perlopentut


DESCRIPTION
       Perl has two simple, built-in ways to open files: the shell way for
       convenience, and the C way for precision.  The shell way also has 2-
       and 3-argument forms, which have different semantics for handling the
       filename.  The choice is yours.

Open a la shell
       Perl's "open" function was designed to mimic the way command-line
       redirection in the shell works.  Here are some basic examples from the
       shell:

           $ myprogram file1 file2 file3
           $ myprogram    <  inputfile
           $ myprogram    >  outputfile
           $ myprogram    >> outputfile
           $ myprogram    |  otherprogram
           $ otherprogram |  myprogram

       And here are some more advanced examples:

           $ otherprogram      | myprogram f1 - f2
           $ otherprogram 2>&1 | myprogram -
           $ myprogram     <&3
           $ myprogram     >&4

       Programmers accustomed to constructs like those above can take comfort
       in learning that Perl directly supports these familiar constructs using
       virtually the same syntax as the shell.

   Simple Opens
       The "open" function takes two arguments: the first is a filehandle, and
       the second is a single string comprising both what to open and how to
       open it.  "open" returns true when it works, and when it fails, returns
       a false value and sets the special variable $! to reflect the system
       error.  If the filehandle was previously opened, it will be implicitly
       closed first.

       For example:

           open(INFO,      "datafile") || die("can't open datafile: $!");
           open(INFO,   "<  datafile") || die("can't open datafile: $!");
           open(RESULTS,">  runstats") || die("can't open runstats: $!");
           open(LOG,    ">> logfile ") || die("can't open logfile:  $!");

       If you prefer the low-punctuation version, you could write that this
       way:

           open INFO,   "<  datafile"  or die "can't open datafile: $!";
           open RESULTS,">  runstats"  or die "can't open runstats: $!";
           open LOG,    ">> logfile "  or die "can't open logfile:  $!";

       A few things to notice.  First, the leading "<" is optional.  If
       omitted, Perl assumes that you want to open the file for reading.

           open INFO,   "<datafile"
           open INFO,   "< datafile"
           open INFO,   "<  datafile"

       Ignoring surrounding whitespace also helps for when you read a filename
       in from a different file, and forget to trim it before opening:

           $filename = <INFO>;         # oops, \n still there
           open(EXTRA, "< $filename") || die "can't open $filename: $!";

       This is not a bug, but a feature.  Because "open" mimics the shell in
       its style of using redirection arrows to specify how to open the file,
       it also does so with respect to extra whitespace around the filename
       itself as well.  For accessing files with naughty names, see
       "Dispelling the Dweomer".

       There is also a 3-argument version of "open", which lets you put the
       special redirection characters into their own argument:

           open( INFO, ">", $datafile ) || die "Can't create $datafile: $!";

       In this case, the filename to open is the actual string in $datafile,
       so you don't have to worry about $datafile containing characters that
       might influence the open mode, or whitespace at the beginning of the
       filename that would be absorbed in the 2-argument version.  Also, any
       reduction of unnecessary string interpolation is a good thing.

   Indirect Filehandles
       "open"'s first argument can be a reference to a filehandle.  As of perl
       5.6.0, if the argument is uninitialized, Perl will automatically create
       a filehandle and put a reference to it in the first argument, like so:

           open( my $in, $infile )   or die "Couldn't read $infile: $!";
           while ( <$in> ) {
               # do something with $_
           }
           close $in;

       Indirect filehandles make namespace management easier.  Since
       filehandles are global to the current package, two subroutines trying
       to open "INFILE" will clash.  With two functions opening indirect
       filehandles like "my $infile", there's no clash and no need to worry
       about future conflicts.

       Another convenient behavior is that an indirect filehandle
       automatically closes when there are no more references to it:

           sub firstline {
               open( my $in, shift ) && return scalar <$in>;
               # no close() required
           }

       Indirect filehandles also make it easy to pass filehandles to and
                   or croak "Could not open '$filename': $!";
               return $h;
           }

   Pipe Opens
       In C, when you want to open a file using the standard I/O library, you
       use the "fopen" function, but when opening a pipe, you use the "popen"
       function.  But in the shell, you just use a different redirection
       character.  That's also the case for Perl.  The "open" call remains the
       same--just its argument differs.

