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. + +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<E<lt>FILEE<gt>> 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 C<$timeleft>, so +calling select() in scalar context just returns C<$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 theC<$timeleft>. If not, they always return +C<$timeleft> equal to the supplied C<$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 E<lt>FHE<gt>) 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. See also C<IPC::SysV> and 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 C<IPC::SysV> and +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, $semflag)>. The number of semaphore +operations is implied by the length of OPSTRING. Returns TRUE if +successful, or FALSE if there is an error. As an example, the +following code waits on semaphore C<$semnum> of semaphore id C<$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 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. +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 setpgrp(2). If the arguments are omitted, it defaults to +C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any +arguments, so only setpgrp C<0,0> is portable. + +=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<END {}>, and C<INIT {}> 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 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 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. +See also C<IPC::SysV> documentation. + +=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 filedescriptor 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<POSIX::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. + +See also the POSIX module's C<sigpause()> 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 example in L<perlipc/"Sockets: Client/Server Communication">. + +=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. + +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 array are to be ordered. (The C<E<lt>=E<gt>> 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. + +In the interests of efficiency the normal calling code for subroutines is +bypassed, with the following effects: the subroutine may not be a +recursive subroutine, and the two elements to be compared are passed into +the subroutine not via C<@_> but as the package global variables C<$a> and +C<$b> (see example below). They are passed by reference, so don't +modify C<$a> and C<$b>. And don't try to declare them as lexicals either. + +You also cannot exit out of the sort block or subroutine using any of the +loop control operators described in L<perlsyn> or with C<goto()>. + +When C<use locale> is in effect, C<sort LIST> sorts LIST according to the +current collation locale. See L<perllocale>. + +Examples: + + # sort lexically + @articles = sort @files; + + # same thing, but with explicit sort routine + @articles = sort {$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; + + # sort using explicit subroutine name + sub byage { + $age{$a} <=> $age{$b}; # presuming numeric + } + @sortedclass = sort byage @class; + + # this sorts the %age hash by value instead of key + # using an in-line function + @eldest = sort { $age{$b} <=> $age{$a} } keys %age; + + sub backwards { $b cmp $a; } + @harry = ('dog','cat','x','Cain','Abel'); + @george = ('gone','chased','yz','Punished','Axed'); + print sort @harry; + # prints AbelCaincatdogx + print sort backwards @harry; + # prints xdogcatCainAbel + print sort @george, 'to', @harry; + # prints AbelAxedCainPunishedcatchaseddoggonetoxyz + + # inefficiently sort by descending numeric compare using + # the first integer after the first = sign, or the + # whole record case-insensitively otherwise + + @new = sort { + ($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 using a Schwartzian Transform (no temps) + @new = map { $_->[0] } + sort { $b->[1] <=> $a->[1] + || + $a->[2] cmp $b->[2] + } map { [$_, /=(\d+)/, uc($_)] } @old; + +If you're using strict, you I<MUST NOT> declare C<$a> +and C<$b> as lexicals. They are package globals. That means +if you're in the C<main> package, it's + + @articles = sort {$main::b <=> $main::a} @files; + +or just + + @articles = sort {$::b <=> $::a} @files; + +but if you're in the C<FooPack> package, it's + + @articles = sort {$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<$x[2]> and +sometimes saying the opposite, for example) the results are not +well-defined. + +=item splice ARRAY,OFFSET,LENGTH,LIST + +=item splice ARRAY,OFFSET,LENGTH + +=item splice ARRAY,OFFSET + +Removes the elements designated by OFFSET and LENGTH from an array, and +replaces them with the elements of LIST, if any. In list context, +returns the elements removed from the array. In scalar context, +returns the last element removed, or C<undef> if no elements are +removed. The array grows or shrinks as necessary. +If OFFSET is negative then it start that far from the end of the array. +If LENGTH is omitted, removes everything from OFFSET onward. +If LENGTH is negative, leave that many elements off the end of the array. +The following equivalences hold (assuming C<$[ == 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..$len],0+@bar,@bar)) { ... } + +=item split /PATTERN/,EXPR,LIMIT + +=item split /PATTERN/,EXPR + +=item split /PATTERN/ + +=item split + +Splits a string into an array of strings, and returns it. By default, +empty leading fields are preserved, and empty trailing ones are deleted. + +If not in list context, returns the number of fields found and splits into +the C<@_> array. (In list context, you can force the split into C<@_> by +using C<??