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diff --git a/contrib/perl5/pod/perlfaq5.pod b/contrib/perl5/pod/perlfaq5.pod deleted file mode 100644 index 4ae7407..0000000 --- a/contrib/perl5/pod/perlfaq5.pod +++ /dev/null @@ -1,1216 +0,0 @@ -=head1 NAME - -perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $) - -=head1 DESCRIPTION - -This section deals with I/O and the "f" issues: filehandles, flushing, -formats, and footers. - -=head2 How do I flush/unbuffer an output filehandle? Why must I do this? - -The C standard I/O library (stdio) normally buffers characters sent to -devices. This is done for efficiency reasons so that there isn't a -system call for each byte. Any time you use print() or write() in -Perl, you go though this buffering. syswrite() circumvents stdio and -buffering. - -In most stdio implementations, the type of output buffering and the size of -the buffer varies according to the type of device. Disk files are block -buffered, often with a buffer size of more than 2k. Pipes and sockets -are often buffered with a buffer size between 1/2 and 2k. Serial devices -(e.g. modems, terminals) are normally line-buffered, and stdio sends -the entire line when it gets the newline. - -Perl does not support truly unbuffered output (except insofar as you can -C<syswrite(OUT, $char, 1)>). What it does instead support is "command -buffering", in which a physical write is performed after every output -command. This isn't as hard on your system as unbuffering, but does -get the output where you want it when you want it. - -If you expect characters to get to your device when you print them there, -you'll want to autoflush its handle. -Use select() and the C<$|> variable to control autoflushing -(see L<perlvar/$|> and L<perlfunc/select>): - - $old_fh = select(OUTPUT_HANDLE); - $| = 1; - select($old_fh); - -Or using the traditional idiom: - - select((select(OUTPUT_HANDLE), $| = 1)[0]); - -Or if don't mind slowly loading several thousand lines of module code -just because you're afraid of the C<$|> variable: - - use FileHandle; - open(DEV, "+</dev/tty"); # ceci n'est pas une pipe - DEV->autoflush(1); - -or the newer IO::* modules: - - use IO::Handle; - open(DEV, ">/dev/printer"); # but is this? - DEV->autoflush(1); - -or even this: - - use IO::Socket; # this one is kinda a pipe? - $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com', - PeerPort => 'http(80)', - Proto => 'tcp'); - die "$!" unless $sock; - - $sock->autoflush(); - print $sock "GET / HTTP/1.0" . "\015\012" x 2; - $document = join('', <$sock>); - print "DOC IS: $document\n"; - -Note the bizarrely hardcoded carriage return and newline in their octal -equivalents. This is the ONLY way (currently) to assure a proper flush -on all platforms, including Macintosh. That's the way things work in -network programming: you really should specify the exact bit pattern -on the network line terminator. In practice, C<"\n\n"> often works, -but this is not portable. - -See L<perlfaq9> for other examples of fetching URLs over the web. - -=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file? - -Those are operations of a text editor. Perl is not a text editor. -Perl is a programming language. You have to decompose the problem into -low-level calls to read, write, open, close, and seek. - -Although humans have an easy time thinking of a text file as being a -sequence of lines that operates much like a stack of playing cards--or -punch cards--computers usually see the text file as a sequence of bytes. -In general, there's no direct way for Perl to seek to a particular line -of a file, insert text into a file, or remove text from a file. - -(There are exceptions in special circumstances. You can add or remove -data at the very end of the file. A sequence of bytes can be replaced -with another sequence of the same length. The C<$DB_RECNO> array -bindings as documented in L<DB_File> also provide a direct way of -modifying a file. Files where all lines are the same length are also -easy to alter.) - -The general solution is to create a temporary copy of the text file with -the changes you want, then copy that over the original. This assumes -no locking. - - $old = $file; - $new = "$file.tmp.$$"; - $bak = "$file.orig"; - - open(OLD, "< $old") or die "can't open $old: $!"; - open(NEW, "> $new") or die "can't open $new: $!"; - - # Correct typos, preserving case - while (<OLD>) { - s/\b(p)earl\b/${1}erl/i; - (print NEW $_) or die "can't write to $new: $!"; - } - - close(OLD) or die "can't close $old: $!"; - close(NEW) or die "can't close $new: $!"; - - rename($old, $bak) or die "can't rename $old to $bak: $!"; - rename($new, $old) or die "can't rename $new to $old: $!"; - -Perl can do this sort of thing for you automatically with the C<-i> -command-line switch or the closely-related C<$^I> variable (see -L<perlrun> for more details). Note that -C<-i> may require a suffix on some non-Unix systems; see the -platform-specific documentation that came with your port. - - # Renumber a series of tests from the command line - perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t - - # form a script - local($^I, @ARGV) = ('.orig', glob("*.c")); - while (<>) { - if ($. == 1) { - print "This line