PERLOPENTUT(1)   Perl Programmers Reference Guide  PERLOPENTUT(1)

NNAAMMEE
       perlopentut - tutorial on opening things in Perl

DDEESSCCRRIIPPTTIIOONN
       Perl has two simple, built-in ways to open files: the
       shell way for convenience, and the C way for precision.
       The choice is yours.

OOppeenn AA llaa sshheellll
       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.

       SSiimmppllee OOppeennss

       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 less-than is
       optional.  If omitted, Perl assumes that you want to open
       the file for reading.

       The other important thing to notice is that, just as in
       the shell, any white space before or after the filename is
       ignored.  This is good, because you wouldn't want these to
       do different things:

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

       Ignoring surround 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 white space around the filename itself as well.  For
       accessing files with naughty names, see the section on
       /"Dispelling the Dweomer.

       PPiippee OOppeennss

       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, C<open) starts
       up a new command and open 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 "cannot fork: $!";
           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 "cannot fork: $!";
           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?  In most systems, such an open will not
       return an error. That's because in the traditional
       fork/exec model, running the other program happens only in
       the forked child process, which means that the failed exec
       can't be reflected in the return value of open.  Only a
       failed fork shows up there.  See the section on Why
       doesn't open() return an error when a pipe open fails? in
       the perlfaq8 manpage to see how to cope with this.
       There's also an explanation in the perlipc manpage.

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

       TThhee MMiinnuuss FFiillee

       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 current running!  This is actually a
       stealth fork hidden inside an open call.  See the section
       on Safe Pipe Opens in the perlipc manpage for details.

       MMiixxiinngg RReeaaddss aanndd WWrriitteess

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

           open(LOGFILE, "+>> /tmp/applog"
               || die "can't open /tmp/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 --ii 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 file name with a ".orig"
       tacked on the end:

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

       This is a short cut for some renaming games that are
       really the best way to update textfiles.  See the second
       question in the perlfaq5 manpage for more details.

       FFiilltteerrss

       One of the most common uses for open is one you never even
       notice.  When you process the ARGV filehandle using
       <ARGV>, Perl actually does an implicit open on each file
       in @ARGV.  Thus a program called like this:

           $ myprogram file1 file2 file3

       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 --nn or --pp 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'
                           : 'ypcat passwd |';

           open(PWD, $pwdinfo)
                       or die "can't open $pwdinfo: $!";

       This sort of thing also comes into play in filter
       processing.  Because <ARGV> processing employs the normal,
       shell-style Perl open, it respects all the special things
       we've already seen:

           $ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile

       That program will read from the file f1, the process cmd1,
       standard input (tmpfile in this case), the f2 file, the
       cmd2 command, and finally the f3 file.

       Yes, this also means that if you have a file named "-"
       (and so on) in your directory, that 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?

OOppeenn AA llaa CC
       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, then 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 white space.  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
           O_APPEND            Append to the file
           O_TRUNC             Truncate the file
           O_NONBLOCK          Non-blocking access

       Less common flags that are sometimes available on some
       operating systems include O_BINARY, O_TEXT, O_SHLOCK,
       O_EXLOCK, O_DEFER, O_SYNC, O_ASYNC, O_DSYNC, O_RSYNC,
       O_NOCTTY, O_NDELAY and O_LARGEFILE.  Consult your open(2)
       manpage or its local equivalent for details.

       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 white space, 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 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);

       PPeerrmmiissssiioonnss AA llaa mmooddee

       If you omit the MASK argument to sysopen, Perl uses the
       octal value 0666.  The normal MASK to use for executables
       and directories should be 0777, and for anything else,
       0666.

       Why so permissive?  Well, it isn't really.  The MASK will
       be modified by your process's current umask.  A umask is a
       number representing disabled permissions bits; that is,
       bits that will not be turned on in the created files'
       permissions field.

       For example, if your umask were 027, then the 020 part
       would disable the group from writing, and the 007 part
       would disable others from reading, writing, or executing.
       Under these conditions, passing sysopen 0666 would create
       a file with mode 0640, since 0666 &~ 027 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.

OObbssccuurree OOppeenn TTrriicckkss
       RRee--OOppeenniinngg FFiilleess ((dduuppss))

       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 simple "&" but rather with a "&=" combination,
       then Perl will not create a completely new descriptor
       opened to the same place using the dup(2) system call.
       Instead, it will just make something of an alias to the
       existing one using the fdopen(3S) library call  This is
       slightly more parsimonious of systems resources, although
       this is less a concern these days.  Here's an example of
       that:

           $fd = $ENV{"MHCONTEXTFD"};
           open(MHCONTEXT, "<&=$fd")   or die "couldn't fdopen $fd: $!";

       If you're using magic <ARGV>, you could even pass in as a
       command line argument in @ARGV something like
       "<&=$MHCONTEXTFD", but we've never seen anyone actually do
       this.