> as the pattern delimiters, but it still returns the list +value.) The use of implicit split to C<@_> is deprecated, however, because +it clobbers your subroutine arguments. + +If EXPR is omitted, splits the C<$_> 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'. + +The LIMIT parameter can be used to split a line partially + + ($login, $passwd, $remainder) = split(/:/, $_, 3); + +When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT +one larger than the number of variables in the list, to avoid +unnecessary work. For the list above LIMIT would have been 4 by +default. In time critical applications it behooves you not to split +into more fields than you really need. + +If the PATTERN contains parentheses, additional array elements are +created from each matching substring in the delimiter. + + split(/([,-])/, "1-10,20", 3); + +produces the list value + + (1, '-', 10, ',', 20) + +If you had the entire header of a normal Unix email message in C<$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(' ', $_)> internally. + +Example: + + open(PASSWD, '/etc/passwd'); + while (<PASSWD>) { + ($login, $passwd, $uid, $gid, + $gcos, $home, $shell) = split(/:/); + #... + } + +(Note that C<$shell> above will still have a newline on it. See L</chop>, +L</chomp>, and L</join>.) + +=item sprintf FORMAT, LIST + +Returns a string formatted by the usual C<printf()> conventions of the +C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)> +on your system for an explanation of the general principles. + +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. + +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) + %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 + +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" + +There is also one Perl-specific flag: + + V interpret integer as Perl's standard integer type + +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. + +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>. + +=item sqrt EXPR + +=item sqrt + +Return the square root of EXPR. If EXPR is omitted, returns square +root of C<$_>. + +=item srand EXPR + +=item srand + +Sets the random number seed for the C<rand()> operator. If EXPR is +omitted, uses a semi-random value 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 ^ $$> or +C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more. + +In fact, it's usually not necessary to call C<srand()> at all, because if +it is not called explicitly, it is called implicitly at the first use of +the C<rand()> operator. However, this was not the case in version of Perl +before 5.004, so if your script will run under older Perl versions, it +should call C<srand()>. + +Note that you need something much more random than the default seed for +cryptographic purposes. Checksumming the compressed output of one or more +rapidly changing operating system status programs is the usual method. For +example: + + srand (time ^ $$ ^ 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 since the epoch + 9 mtime last modify time since the epoch + 10 ctime inode change time (NOT creation time!) 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.) + +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<_>. + +=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 $file, "\n"; + } + +=item sub BLOCK + +=item sub NAME + +=item sub NAME BLOCK + +This is subroutine definition, not a real function I<per se>. With just a +NAME (and possibly prototypes), it's just a forward declaration. Without +a NAME, it's an anonymous function declaration, and does actually return a +value: the CODE ref of the closure you just created. See L<perlsub> and +L<perlref> for details. + +=item substr EXPR,OFFSET,LEN,REPLACEMENT + +=item substr EXPR,OFFSET,LEN + +=item substr EXPR,OFFSET + +Extracts a substring out of EXPR and returns it. First character is at +offset C<0>, or whatever you've set C<$[> 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 LEN is omitted, returns +everything to the end of the string. If LEN is negative, leaves that +many characters off the end of the string. + +If you specify a substring that is partly outside the string, the part +within the string is returned. If the substring is totally outside +the string a warning is produced. + +You can use the C<substr()> function +as an lvalue, in which case EXPR must be an lvalue. If you assign +something shorter than LEN, the string will shrink, and if you assign +something longer than LEN, 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()>. + +An alternative to using C<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. + +=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>. +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 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>. The C<umask> value is a number representing +disabled permissions bits--if your C<umask> were C<027> (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>). + +If you find this C<umask()> talk confusing, here's some advice: supply a +creation mode of C<0666> for regular files 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. In short, +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. + +The C<IO::File> module provides a more object-oriented approach, if you're +into that kind of thing. + +=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()>, or C<tell()> 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. + +=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()>, or C<tell()> 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 use the +constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the C<IO::Seekable> +or the POSIX 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. + +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`">. + +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 + +Attempts to write LENGTH bytes of data from variable SCALAR to the +specified FILEHANDLE, using the system call write(2). It bypasses +stdio, so mixing this with reads (other than C<sysread())>, C<print()>, +C<write()>, C<seek()>, or C<tell()> 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. FILEHANDLE may be an +expression whose value gives the name of the actual filehandle. If +FILEHANDLE is omitted, assumes the file last read. + +=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<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 + DESTROY this + FETCH this, key + STORE this, key, value + DELETE this, key + EXISTS this, key + FIRSTKEY this + NEXTKEY this, lastkey + +A class implementing an ordinary