should appear at the top of each file\n"; - } - s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case - print; - close ARGV if eof; # Reset $. - } - -If you need to seek to an arbitrary line of a file that changes -infrequently, you could build up an index of byte positions of where -the line ends are in the file. If the file is large, an index of -every tenth or hundredth line end would allow you to seek and read -fairly efficiently. If the file is sorted, try the look.pl library -(part of the standard perl distribution). - -In the unique case of deleting lines at the end of a file, you -can use tell() and truncate(). The following code snippet deletes -the last line of a file without making a copy or reading the -whole file into memory: - - open (FH, "+< $file"); - while ( <FH> ) { $addr = tell(FH) unless eof(FH) } - truncate(FH, $addr); - -Error checking is left as an exercise for the reader. - -=head2 How do I count the number of lines in a file? - -One fairly efficient way is to count newlines in the file. The -following program uses a feature of tr///, as documented in L<perlop>. -If your text file doesn't end with a newline, then it's not really a -proper text file, so this may report one fewer line than you expect. - - $lines = 0; - open(FILE, $filename) or die "Can't open `$filename': $!"; - while (sysread FILE, $buffer, 4096) { - $lines += ($buffer =~ tr/\n//); - } - close FILE; - -This assumes no funny games with newline translations. - -=head2 How do I make a temporary file name? - -Use the C<new_tmpfile> class method from the IO::File module to get a -filehandle opened for reading and writing. Use it if you don't -need to know the file's name: - - use IO::File; - $fh = IO::File->new_tmpfile() - or die "Unable to make new temporary file: $!"; - -If you do need to know the file's name, you can use the C<tmpnam> -function from the POSIX module to get a filename that you then open -yourself: - - - use Fcntl; - use POSIX qw(tmpnam); - - # try new temporary filenames until we get one that didn't already - # exist; the check should be unnecessary, but you can't be too careful - do { $name = tmpnam() } - until sysopen(FH, $name, O_RDWR|O_CREAT|O_EXCL); - - # install atexit-style handler so that when we exit or die, - # we automatically delete this temporary file - END { unlink($name) or die "Couldn't unlink $name : $!" } - - # now go on to use the file ... - -If you're committed to creating a temporary file by hand, use the -process ID and/or the current time-value. If you need to have many -temporary files in one process, use a counter: - - BEGIN { - use Fcntl; - my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP}; - my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); - sub temp_file { - local *FH; - my $count = 0; - until (defined(fileno(FH)) || $count++ > 100) { - $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; - sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); - } - if (defined(fileno(FH)) - return (*FH, $base_name); - } else { - return (); - } - } - } - -=head2 How can I manipulate fixed-record-length files? - -The most efficient way is using pack() and unpack(). This is faster than -using substr() when taking many, many strings. It is slower for just a few. - -Here is a sample chunk of code to break up and put back together again -some fixed-format input lines, in this case from the output of a normal, -Berkeley-style ps: - - # sample input line: - # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what - $PS_T = 'A6 A4 A7 A5 A*'; - open(PS, "ps|"); - print scalar <PS>; - while (<PS>) { - ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_); - for $var (qw!pid tt stat time command!) { - print "$var: <$$var>\n"; - } - print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command), - "\n"; - } - -We've used C<$$var> in a way that forbidden by C<use strict 'refs'>. -That is, we've promoted a string to a scalar variable reference using -symbolic references. This is ok in small programs, but doesn't scale -well. It also only works on global variables, not lexicals. - -=head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles? - -The fastest, simplest, and most direct way is to localize the typeglob -of the filehandle in question: - - local *TmpHandle; - -Typeglobs are fast (especially compared with the alternatives) and -reasonably easy to use, but they also have one subtle drawback. If you -had, for example, a function named TmpHandle(), or a variable named -%TmpHandle, you just hid it from yourself. - - sub findme { - local *HostFile; - open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!"; - local $_; # <- VERY IMPORTANT - while (<HostFile>) { - print if /\b127\.(0\.0\.)