       If the leading character is a pipe symbol, "open" starts up a new
       command and opens a write-only filehandle leading into that command.
       This lets you write into that handle and have what you write show up on
       that command's standard input.  For example:

           open(PRINTER, "| lpr -Plp1")    || die "can't run lpr: $!";
           print PRINTER "stuff\n";
           close(PRINTER)                  || die "can't close lpr: $!";

       If the trailing character is a pipe, you start up a new command and
       open a read-only filehandle leading out of that command.  This lets
       whatever that command writes to its standard output show up on your
       handle for reading.  For example:

           open(NET, "netstat -i -n |")    || die "can't fork netstat: $!";
           while (<NET>) { }               # do something with input
           close(NET)                      || die "can't close netstat: $!";

       What happens if you try to open a pipe to or from a non-existent
       command?  If possible, Perl will detect the failure and set $! as
       usual.  But if the command contains special shell characters, such as
       ">" or "*", called 'metacharacters', Perl does not execute the command
       directly.  Instead, Perl runs the shell, which then tries to run the
       command.  This means that it's the shell that gets the error
       indication.  In such a case, the "open" call will only indicate failure
       if Perl can't even run the shell.  See "How can I capture STDERR from
       an external command?" in perlfaq8 to see how to cope with this.
       There's also an explanation in perlipc.

       If you would like to open a bidirectional pipe, the IPC::Open2 library
       will handle this for you.  Check out "Bidirectional Communication with
       Another Process" in perlipc

       perl-5.6.x introduced a version of piped open that executes a process
       based on its command line arguments without relying on the shell.
       (Similar to the "system(@LIST)" notation.) This is safer and faster
       than executing a single argument pipe-command, but does not allow
       special shell constructs. (It is also not supported on Microsoft
       Windows, Mac OS Classic or RISC OS.)

       Here's an example of "open '-|'", which prints a random Unix fortune
       cookie as uppercase:

               or die "can't run lpr: $!";
           print {$printer} "stuff\n";
           close($printer)
               or die "can't close lpr: $!";

   The Minus File
       Again following the lead of the standard shell utilities, Perl's "open"
       function treats a file whose name is a single minus, "-", in a special
       way.  If you open minus for reading, it really means to access the
       standard input.  If you open minus for writing, it really means to
       access the standard output.

       If minus can be used as the default input or default output, what
       happens if you open a pipe into or out of minus?  What's the default
       command it would run?  The same script as you're currently running!
       This is actually a stealth "fork" hidden inside an "open" call.  See
       "Safe Pipe Opens" in perlipc for details.

   Mixing Reads and Writes
       It is possible to specify both read and write access.  All you do is
       add a "+" symbol in front of the redirection.  But as in the shell,
       using a less-than on a file never creates a new file; it only opens an
       existing one.  On the other hand, using a greater-than always clobbers
       (truncates to zero length) an existing file, or creates a brand-new one
       if there isn't an old one.  Adding a "+" for read-write doesn't affect
       whether it only works on existing files or always clobbers existing
       ones.

           open(WTMP, "+< /usr/adm/wtmp")
               || die "can't open /usr/adm/wtmp: $!";

           open(SCREEN, "+> lkscreen")
               || die "can't open lkscreen: $!";

           open(LOGFILE, "+>> /var/log/applog")
               || die "can't open /var/log/applog: $!";

       The first one won't create a new file, and the second one will always
       clobber an old one.  The third one will create a new file if necessary
       and not clobber an old one, and it will allow you to read at any point
       in the file, but all writes will always go to the end.  In short, the
       first case is substantially more common than the second and third
       cases, which are almost always wrong.  (If you know C, the plus in
       Perl's "open" is historically derived from the one in C's fopen(3S),
       which it ultimately calls.)

