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perlfaq5 ()
  • >> perlfaq5 (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlfaq5 (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    NAME

         perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date:
         1999/05/23 16:08:30 $)
    
    
    

    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 `$|' variable to control autoflushing (see
         perlvar/$ 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's 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?
    
         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 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.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 `-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($^I, @ARGV) = ('.orig', glob("*.c"));
             while (<>) {
                 if ($. == 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.
    
         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 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 `$$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
         preface them with a star, as in func(*STDIN).  See the
         Passing Filehandles entry 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 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.
    
         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 `<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 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 Accessing Formatting Internals entry in 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 scalar 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.
    
             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 successful create or unlink the same
         file!  Therefore O_EXCL isn't so exclusive as you might
         wish.
    
         See also the new the perlopentut manpage if you have it (new
         for 5.6).
    
         Why do I sometimes get an "Argument list too long" when I
         use <*>?
    
         The `<>' 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 `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.
         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;
                 s#^([^./])#./$1#;
                 $_ .= "\0";
                 return $_;
             }
    
             $badpath = "<<sysopen(), though:
    
             use Fcntl;
             $badpath = "<<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.  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 semantics as a real rename(), though,
         which preserves metainformation like permissions,
         timestamps, inode info, etc.
    
         The newer version of File::Copy exports 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.  But even this is
             dubious at best.  See the flock entry of the perlfunc
             manpage, and the 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 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.  (But if you're
             not, you should as always feel perfectly free to write
             for your own system's idiosyncrasies (sometimes called
             "features").  Slavish adherence to portability concerns
             shouldn't get in the way of your getting your job done.)
    
             For more information on file locking, see also the File
             Locking entry in the perlopentut manpage if you have it
             (new for 5.6).
    
         Why 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 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 ':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: $!";
             # Perl as of 5.004 automatically flushes before unlocking
             flock(FH, LOCK_UN)                           or die "can't flock 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.  :-)
    
         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.
    
         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:\fIs0: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:\fIs0:strftime() approach has the benefit of
         being, in theory, independent of the current locale.  See
         the perllocale manpage for details.
    
         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, 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 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 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 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> };
    
    
         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.
    
         Note that a blank line must have no blanks in it.  Thus
         `"fred\n \nstuff\n\n"' is one paragraph, but
         `"fred\n\nstuff\n\n"' is two.
    
         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 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 version 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 $c.  If `ord($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 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
         h2ph tool that comes with Perl tries to convert C include
         files to Perl code, which can be `require'd.  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.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 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.
    
         There's also a File::Tail module from CPAN.
    
         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;
    
         Or just use the fdopen(3S) feature of open():
    
             {
                 local *F;
                 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
                 close F;
             }
    
    
         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 Quote and Quote-like Operators
         entry 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 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 `/' 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.  POSIX paths
         are more portable, too.
    
         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
         even to legacy systems.  Your port may include proprietary
         globbing functions as well.  Check its documentation for
         details.
    
         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.
    
         Why do I get weird spaces when I print an array of lines?
    
         Saying
    
             print "@lines\n";
    
         joins together the elements of `@lines' with a space between
         them.  If `@lines' were `("little", "fluffy", "clouds")'
         then the above statement would print:
    
             little fluffy clouds
    
         but if each element of `@lines' was a line of text, ending a
         newline character `("little\n", "fluffy\n", "clouds\n")'
         then it would print:
    
             little
              fluffy
              clouds
    
         If your array contains lines, just print them:
    
             print @lines;
    
    
    

    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 the perlfaq manpage.
    
         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.
    
    
    
    


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