?1\b/; - } - # *HostFile automatically closes/disappears here - } - -Here's how to use typeglobs in a loop to open and store a bunch of -filehandles. We'll use as values of the hash an ordered -pair to make it easy to sort the hash in insertion order. - - @names = qw(motd termcap passwd hosts); - my $i = 0; - foreach $filename (@names) { - local *FH; - open(FH, "/etc/$filename") || die "$filename: $!"; - $file{$filename} = [ $i++, *FH ]; - } - - # Using the filehandles in the array - foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) { - my $fh = $file{$name}[1]; - my $line = <$fh>; - print "$name $. $line"; - } - -For passing filehandles to functions, the easiest way is to -preface them with a star, as in func(*STDIN). -See L<perlfaq7/"Passing Filehandles"> for details. - -If you want to create many anonymous handles, you should check out the -Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent -code with Symbol::gensym, which is reasonably light-weight: - - foreach $filename (@names) { - use Symbol; - my $fh = gensym(); - open($fh, "/etc/$filename") || die "open /etc/$filename: $!"; - $file{$filename} = [ $i++, $fh ]; - } - -Here's using the semi-object-oriented FileHandle module, which certainly -isn't light-weight: - - use FileHandle; - - foreach $filename (@names) { - my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!"; - $file{$filename} = [ $i++, $fh ]; - } - -Please understand that whether the filehandle happens to be a (probably -localized) typeglob or an anonymous handle from one of the modules -in no way affects the bizarre rules for managing indirect handles. -See the next question. - -=head2 How can I use a filehandle indirectly? - -An indirect filehandle is using something other than a symbol -in a place that a filehandle is expected. Here are ways -to get indirect filehandles: - - $fh = SOME_FH; # bareword is strict-subs hostile - $fh = "SOME_FH"; # strict-refs hostile; same package only - $fh = *SOME_FH; # typeglob - $fh = \*SOME_FH; # ref to typeglob (bless-able) - $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob - -Or, you can use the C<new> method from the FileHandle or IO modules to -create an anonymous filehandle, store that in a scalar variable, -and use it as though it were a normal filehandle. - - use FileHandle; - $fh = FileHandle->new(); - - use IO::Handle; # 5.004 or higher - $fh = IO::Handle->new(); - -Then use any of those as you would a normal filehandle. Anywhere that -Perl is expecting a filehandle, an indirect filehandle may be used -instead. An indirect filehandle is just a scalar variable that contains -a filehandle. Functions like C<print>, C<open>, C<seek>, or -the C<< <FH> >> diamond operator will accept either a read filehandle -or a scalar variable containing one: - - ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); - print $ofh "Type it: "; - $got = <$ifh> - print $efh "What was that: $got"; - -If you're passing a filehandle to a function, you can write -the function in two ways: - - sub accept_fh { - my $fh = shift; - print $fh "Sending to indirect filehandle\n"; - } - -Or it can localize a typeglob and use the filehandle directly: - - sub accept_fh { - local *FH = shift; - print FH "Sending to localized filehandle\n"; - } - -Both styles work with either objects or typeglobs of real filehandles. -(They might also work with strings under some circumstances, but this -is risky.) - - accept_fh(*STDOUT); - accept_fh($handle); - -In the examples above, we assigned the filehandle to a scalar variable -before using it. That is because only simple scalar variables, not -expressions or subscripts of hashes or arrays, can be used with -built-ins like C<print>, C<printf>, or the diamond operator. Using -something other than a simple scalar varaible as a filehandle is -illegal and won't even compile: - - @fd = (*STDIN, *STDOUT, *STDERR); - print $fd[1] "Type it: "; # WRONG - $got = <$fd[0]> # WRONG - print $fd[2] "What was that: $got"; # WRONG - -With C<print> and C<printf>, you get around this by using a block and -an expression where you would place the filehandle: - - print { $fd[1] } "funny stuff\n"; - printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; - # Pity the poor deadbeef. - -That block is a proper block like any other, so you can put more -complicated code there. This sends the message out to one of two places: - - $ok = -x "/bin/cat"; - print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; - print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; - -This approach of treating C<print> and C<printf> like object methods -calls doesn't work for the diamond operator. That's because it's a -real operator, not just a function with a comma-less argument. Assuming -you've been storing typeglobs in your structure as we did above, you -can use the built-in function named C<readline> to reads a record just -as C<< <> >> does. Given the initialization shown above for @fd, this -would work, but only because readline() require a typeglob. It doesn't -work with objects or strings, which might be a bug we haven't fixed yet. - - $got = readline($fd[0]); - -Let it be noted that the flakiness of indirect filehandles is not -related to whether they're strings, typeglobs, objects, or anything else. -It's the syntax of the fundamental operators. Playing the object -game doesn't help you at all here. - -=head2 How can I set up a footer format to be used with write()? - -There's no builtin way to do this, but L<perlform> has a couple of -techniques to make it possible for the intrepid hacker. - -=head2 How can I write() into a string? - -See L<perlform/"Accessing Formatting Internals"> for an swrite() function. - -=head2 How can I output my numbers with commas added? - -This one will do it for you: - - sub commify { - local $_ = shift; - 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; - return $_; - } - - $n = 23659019423.2331; - print "GOT: ", commify($n), "\n"; - - GOT: 23,659,019,423.2331 - -You can't just: - - s/^([-+]?