       In fact, when it comes to updating a file, unless you're working on a
       binary file as in the WTMP case above, you probably don't want to use
       this approach for updating.  Instead, Perl's -i flag comes to the
       rescue.  The following command takes all the C, C++, or yacc source or
       header files and changes all their foo's to bar's, leaving the old
       version in the original filename with a ".orig" tacked on the end:

           $ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]

       can have all its files opened and processed one at a time using a
       construct no more complex than:

           while (<>) {
               # do something with $_
           }

       If @ARGV is empty when the loop first begins, Perl pretends you've
       opened up minus, that is, the standard input.  In fact, $ARGV, the
       currently open file during "<ARGV>" processing, is even set to "-" in
       these circumstances.

       You are welcome to pre-process your @ARGV before starting the loop to
       make sure it's to your liking.  One reason to do this might be to
       remove command options beginning with a minus.  While you can always
       roll the simple ones by hand, the Getopts modules are good for this:

           use Getopt::Std;

           # -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
           getopts("vDo:");

           # -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
           getopts("vDo:", \%args);

       Or the standard Getopt::Long module to permit named arguments:

           use Getopt::Long;
           GetOptions( "verbose"  => \$verbose,        # --verbose
                       "Debug"    => \$debug,          # --Debug
                       "output=s" => \$output );
                   # --output=somestring or --output somestring

       Another reason for preprocessing arguments is to make an empty argument
       list default to all files:

           @ARGV = glob("*") unless @ARGV;

       You could even filter out all but plain, text files.  This is a bit
       silent, of course, and you might prefer to mention them on the way.

           @ARGV = grep { -f && -T } @ARGV;

       If you're using the -n or -p command-line options, you should put
       changes to @ARGV in a "BEGIN{}" block.

       Remember that a normal "open" has special properties, in that it might
       call fopen(3S) or it might called popen(3S), depending on what its
       argument looks like; that's why it's sometimes called "magic open".
       Here's an example:

           $pwdinfo = `domainname` =~ /^(\(none\))?$/
                           ? '< /etc/passwd'

       input (tmpfile in this case), the f2 file, the cmd2 command, and
       finally the f3 file.

       Yes, this also means that if you have files named "-" (and so on) in
       your directory, they won't be processed as literal files by "open".
       You'll need to pass them as "./-", much as you would for the rm
       program, or you could use "sysopen" as described below.

       One of the more interesting applications is to change files of a
       certain name into pipes.  For example, to autoprocess gzipped or
       compressed files by decompressing them with gzip:

           @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc $_ |" : $_  } @ARGV;

       Or, if you have the GET program installed from LWP, you can fetch URLs
       before processing them:

           @ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;

       It's not for nothing that this is called magic "<ARGV>".  Pretty nifty,
       eh?

Open a la C
       If you want the convenience of the shell, then Perl's "open" is
       definitely the way to go.  On the other hand, if you want finer
       precision than C's simplistic fopen(3S) provides you should look to
       Perl's "sysopen", which is a direct hook into the open(2) system call.
       That does mean it's a bit more involved, but that's the price of
       precision.

       "sysopen" takes 3 (or 4) arguments.

           sysopen HANDLE, PATH, FLAGS, [MASK]

       The HANDLE argument is a filehandle just as with "open".  The PATH is a
       literal path, one that doesn't pay attention to any greater-thans or
       less-thans or pipes or minuses, nor ignore whitespace.  If it's there,
       it's part of the path.  The FLAGS argument contains one or more values
       derived from the Fcntl module that have been or'd together using the
       bitwise "|" operator.  The final argument, the MASK, is optional; if
       present, it is combined with the user's current umask for the creation
       mode of the file.  You should usually omit this.

       Although the traditional values of read-only, write-only, and read-
       write are 0, 1, and 2 respectively, this is known not to hold true on
       some systems.  Instead, it's best to load in the appropriate constants
       first from the Fcntl module, which supplies the following standard
       flags:

           O_RDONLY            Read only
           O_WRONLY            Write only
           O_RDWR              Read and write
           O_CREAT             Create the file if it doesn't exist
           O_EXCL              Fail if the file already exists

       Here's how to use "sysopen" to emulate the simple "open" calls we had
       before.  We'll omit the "|| die $!" checks for clarity, but make sure
       you always check the return values in real code.  These aren't quite
       the same, since "open" will trim leading and trailing whitespace, but
       you'll get the idea.

