perlfaq5

PERLFAQ5(1)            Perl Programmers Reference Guide            PERLFAQ5(1)



NAME
       perlfaq5 - Files and Formats ($Revision: 1.24 $, $Date: 1998/07/05
       15:07:20 $)

DESCRIPTION
       This section deals with I/O and the "f" issues: filehandles, flushing,
       formats, and footers.

       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 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 $⎪
       variable to control autoflushing (see the section on $⎪ in the perlvar
       manpage and the select entry in the perlfunc manpage):

           $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 $⎪ 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 the way things work in
       network programming: you really should specify the exact bit pattern on
       the network line terminator.  In practice, "\n\n" often works, but this
       is not portable.

       See the perlfaq9 manpage for other examples of fetching URLs over the
       web.

       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?

       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
       at the very end of the file.  Another is replacing a sequence of bytes
       with another sequence of the same length.  Another is using the
       $DB_RECNO array bindings as documented in the DB_File manpage.  Yet
       another is manipulating files with all lines the same length.)

       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.bak";

           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 -i
       command-line switch or the closely-related $^I variable (see the
       perlrun manpage for more details).  Note that -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($^I, @ARGV) = ('.bak', glob("*.c"));
           while (<>) {
               if ($. == 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.

       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 the perlop
       manpage.  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.

       How do I make a temporary file name?

       Use the new_tmpfile class method from the IO::File module to get a
       filehandle opened for reading and writing.  Use this 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: $!";

       Or you can use the tmpnam function from the POSIX module to get a
       filename that you then open yourself.  Use this if you do need to know
       the file's name.

           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 doing this 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 ();
                   }
               }
           }


       How can I manipulate fixed-record-length files?

       The most efficient way is using pack() and unpack().  This is faster
       than using substr() when take 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 $$var in a way that forbidden by 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.

       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 this 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 prefer them
       with a star, as in func(*STDIN).  See the section on Passing
       Filehandles in the perlfaq7 manpage 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 ];
           }

       Or here using the semi-object-oriented FileHandle, 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.

       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 those:

           $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 to use the 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 print, open, seek, or the functions or
       the <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";

       Of 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 into hashes or arrays, can be used with
       built-ins like print, printf, or the diamond operator.  These are
       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 print and 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 print and 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 readline to reads a record just as
       <> 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.

       How can I set up a footer format to be used with write()?

       There's no builtin way to do this, but the perlform manpage has a
       couple of techniques to make it possible for the intrepid hacker.

       How can I write() into a string?

       See the perlform manpage for an swrite() function.

       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 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 reverse $input;
           }


       How can I translate tildes (~) in a filename?

       Use the <> (glob()) operator, documented in the perlfunc manpage.  This
       requires that you have a shell installed that groks tildes, meaning csh
       or tcsh or (some versions of) ksh, and thus may have portability
       problems.  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;


       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
       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.  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
       successful create or unlink the same file!  Therefore O_EXCL isn't so
       exclusive as you might wish.

       Why do I sometimes get an ""Argument list too long"" when I use <*>?

       The <> operator performs a globbing operation (see above).  By default
       glob() forks csh(1) to do the actual glob expansion, but csh can't
       handle more than 127 items and so gives the error message 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 do the glob yourself with Dirhandles and
       patterns, or use a module like Glob::KGlob, one that doesn't use the
       shell to do globbing.

       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 leak and/or unpredictable behavior.  It's best
       therefore to use glob() only in list context.

       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 this.  It
       makes 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 $_  = shift;
               return m#^/#
                       ? "$_\0"
                       : "./$_\0";
           }

           $fn = safe_filename("<<<something really wicked   ");
           open(FH, "> $fn") or "couldn't open $fn: $!";

       You could also use the sysopen() function (see the sysopen entry in the
       perlfunc manpage).

       How can I reliably rename a file?

       Well, usually you just use Perl's rename() function.  But that may not
       work everywhere, in particular, renaming files across file systems.  If
       your operating system supports a 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 semantics as a
       real rename(), though, which preserves metainformation like
       permissions, timestamps, inode info, etc.

       The newer version of File::Copy export a move() function.

       How can I lock a file?

       Perl's builtin flock() function (see the perlfunc manpage 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():

       1   Produces a fatal error if none of the three system calls (or their
           close equivalent) exists.

       2   lockf(3) does not provide shared locking, and requires that the
           filehandle be open for writing (or appending, or read/writing).

       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.  See the flock entry of the perlfunc manpage, and
           the INSTALL file in the source distribution for information on
           building Perl to do this.

       What can't I just open(FH, "">file.lock")?

       A common bit of code 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 involving link() have been suggested, but
       these tend to involve busy-wait, which is also subdesirable.

       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.  Better to pick a random number.
       It's more realistic.

       Anyway, this is what you can do if you can't help yourself.

           use Fcntl;
           sysopen(FH, "numfile", O_RDWR⎪O_CREAT)       or die "can't open numfile: $!";
           flock(FH, 2)                                 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: $!";
           # DO NOT UNLOCK THIS UNTIL YOU CLOSE
           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.  :-)

       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, $recno * $RECSIZE, 0);
           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.

       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 -M, -A,
       or -C filetest operations as documented in the perlfunc manpage.  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):

           use File::stat;
           use Time::localtime;
           $date_string = ctime(stat($file)->mtime);
           print "file $file updated at $date_string\n";

       Error checking is left as an exercise for the reader.

       How do I set a file's timestamp in perl?

       You use the utime() function documented in the utime entry in the
       perlfunc manpage.  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 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 it on those
       platforms.

       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.

       How can I read in a file by paragraphs?

       Use the $\ variable (see the perlvar manpage for details).  You can
       either set it to "" to eliminate empty paragraphs ("abc\n\n\n\ndef",
       for instance, gets treated as two paragraphs and not three), or "\n\n"
       to accept empty paragraphs.

       How can I read a single character from a file?  From the keyboard?

       You can use the builtin 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 the getc
       entry in the perlfunc manpage.

       If your system supports 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:

           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 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 $c.  If ord($c) == 0, you have a two byte code, which
       means you hit a special key.  Read another byte with
       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.

       How can I tell if 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.  It now even has limited support for
       closed, proprietary (read: not open systems, 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 h2ph tool that comes with Perl tries to convert C include files to
       Perl code, which can be required.  FIONREAD ends up defined as a
       function in the 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 h2ph wasn't installed or doesn't work for you, you can 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
           % ./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 sockets,
       pipes, and tty devices work, but not files.

       How do I do a tail -f in perl?

       First try

           seek(GWFILE, 0, 1);

       The statement 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.

       How do I dup() a filehandle in Perl?

       If you check the open entry in the perlfunc manpage, 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.

       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;


       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 the section on
       Quote and Quote-like Operators in the perlop manpage.  Unsurprisingly,
       you don't have a file called "c:(tab)emp(formfeed)oo" or
       "c:(tab)emp(formfeed)oo.exe" on your 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 / and \ 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.

       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 glob("*") to get all (non-hidden)
       files.  This makes glob() portable.

       Why does Perl let me delete read-only files?  Why does -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.

       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
       its correctness.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997, 1998 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 Licence.  For separate distributions of
       all or part of this FAQ outside of that, see the perlfaq manpage.

       Irrespective of its distribution, all code examples here are 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.












































3rd Berkeley Distribution    perl 5.005, patch 02                  PERLFAQ5(1)