\d+)(\d{3})/$1,$2/g; - -because you have to put the comma in and then recalculate your -position. - -Alternatively, this code commifies all numbers in a line regardless of -whether they have decimal portions, are preceded by + or -, or -whatever: - - # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca> - sub commify { - my $input = shift; - $input = reverse $input; - $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g; - return scalar reverse $input; - } - -=head2 How can I translate tildes (~) in a filename? - -Use the <> (glob()) operator, documented in L<perlfunc>. Older -versions of Perl require that you have a shell installed that groks -tildes. Recent perl versions have this feature built in. The -Glob::KGlob module (available from CPAN) gives more portable glob -functionality. - -Within Perl, you may use this directly: - - $filename =~ s{ - ^ ~ # find a leading tilde - ( # save this in $1 - [^/] # a non-slash character - * # repeated 0 or more times (0 means me) - ) - }{ - $1 - ? (getpwnam($1))[7] - : ( $ENV{HOME} || $ENV{LOGDIR} ) - }ex; - -=head2 How come when I open a file read-write it wipes it out? - -Because you're using something like this, which truncates the file and -I<then> gives you read-write access: - - open(FH, "+> /path/name"); # WRONG (almost always) - -Whoops. You should instead use this, which will fail if the file -doesn't exist. - - open(FH, "+< /path/name"); # open for update - -Using ">" always clobbers or creates. Using "<" never does -either. The "+" doesn't change this. - -Here are examples of many kinds of file opens. Those using sysopen() -all assume - - use Fcntl; - -To open file for reading: - - open(FH, "< $path") || die $!; - sysopen(FH, $path, O_RDONLY) || die $!; - -To open file for writing, create new file if needed or else truncate old file: - - open(FH, "> $path") || die $!; - sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; - sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; - -To open file for writing, create new file, file must not exist: - - sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; - sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; - -To open file for appending, create if necessary: - - open(FH, ">> $path") || die $!; - sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; - sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; - -To open file for appending, file must exist: - - sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; - -To open file for update, file must exist: - - open(FH, "+< $path") || die $!; - sysopen(FH, $path, O_RDWR) || die $!; - -To open file for update, create file if necessary: - - sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; - sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; - -To open file for update, file must not exist: - - sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; - sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; - -To open a file without blocking, creating if necessary: - - sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT) - or die "can't open /tmp/somefile: $!": - -Be warned that neither creation nor deletion of files is guaranteed to -be an atomic operation over NFS. That is, two processes might both -successfully create or unlink the same file! Therefore O_EXCL -isn't as exclusive as you might wish. - -See also the new L<perlopentut> if you have it (new for 5.6). - -=head2 Why do I sometimes get an "Argument list too long" when I use <*>? - -The C<< <> >> operator performs a globbing operation (see above). -In Perl versions earlier than v5.6.0, the internal glob() operator forks -csh(1) to do the actual glob expansion, but -csh can't handle more than 127 items and so gives the error message -C<Argument list too long>. People who installed tcsh as csh won't -have this problem, but their users may be surprised by it. - -To get around this, either upgrade to Perl v5.6.0 or later, do the glob -yourself with readdir() and patterns, or use a module like Glob::KGlob, -one that doesn't use the shell to do globbing. - -=head2 Is there a leak/bug in glob()? - -Due to the current implementation on some operating systems, when you -use the glob() function or its angle-bracket alias in a scalar -context, you may cause a memory leak and/or unpredictable behavior. It's -best therefore to use glob() only in list context. - -=head2 How can I open a file with a leading ">" or trailing blanks? - -Normally perl ignores trailing blanks in filenames, and interprets -certain leading characters (or a trailing "|") to mean something -special. To avoid this, you might want to use a routine like the one below. -It turns incomplete pathnames into explicit relative ones, and tacks a -trailing null byte on the name to make perl leave it alone: - - sub safe_filename { - local $_ = shift; - s#^([^./])#./$1#; - $_ .