       To open a file for reading:

           open(FH, "< $path");
           sysopen(FH, $path, O_RDONLY);

       To open a file for writing, creating a new file if needed or else
       truncating an old file:

           open(FH, "> $path");
           sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);

       To open a file for appending, creating one if necessary:

           open(FH, ">> $path");
           sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);

       To open a file for update, where the file must already exist:

           open(FH, "+< $path");
           sysopen(FH, $path, O_RDWR);

       And here are things you can do with "sysopen" that you cannot do with a
       regular "open".  As you'll see, it's just a matter of controlling the
       flags in the third argument.

       To open a file for writing, creating a new file which must not
       previously exist:

           sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);

       To open a file for appending, where that file must already exist:

           sysopen(FH, $path, O_WRONLY | O_APPEND);

       To open a file for update, creating a new file if necessary:

           sysopen(FH, $path, O_RDWR | O_CREAT);

       To open a file for update, where that file must not already exist:

           sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);

       To open a file without blocking, creating one if necessary:

           sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);

   Permissions a la mode
       If you omit the MASK argument to "sysopen", Perl uses the octal value
       is 0640.

       You should seldom use the MASK argument to "sysopen()".  That takes
       away the user's freedom to choose what permission new files will have.
       Denying choice is almost always a bad thing.  One exception would be
       for cases where sensitive or private data is being stored, such as with
       mail folders, cookie files, and internal temporary files.

Obscure Open Tricks
   Re-Opening Files (dups)
       Sometimes you already have a filehandle open, and want to make another
       handle that's a duplicate of the first one.  In the shell, we place an
       ampersand in front of a file descriptor number when doing redirections.
       For example, "2>&1" makes descriptor 2 (that's STDERR in Perl) be
       redirected into descriptor 1 (which is usually Perl's STDOUT).  The
       same is essentially true in Perl: a filename that begins with an
       ampersand is treated instead as a file descriptor if a number, or as a
       filehandle if a string.

           open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
           open(MHCONTEXT, "<&4")     || die "couldn't dup fd4: $!";

       That means that if a function is expecting a filename, but you don't
       want to give it a filename because you already have the file open, you
       can just pass the filehandle with a leading ampersand.  It's best to
       use a fully qualified handle though, just in case the function happens
       to be in a different package:

           somefunction("&main::LOGFILE");

       This way if somefunction() is planning on opening its argument, it can
       just use the already opened handle.  This differs from passing a
       handle, because with a handle, you don't open the file.  Here you have
       something you can pass to open.

       If you have one of those tricky, newfangled I/O objects that the C++
       folks are raving about, then this doesn't work because those aren't a
       proper filehandle in the native Perl sense.  You'll have to use
       fileno() to pull out the proper descriptor number, assuming you can:

           use IO::Socket;
           $handle = IO::Socket::INET->new("www.perl.com:80");
           $fd = $handle->fileno;
           somefunction("&$fd");  # not an indirect function call

       It can be easier (and certainly will be faster) just to use real
       filehandles though:

           use IO::Socket;
           local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
           die "can't connect" unless defined(fileno(REMOTE));
           somefunction("&main::REMOTE");

       If the filehandle or descriptor number is preceded not just with a
       never seen anyone actually do this.

   Dispelling the Dweomer
       Perl is more of a DWIMmer language than something like Java--where DWIM
       is an acronym for "do what I mean".  But this principle sometimes leads
       to more hidden magic than one knows what to do with.  In this way, Perl
       is also filled with dweomer, an obscure word meaning an enchantment.
       Sometimes, Perl's DWIMmer is just too much like dweomer for comfort.

       If magic "open" is a bit too magical for you, you don't have to turn to
       "sysopen".  To open a file with arbitrary weird characters in it, it's
       necessary to protect any leading and trailing whitespace.  Leading
       whitespace is protected by inserting a "./" in front of a filename that
       starts with whitespace.  Trailing whitespace is protected by appending
       an ASCII NUL byte ("\0") at the end of the string.