       DDiissppeelllliinngg tthhee DDwweeoommeerr

       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 off 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 proprietary 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.

       PPaatthhss aass OOppeennss

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

           Some warning at scriptname line 29, <FH> chunk 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,
       not the handle?

       If you aren't running with strict refs, or if you've turn
       them off temporarily, then all you have to do is this:

           open($path, "< $path") || die "can't open $path: $!";
           while (<$path>) {
               # whatever
           }

       Since you're using the pathname of the file as its handle,
       you'll get warnings more like

           Some warning at scriptname line 29, </etc/motd> chunk 7.

       SSiinnggllee AArrgguummeenntt OOppeenn

       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.

       PPllaayyiinngg wwiitthh SSTTDDIINN aanndd SSTTDDOOUUTT

       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 read 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);
           while (<>) {
               print;
           }

           sub head {
               my $lines = shift || 20;
               return unless $pid = open(STDOUT, "|-");
               die "cannot fork: $!" unless defined $pid;
               while (<STDIN>) {
                   print;
                   last if --$lines < 0;
               }
               exit;
           }

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

OOtthheerr II//OO IIssssuueess
       These topics aren't really arguments related to open or
       sysopen, but they do affect what you do with your open
       files.

       OOppeenniinngg NNoonn--FFiillee FFiilleess

       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 textfiles.  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, add 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($file))) {
                   print "$file points to $whither\n";
               } else {
                   print "$file points nowhere: $!\n";
               }
           }

       Named pipes are a different matter.  You pretend they're
       regular files, but their opens will normally block until
       there is both a reader and a writer.  You can read more
       about them in the section on Named Pipes in the perlipc
       manpage.  Unix-domain sockets are rather different beasts
       as well; they're described in the section on Unix-Domain
       TCP Clients and Servers in the perlipc manpage.

       When it comes to opening devices, it can be easy and it
       can 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 the section on How do I read and write the
       serial port? in the perlfaq8 manpage 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 a socket, you can set them to be non-blocking
       using fcntl:

           use Fcntl;
           fcntl(Connection, F_SETFL, O_NONBLOCK)
               or die "can't set non blocking: $!";

       Rather than losing yourself in a morass of twisting,
       turning ioctls, 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 the POSIX manpage,
       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.

       What else can you open?  To open a connection using
       sockets, you won't use one of Perl's two open functions.
       See the section on Sockets: Client/Server Communication in
       the perlipc manpage 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.sn.no/libwww-perl/');

       BBiinnaarryy FFiilleess

       On certain legacy systems with what could charitably be
       called terminally convoluted (some would say broken) I/O
       models, a file isn't a file--at least, not with respect to
       the C standard I/O library.  On these old systems whose
       libraries (but not kernels) distinguish between text and
       binary streams, to get files to behave properly you'll
       have to bend over backwards to avoid nasty problems.  On
       such infelicitous systems, sockets and pipes are already
       opened in binary mode, and there is currently no way to
       turn that off.  With files, you have more options.

       Another option is to use the binmode function on the
       appropriate handles before doing regular I/O on them:

           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 binmodeing 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 system non-standard 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, Plan9, and Inferno.

       FFiillee LLoocckkiinngg

       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 others 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.
       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 WindowsNT.  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 request for a shared lock will be granted as
       soon as there is no exclusive locker.  A request for a
       exclusive lock will be granted as soon as there is no
       locker of any kind.  Locks are on file descriptors, not
       file names.  You can't lock a file until you open it, and
       you can't hold on to a lock once the file has been closed.

       Here's how to get a blocking shared lock on a file,
       typically used for reading:

           use 5.004;
           use Fcntl qw(:DEFAULT :flock);
           open(FH, "< filename")  or die "can't open filename: $!";
           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: $!";

           truncate(FH, tell(FH))
               or die "can't truncate numfile: $!";
           close(FH)
               or die "can't close numfile: $!";

SSEEEE AALLSSOO
       The open and sysopen function in perlfunc(1); the standard
       open(2), dup(2), fopen(3), and fdopen(3) manpages; the
       POSIX documentation.

AAUUTTHHOORR aanndd CCOOPPYYRRIIGGHHTT
       Copyright 1998 Tom Christiansen.

       When included as part of the Standard Version of Perl, or
       as part of its complete documentation whether printed or
       otherwise, this work may be distributed only under the
       terms of Perl's Artistic License.  Any distribution of
       this file or derivatives thereof outside of that package
       require that special arrangements be made with copyright
       holder.

       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.

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

24/Jan/1999            perl 5.005, patch 03                     1