= "\0"; - return $_; - } - - $badpath = "<<<something really wicked "; - $fn = safe_filename($badpath"); - open(FH, "> $fn") or "couldn't open $badpath: $!"; - -This assumes that you are using POSIX (portable operating systems -interface) paths. If you are on a closed, non-portable, proprietary -system, you may have to adjust the C<"./"> above. - -It would be a lot clearer to use sysopen(), though: - - use Fcntl; - $badpath = "<<<something really wicked "; - sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC) - or die "can't open $badpath: $!"; - -For more information, see also the new L<perlopentut> if you have it -(new for 5.6). - -=head2 How can I reliably rename a file? - -Well, usually you just use Perl's rename() function. That may not -work everywhere, though, particularly when renaming files across file systems. -Some sub-Unix systems have broken ports that corrupt the semantics of -rename()--for example, WinNT does this right, but Win95 and Win98 -are broken. (The last two parts are not surprising, but the first is. :-) - -If your operating system supports a proper mv(1) program or its moral -equivalent, this works: - - rename($old, $new) or system("mv", $old, $new); - -It may be more compelling to use the File::Copy module instead. You -just copy to the new file to the new name (checking return values), -then delete the old one. This isn't really the same semantically as a -real rename(), though, which preserves metainformation like -permissions, timestamps, inode info, etc. - -Newer versions of File::Copy exports a move() function. - -=head2 How can I lock a file? - -Perl's builtin flock() function (see L<perlfunc> for details) will call -flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and -later), and lockf(3) if neither of the two previous system calls exists. -On some systems, it may even use a different form of native locking. -Here are some gotchas with Perl's flock(): - -=over 4 - -=item 1 - -Produces a fatal error if none of the three system calls (or their -close equivalent) exists. - -=item 2 - -lockf(3) does not provide shared locking, and requires that the -filehandle be open for writing (or appending, or read/writing). - -=item 3 - -Some versions of flock() can't lock files over a network (e.g. on NFS file -systems), so you'd need to force the use of fcntl(2) when you build Perl. -But even this is dubious at best. See the flock entry of L<perlfunc> -and the F<INSTALL> file in the source distribution for information on -building Perl to do this. - -Two potentially non-obvious but traditional flock semantics are that -it waits indefinitely until the lock is granted, and that its locks are -I<merely advisory>. Such discretionary locks are more flexible, but -offer fewer guarantees. This means that files locked with flock() may -be modified by programs that do not also use flock(). Cars that stop -for red lights get on well with each other, but not with cars that don't -stop for red lights. See the perlport manpage, your port's specific -documentation, or your system-specific local manpages for details. It's -best to assume traditional behavior if you're writing portable programs. -(If you're not, you should as always feel perfectly free to write -for your own system's idiosyncrasies (sometimes called "features"). -Slavish adherence to portability concerns shouldn't get in the way of -your getting your job done.) - -For more information on file locking, see also -L<perlopentut/"File Locking"> if you have it (new for 5.6). - -=back - -=head2 Why can't I just open(FH, ">file.lock")? - -A common bit of code B<NOT TO USE> is this: - - sleep(3) while -e "file.lock"; # PLEASE DO NOT USE - open(LCK, "> file.lock"); # THIS BROKEN CODE - -This is a classic race condition: you take two steps to do something -which must be done in one. That's why computer hardware provides an -atomic test-and-set instruction. In theory, this "ought" to work: - - sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) - or die "can't open file.lock: $!": - -except that lamentably, file creation (and deletion) is not atomic -over NFS, so this won't work (at least, not every time) over the net. -Various schemes involving link() have been suggested, but -these tend to involve busy-wait, which is also subdesirable. - -=head2 I still don't get locking. I just want to increment the number in the file. How can I do this? - -Didn't anyone ever tell you web-page hit counters were useless? -They don't count number of hits, they're a waste of time, and they serve -only to stroke the writer's vanity. It's better to pick a random number; -they're more realistic. - -Anyway, this is what you can do if you can't help yourself. - - use Fcntl qw(:DEFAULT :flock); - sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; - flock(FH, LOCK_EX) or die "can't flock numfile: $!"; - $num = <FH> || 0; - seek(FH, 0, 0) or die "can't rewind numfile: $!"; - truncate(FH, 0) or die "can't truncate numfile: $!"; - (print FH $num+1, "\n") or die "can't write numfile: $!"; - close FH or die "can't close numfile: $!"; - -Here's a much better web-page hit counter: - - $hits = int( (time() - 850_000_000) / rand(1_000) ); - -If the count doesn't impress your friends, then the code might. :-) - -=head2 How do I randomly update a binary file? - -If you're just trying to patch a binary, in many cases something as -simple as