           $file =~ s#^(\s)#./$1#;
           open(FH, "< $file\0")   || die "can't open $file: $!";

       This assumes, of course, that your system considers dot the current
       working directory, slash the directory separator, and disallows ASCII
       NULs within a valid filename.  Most systems follow these conventions,
       including all POSIX systems as well as proprietary Microsoft systems.
       The only vaguely popular system that doesn't work this way is the
       "Classic" Macintosh system, which uses a colon where the rest of us use
       a slash.  Maybe "sysopen" isn't such a bad idea after all.

       If you want to use "<ARGV>" processing in a totally boring and non-
       magical way, you could do this first:

           #   "Sam sat on the ground and put his head in his hands.
           #   'I wish I had never come here, and I don't want to see
           #   no more magic,' he said, and fell silent."
           for (@ARGV) {
               s#^([^./])#./$1#;
               $_ .= "\0";
           }
           while (<>) {
               # now process $_
           }

       But be warned that users will not appreciate being unable to use "-" to
       mean standard input, per the standard convention.

   Paths as Opens
       You've probably noticed how Perl's "warn" and "die" functions can
       produce messages like:

           Some warning at scriptname line 29, <FH> line 7.

       That's because you opened a filehandle FH, and had read in seven
       records from it.  But what was the name of the file, rather than the
       handle?


   Single Argument Open
       Remember how we said that Perl's open took two arguments?  That was a
       passive prevarication.  You see, it can also take just one argument.
       If and only if the variable is a global variable, not a lexical, you
       can pass "open" just one argument, the filehandle, and it will get the
       path from the global scalar variable of the same name.

           $FILE = "/etc/motd";
           open FILE or die "can't open $FILE: $!";
           while (<FILE>) {
               # whatever
           }

       Why is this here?  Someone has to cater to the hysterical porpoises.
       It's something that's been in Perl since the very beginning, if not
       before.

   Playing with STDIN and STDOUT
       One clever move with STDOUT is to explicitly close it when you're done
       with the program.

           END { close(STDOUT) || die "can't close stdout: $!" }

       If you don't do this, and your program fills up the disk partition due
       to a command line redirection, it won't report the error exit with a
       failure status.

       You don't have to accept the STDIN and STDOUT you were given.  You are
       welcome to reopen them if you'd like.

           open(STDIN, "< datafile")
               || die "can't open datafile: $!";

           open(STDOUT, "> output")
               || die "can't open output: $!";

       And then these can be accessed directly or passed on to subprocesses.
       This makes it look as though the program were initially invoked with
       those redirections from the command line.

       It's probably more interesting to connect these to pipes.  For example:

           $pager = $ENV{PAGER} || "(less || more)";
           open(STDOUT, "| $pager")
               || die "can't fork a pager: $!";

       This makes it appear as though your program were called with its stdout
       already piped into your pager.  You can also use this kind of thing in
       conjunction with an implicit fork to yourself.  You might do this if
       you would rather handle the post processing in your own program, just
       in a different process:

           head(100);
               exit;
           }

       This technique can be applied to repeatedly push as many filters on
       your output stream as you wish.

Other I/O Issues
       These topics aren't really arguments related to "open" or "sysopen",
       but they do affect what you do with your open files.

   Opening Non-File Files
       When is a file not a file?  Well, you could say when it exists but
       isn't a plain file.   We'll check whether it's a symbolic link first,
       just in case.

           if (-l $file || ! -f _) {
               print "$file is not a plain file\n";
           }

       What other kinds of files are there than, well, files?  Directories,
       symbolic links, named pipes, Unix-domain sockets, and block and
       character devices.  Those are all files, too--just not plain files.
       This isn't the same issue as being a text file. Not all text files are
       plain files.  Not all plain files are text files.  That's why there are
       separate "-f" and "-T" file tests.

       To open a directory, you should use the "opendir" function, then
       process it with "readdir", carefully restoring the directory name if
       necessary:

           opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
           while (defined($file = readdir(DIR))) {
               # do something with "$dirname/$file"
           }
           closedir(DIR);

       If you want to process directories recursively, it's better to use the
       File::Find module.  For example, this prints out all files recursively
       and adds a slash to their names if the file is a directory.