array should have the following methods: + + TIEARRAY classname, LIST + DESTROY this + FETCH this, key + STORE this, key, value + [others TBD] + +A class implementing a scalar should have the following methods: + + TIESCALAR classname, LIST + DESTROY this + FETCH this, + STORE this, value + +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()>. + +=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>. + +If EXPR is omitted, uses C<$_>. + +=item ucfirst EXPR + +=item ucfirst + +Returns the value of EXPR with the first character uppercased. This is +the internal function implementing the C<\u> escape in double-quoted strings. +Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. + +If EXPR is omitted, uses C<$_>. + +=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. + +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 representing a +structure and expands it out into a list value, returning the array +value. (In scalar context, it returns merely the first value +produced.) 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, you may prefix a field with a %E<lt>numberE<gt> to indicate that +you want a E<lt>numberE<gt>-bit checksum of the items instead of the items +themselves. Default is a 16-bit checksum. For example, the following +computes the same number as the System V sum program: + + while (<>) { + $checksum += unpack("%16C*", $_); + } + $checksum %= 65536; + +The following efficiently counts the number of set bits in a bit vector: + + $setbits = unpack("%32b*", $selectmask); + +=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 LIST + +=item use Module + +=item use Module VERSION LIST + +=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. + +If the first argument to C<use> is a number, it is treated as a version +number instead of a module name. If the version of the Perl interpreter +is less than VERSION, then an error message is printed and Perl exits +immediately. 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 error is currently silently ignored. This +may change to a fatal error in a future version. + +If you don't want your namespace altered, explicitly supply an 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>. (Note that there is not a +comma after VERSION!) + +Because this is a wide-open interface, pragmas (compiler directives) +are also implemented this way. Currently implemented pragmas are: + + use integer; + use diagnostics; + use sigtrap qw(SEGV BUS); + use strict qw(subs vars refs); + use subs qw(afunc blurfl); + +Some of these 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'; + +If no C<unimport()> method can be found the call fails with a fatal error. + +See L<perlmod> for a list of standard modules and pragmas. + +=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 modification 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, $now, @ARGV; + +=item values HASH + +Returns a list consisting of all the values of the named hash. (In a +scalar context, returns the number of values.) The values are +returned in an apparently random order, but it is the same order as +either the C<keys()> or C<each()> function would produce on the same hash. +As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>, +and C<sort()>. + +=item vec EXPR,OFFSET,BITS + +Treats the string in EXPR as a vector of unsigned integers, and +returns the value of the bit field specified by OFFSET. BITS specifies +the number of bits that are reserved for each entry in the bit +vector. This must be a power of two from 1 to 32. C<vec()> may also be +assigned to, in which case parentheses are needed to give the expression +the correct precedence as in + + vec($image, $max_x * $x + $y, 8) = 3; + +Vectors created with C<vec()> can also be manipulated with the logical +operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is +desired when both operands are strings. + +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' + 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 array of 0's and 1's, use these: + + $bits = unpack("b*", $vector); + @bits = split(//, unpack("b*", $vector)); + +If you know the exact length in bits, it can be used in place of the C<*>. + +=item wait + +Waits for a child process to terminate and returns the pid of the +deceased process, or C<-1> if there are no child processes. The status is +returned in C<$?>. + +=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. The +status is returned in C<$?>. If you say + + use POSIX ":sys_wait_h"; + #... + waitpid(-1,&WNOHANG); + +then you can do a non-blocking wait for any process. Non-blocking wait +is available on machines supporting either the waitpid(2) or +wait4(2) system calls. However, waiting for a particular pid with +FLAGS of C<0> is implemented everywhere. (Perl emulates the system call +by remembering the status values of processes that have exited but have +not been harvested by the Perl script yet.) + +See L<perlipc> for other examples. + +=item wantarray + +Returns TRUE if the context of the currently executing subroutine is +looking for a list value. Returns FALSE if the context is looking +for a scalar. Returns the undefined value if the context is looking +for no value (void context). + + return unless defined wantarray; # don't bother doing more + my @a = complex_calculation(); + return wantarray ? @a : "@a"; + +=item warn LIST + +Produces a message on STDERR just like C<die()>, but doesn't exit or throw +an exception. + +If LIST is empty and C<$@> 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 $foo!"; # does show up + +See L<perlvar> for details on setting C<%SIG> entries, and for more +examples. + +=item write FILEHANDLE + +=item write EXPR + +=item write + +Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, +using the format associated with that file. By default the format for +a file is the one having the same name as the filehandle, but the +format for the current output channel (see the C<select()> function) may be set +explicitly by assigning the name of the format to the C<$~> 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 |