this works: - - perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs - -However, if you have fixed sized records, then you might do something more -like this: - - $RECSIZE = 220; # size of record, in bytes - $recno = 37; # which record to update - open(FH, "+<somewhere") || die "can't update somewhere: $!"; - seek(FH, $recno * $RECSIZE, 0); - read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; - # munge the record - seek(FH, -$RECSIZE, 1); - print FH $record; - close FH; - -Locking and error checking are left as an exercise for the reader. -Don't forget them or you'll be quite sorry. - -=head2 How do I get a file's timestamp in perl? - -If you want to retrieve the time at which the file was last read, -written, or had its meta-data (owner, etc) changed, you use the B<-M>, -B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These -retrieve the age of the file (measured against the start-time of your -program) in days as a floating point number. To retrieve the "raw" -time in seconds since the epoch, you would call the stat function, -then use localtime(), gmtime(), or POSIX::strftime() to convert this -into human-readable form. - -Here's an example: - - $write_secs = (stat($file))[9]; - printf "file %s updated at %s\n", $file, - scalar localtime($write_secs); - -If you prefer something more legible, use the File::stat module -(part of the standard distribution in version 5.004 and later): - - # error checking left as an exercise for reader. - use File::stat; - use Time::localtime; - $date_string = ctime(stat($file)->mtime); - print "file $file updated at $date_string\n"; - -The POSIX::strftime() approach has the benefit of being, -in theory, independent of the current locale. See L<perllocale> -for details. - -=head2 How do I set a file's timestamp in perl? - -You use the utime() function documented in L<perlfunc/utime>. -By way of example, here's a little program that copies the -read and write times from its first argument to all the rest -of them. - - if (@ARGV < 2) { - die "usage: cptimes timestamp_file other_files ...\n"; - } - $timestamp = shift; - ($atime, $mtime) = (stat($timestamp))[8,9]; - utime $atime, $mtime, @ARGV; - -Error checking is, as usual, left as an exercise for the reader. - -Note that utime() currently doesn't work correctly with Win95/NT -ports. A bug has been reported. Check it carefully before using -utime() on those platforms. - -=head2 How do I print to more than one file at once? - -If you only have to do this once, you can do this: - - for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } - -To connect up to one filehandle to several output filehandles, it's -easiest to use the tee(1) program if you have it, and let it take care -of the multiplexing: - - open (FH, "| tee file1 file2 file3"); - -Or even: - - # make STDOUT go to three files, plus original STDOUT - open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n"; - print "whatever\n" or die "Writing: $!\n"; - close(STDOUT) or die "Closing: $!\n"; - -Otherwise you'll have to write your own multiplexing print -function--or your own tee program--or use Tom Christiansen's, -at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz , which is -written in Perl and offers much greater functionality -than the stock version. - -=head2 How can I read in an entire file all at once? - -The customary Perl approach for processing all the lines in a file is to -do so one line at a time: - - open (INPUT, $file) || die "can't open $file: $!"; - while (<INPUT>) { - chomp; - # do something with $_ - } - close(INPUT) || die "can't close $file: $!"; - -This is tremendously more efficient than reading the entire file into -memory as an array of lines and then processing it one element at a time, -which is often--if not almost always--the wrong approach. Whenever -you see someone do this: - - @lines = <INPUT>; - -you should think long and hard about why you need everything loaded -at once. It's just not a scalable solution. You might also find it -more fun to use the standard DB_File module's $DB_RECNO bindings, -which allow you to tie an array to a file so that accessing an element -the array actually accesses the corresponding line in the file. - -On very rare occasion, you may have an algorithm that demands that -the entire file be in memory at once as one scalar. The simplest solution -to that is - - $var = `cat $file`; - -Being in scalar context, you get the whole thing. In list context, -you'd get a list of all the lines: - - @lines = `cat $file`; - -This tiny but expedient solution is neat, clean, and portable to -all systems on which decent tools have been installed. For those -who prefer not to use the toolbox, you can of course read the file -manually, although this makes for more complicated code. - - { - local(*INPUT, $/); - open (INPUT, $file) || die "can't open $file: $!"; - $var = <INPUT>; - } - -That temporarily undefs your record separator, and will automatically -close the file at block exit. If the file is already open, just use this: - - $var = do { local $/; <INPUT> }; - -=head2 How can I read in a file by paragraphs? - -Use the C<$/> variable (see L<perlvar> for details). You can either -set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, -for instance, gets treated as two paragraphs and not three), or -C<"\n\n"> to accept empty paragraphs. - -Note that a blank line must have no blanks in it. Thus C<"fred\n -\nstuff\n\n"> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. - -=head2 How can I read a single character from a file? From the keyboard? - -You can use the builtin C<getc()> function for most filehandles, but -it won't (easily) work on a terminal device. For STDIN, either use -the Term::ReadKey module from CPAN or use the sample code in -L<perlfunc/getc>. - -If your system supports the portable operating system programming -interface (POSIX), you can use the following code, which you'll note -turns off echo processing as well. - - #!