           @ARGV = qw(.) unless @ARGV;
           use File::Find;
           find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;

       This finds all bogus symbolic links beneath a particular directory:

           find sub { print "$File::Find::name\n" if -l && !-e }, $dir;

       As you see, with symbolic links, you can just pretend that it is what
       it points to.  Or, if you want to know what it points to, then
       "readlink" is called for:

           if (-l $file) {
               if (defined($whither = readlink($file))) {

       When it comes to opening devices, it can be easy and it can be tricky.
       We'll assume that if you're opening up a block device, you know what
       you're doing.  The character devices are more interesting.  These are
       typically used for modems, mice, and some kinds of printers.  This is
       described in "How do I read and write the serial port?" in perlfaq8
       It's often enough to open them carefully:

           sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
                       # (O_NOCTTY no longer needed on POSIX systems)
               or die "can't open /dev/ttyS1: $!";
           open(TTYOUT, "+>&TTYIN")
               or die "can't dup TTYIN: $!";

           $ofh = select(TTYOUT); $| = 1; select($ofh);

           print TTYOUT "+++at\015";
           $answer = <TTYIN>;

       With descriptors that you haven't opened using "sysopen", such as
       sockets, you can set them to be non-blocking using "fcntl":

           use Fcntl;
           my $old_flags = fcntl($handle, F_GETFL, 0)
               or die "can't get flags: $!";
           fcntl($handle, F_SETFL, $old_flags | O_NONBLOCK)
               or die "can't set non blocking: $!";

       Rather than losing yourself in a morass of twisting, turning "ioctl"s,
       all dissimilar, if you're going to manipulate ttys, it's best to make
       calls out to the stty(1) program if you have it, or else use the
       portable POSIX interface.  To figure this all out, you'll need to read
       the termios(3) manpage, which describes the POSIX interface to tty
       devices, and then POSIX, which describes Perl's interface to POSIX.
       There are also some high-level modules on CPAN that can help you with
       these games.  Check out Term::ReadKey and Term::ReadLine.

   Opening Sockets
       What else can you open?  To open a connection using sockets, you won't
       use one of Perl's two open functions.  See "Sockets: Client/Server
       Communication" in perlipc for that.  Here's an example.  Once you have
       it, you can use FH as a bidirectional filehandle.

           use IO::Socket;
           local *FH = IO::Socket::INET->new("www.perl.com:80");

       For opening up a URL, the LWP modules from CPAN are just what the
       doctor ordered.  There's no filehandle interface, but it's still easy
       to get the contents of a document:

           use LWP::Simple;
           $doc = get('http://www.cpan.org/');

   Binary Files

           binmode(STDIN);
           binmode(STDOUT);
           while (<STDIN>) { print }

       Passing "sysopen" a non-standard flag option will also open the file in
       binary mode on those systems that support it.  This is the equivalent
       of opening the file normally, then calling "binmode" on the handle.

           sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
               || die "can't open records.data: $!";

       Now you can use "read" and "print" on that handle without worrying
       about the non-standard system I/O library breaking your data.  It's not
       a pretty picture, but then, legacy systems seldom are.  CP/M will be
       with us until the end of days, and after.

       On systems with exotic I/O systems, it turns out that, astonishingly
       enough, even unbuffered I/O using "sysread" and "syswrite" might do
       sneaky data mutilation behind your back.

           while (sysread(WHENCE, $buf, 1024)) {
               syswrite(WHITHER, $buf, length($buf));
           }

       Depending on the vicissitudes of your runtime system, even these calls
       may need "binmode" or "O_BINARY" first.  Systems known to be free of
       such difficulties include Unix, the Mac OS, Plan 9, and Inferno.

   File Locking
       In a multitasking environment, you may need to be careful not to
       collide with other processes who want to do I/O on the same files as
       you are working on.  You'll often need shared or exclusive locks on
       files for reading and writing respectively.  You might just pretend
       that only exclusive locks exist.