/usr/bin/perl -w - use strict; - $| = 1; - for (1..4) { - my $got; - print "gimme: "; - $got = getone(); - print "--> $got\n"; - } - exit; - - BEGIN { - use POSIX qw(:termios_h); - - my ($term, $oterm, $echo, $noecho, $fd_stdin); - - $fd_stdin = fileno(STDIN); - - $term = POSIX::Termios->new(); - $term->getattr($fd_stdin); - $oterm = $term->getlflag(); - - $echo = ECHO | ECHOK | ICANON; - $noecho = $oterm & ~$echo; - - sub cbreak { - $term->setlflag($noecho); - $term->setcc(VTIME, 1); - $term->setattr($fd_stdin, TCSANOW); - } - - sub cooked { - $term->setlflag($oterm); - $term->setcc(VTIME, 0); - $term->setattr($fd_stdin, TCSANOW); - } - - sub getone { - my $key = ''; - cbreak(); - sysread(STDIN, $key, 1); - cooked(); - return $key; - } - - } - - END { cooked() } - -The Term::ReadKey module from CPAN may be easier to use. Recent versions -include also support for non-portable systems as well. - - use Term::ReadKey; - open(TTY, "</dev/tty"); - print "Gimme a char: "; - ReadMode "raw"; - $key = ReadKey 0, *TTY; - ReadMode "normal"; - printf "\nYou said %s, char number %03d\n", - $key, ord $key; - -For legacy DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following: - -To put the PC in "raw" mode, use ioctl with some magic numbers gleaned -from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes -across the net every so often): - - $old_ioctl = ioctl(STDIN,0,0); # Gets device info - $old_ioctl &= 0xff; - ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5 - -Then to read a single character: - - sysread(STDIN,$c,1); # Read a single character - -And to put the PC back to "cooked" mode: - - ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode. - -So now you have $c. If C<ord($c) == 0>, you have a two byte code, which -means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>, -and that value tells you what combination it was according to this -table: - - # PC 2-byte keycodes = ^@ + the following: - - # HEX KEYS - # --- ---- - # 0F SHF TAB - # 10-19 ALT QWERTYUIOP - # 1E-26 ALT ASDFGHJKL - # 2C-32 ALT ZXCVBNM - # 3B-44 F1-F10 - # 47-49 HOME,UP,PgUp - # 4B LEFT - # 4D RIGHT - # 4F-53 END,DOWN,PgDn,Ins,Del - # 54-5D SHF F1-F10 - # 5E-67 CTR F1-F10 - # 68-71 ALT F1-F10 - # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME - # 78-83 ALT 1234567890-= - # 84 CTR PgUp - -This is all trial and error I did a long time ago; I hope I'm reading the -file that worked... - -=head2 How can I tell whether there's a character waiting on a filehandle? - -The very first thing you should do is look into getting the Term::ReadKey -extension from CPAN. As we mentioned earlier, it now even has limited -support for non-portable (read: not open systems, closed, proprietary, -not POSIX, not Unix, etc) systems. - -You should also check out the Frequently Asked Questions list in -comp.unix.* for things like this: the answer is essentially the same. -It's very system dependent. Here's one solution that works on BSD -systems: - - sub key_ready { - my($rin, $nfd); - vec($rin, fileno(STDIN), 1) = 1; - return $nfd = select($rin,undef,undef,0); - } - -If you want to find out how many characters are waiting, there's -also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that -comes with Perl tries to convert C include files to Perl code, which -can be C<require>d. FIONREAD ends up defined as a function in the -I<sys/ioctl.ph> file: - - require 'sys/ioctl.ph'; - - $size = pack("L", 0); - ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; - $size = unpack("L", $size); - -If I<h2ph> wasn't installed or doesn't work for you, you can -I<grep> the include files by hand: - - % grep FIONREAD /usr/include/*/* - /usr/include/asm/ioctls.h:#define FIONREAD 0x541B - -Or write a small C program using the editor of champions: - - % cat > fionread.c - #include <sys/ioctl.h> - main() { - printf("%#08x\n", FIONREAD); - } - ^D - % cc -o fionread fionread.c - % ./fionread - 0x4004667f - -And then hard-code it, leaving porting as an exercise to your successor. - - $FIONREAD = 0x4004667f; # XXX: opsys dependent - - $size = pack("L", 0); - ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; - $size = unpack("L", $size); - -FIONREAD requires a filehandle connected to a stream, meaning that sockets, -pipes, and tty devices work, but I<not> files. - -=head2 How do I do a C<tail -f> in perl? - -First try - - seek(GWFILE, 0, 1); - -The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, -but it does clear the end-of-file condition on the handle, so that the -next <GWFILE> makes Perl try again to read something. - -If that doesn't work (it relies on features of your stdio implementation), -then you need something more like this: - - for (;;) { - for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { - # search for some stuff and put it into files - } - # sleep for a while - seek(GWFILE, $curpos, 0); # seek to where we had been - } - -If this still doesn't work, look into the POSIX module. POSIX defines -the clearerr() method, which can remove the end of file condition on a -filehandle. The method: read until end of file, clearerr(), read some -more. Lather, rinse, repeat. - -There's also a File::Tail module from CPAN. - -=head2 How do I dup() a filehandle in Perl? - -If you check L<perlfunc/open>, you'll see that several of the ways -to call open() should do the trick. For example: - - open(LOG, ">>/tmp/logfile"); - open(STDERR, ">&LOG"); - -Or even with a literal numeric descriptor: - - $fd = $ENV{MHCONTEXTFD}; - open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) - -Note that "<&STDIN" makes a copy, but "<&=STDIN" make -an alias. That means if you close an aliased handle, all -aliases become inaccessible. This is not true with -a copied one. - -Error checking, as always, has been left as an exercise for the reader. - -=head2 How do I close a file descriptor by number? - -This should rarely be necessary, as the Perl close() function is to be -used for things that Perl opened itself, even if it was a dup of a -numeric descriptor as with MHCONTEXT above. But if you really have -to, you may be able to do this: - - require 'sys/syscall.ph'; - $rc = syscall(&SYS_close, $fd + 0); # must force numeric - die "can't sysclose $fd: $!" unless $rc == -1; - -Or, just use the fdopen(3S) feature of open(): - - { - local *F; - open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; - close F; - } - -=head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work? - -Whoops! You just put a tab and a formfeed into that filename! -Remember that within double quoted strings ("like\this"), the -backslash is an escape character. The full list of these is in -L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't -have a file called "c:(tab)emp(formfeed)oo" or -"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. - -Either single-quote your strings, or (preferably) use forward slashes. -Since all DOS and Windows versions since something like MS-DOS 2.0 or so -have treated C</> and C<\> the same in a path, you might as well use the -one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, -awk, Tcl, Java, or Python, just to mention a few. POSIX paths -are more portable, too. - -=head2 Why doesn't glob("*.*") get all the files? - -Because even on non-Unix ports, Perl's glob function follows standard -Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) -files. This makes glob() portable even to legacy systems. Your -port may include proprietary globbing functions as well. Check its -documentation for details. - -=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? - -This is elaborately and painstakingly described in the "Far More Than -You Ever Wanted To Know" in -http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms . - -The executive summary: learn how your filesystem works. The -permissions on a file say what can happen to the data in that file. -The permissions on a directory say what can happen to the list of -files in that directory. If you delete a file, you're removing its -name from the directory (so the operation depends on the permissions -of the directory, not of the file). If you try to write to the file, -the permissions of the file govern whether you're allowed to. - -=head2 How do I select a random line from a file? - -Here's an algorithm from the Camel Book: - - srand; - rand($.) < 1 && ($line = $_) while <>; - -This has a significant advantage in space over reading the whole -file in. A simple proof by induction is available upon -request if you doubt the algorithm's correctness. - -=head2 Why do I get weird spaces when I print an array of lines? - -Saying - - print "@lines\n"; - -joins together the elements of C<@lines> with a space between them. -If C<@lines> were C<("little", "fluffy", "clouds")> then the above -statement would print - - little fluffy clouds - -but if each element of C<@lines> was a line of text, ending a newline -character C<("little\n", "fluffy\n", "clouds\n")> then it would print: - - little - fluffy - clouds - -If your array contains lines, just print them: - - print @lines; - -=head1 AUTHOR AND COPYRIGHT - -Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. -All rights reserved. - -When included as an integrated part of the Standard Distribution -of Perl or of its documentation (printed or otherwise), this works is -covered under Perl's Artistic License. For separate distributions of -all or part of this FAQ outside of that, see L<perlfaq>. - -Irrespective of its distribution, all code examples here are in the public -domain. You are permitted and encouraged to use this code and any -derivatives thereof in your own programs for fun or for profit as you -see fit. A simple comment in the code giving credit to the FAQ would -be courteous but is not required. |