       Never use the existence of a file "-e $file" as a locking indication,
       because there is a race condition between the test for the existence of
       the file and its creation.  It's possible for another process to create
       a file in the slice of time between your existence check and your
       attempt to create the file.  Atomicity is critical.

       Perl's most portable locking interface is via the "flock" function,
       whose simplicity is emulated on systems that don't directly support it
       such as SysV or Windows.  The underlying semantics may affect how it
       all works, so you should learn how "flock" is implemented on your
       system's port of Perl.

       File locking does not lock out another process that would like to do
       I/O.  A file lock only locks out others trying to get a lock, not
       processes trying to do I/O.  Because locks are advisory, if one process
       uses locking and another doesn't, all bets are off.

       By default, the "flock" call will block until a lock is granted.  A
           flock(FH, LOCK_SH)      or die "can't lock filename: $!";
           # now read from FH

       You can get a non-blocking lock by using "LOCK_NB".

           flock(FH, LOCK_SH | LOCK_NB)
               or die "can't lock filename: $!";

       This can be useful for producing more user-friendly behaviour by
       warning if you're going to be blocking:

           use 5.004;
           use Fcntl qw(:DEFAULT :flock);
           open(FH, "< filename")  or die "can't open filename: $!";
           unless (flock(FH, LOCK_SH | LOCK_NB)) {
               $| = 1;
               print "Waiting for lock...";
               flock(FH, LOCK_SH)  or die "can't lock filename: $!";
               print "got it.\n"
           }
           # now read from FH

       To get an exclusive lock, typically used for writing, you have to be
       careful.  We "sysopen" the file so it can be locked before it gets
       emptied.  You can get a nonblocking version using "LOCK_EX | LOCK_NB".

           use 5.004;
           use Fcntl qw(:DEFAULT :flock);
           sysopen(FH, "filename", O_WRONLY | O_CREAT)
               or die "can't open filename: $!";
           flock(FH, LOCK_EX)
               or die "can't lock filename: $!";
           truncate(FH, 0)
               or die "can't truncate filename: $!";
           # now write to FH

       Finally, due to the uncounted millions who cannot be dissuaded from
       wasting cycles on useless vanity devices called hit counters, here's
       how to increment a number in a file safely:

           use Fcntl qw(:DEFAULT :flock);

           sysopen(FH, "numfile", O_RDWR | O_CREAT)
               or die "can't open numfile: $!";
           # autoflush FH
           $ofh = select(FH); $| = 1; select ($ofh);
           flock(FH, LOCK_EX)
               or die "can't write-lock numfile: $!";

           $num = <FH> || 0;
           seek(FH, 0, 0)
               or die "can't rewind numfile : $!";
           print FH $num+1, "\n"
               or die "can't write numfile: $!";

       transformations on the data.  Such transformations may include
       compression and decompression, encryption and decryption, and
       transforming between various character encodings.

       Full discussion about the features of PerlIO is out of scope for this
       tutorial, but here is how to recognize the layers being used:

       o   The three-(or more)-argument form of "open" is being used and the
           second argument contains something else in addition to the usual
           '<', '>', '>>', '|' and their variants, for example:

               open(my $fh, "<:crlf", $fn);

       o   The two-argument form of "binmode" is being used, for example

               binmode($fh, ":encoding(utf16)");

       For more detailed discussion about PerlIO see PerlIO; for more detailed
       discussion about Unicode and I/O see perluniintro.

SEE ALSO
       The "open" and "sysopen" functions in perlfunc(1); the system open(2),
       dup(2), fopen(3), and fdopen(3) manpages; the POSIX documentation.

AUTHOR and COPYRIGHT
       Copyright 1998 Tom Christiansen.

       This documentation is free; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in these files are
       hereby placed into the public domain.  You are permitted and encouraged
       to use this code in your own programs for fun or for profit as you see
       fit.  A simple comment in the code giving credit would be courteous but
       is not required.

HISTORY
       First release: Sat Jan  9 08:09:11 MST 1999



perl v5.14.2                      2011-09-19                    PERLOPENTUT(1)
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