perlopentut.pod

To: pumpkings@jhereg.perl.com
Message-Id: <199901091513.IAA17512@jhereg.perl.com>

p4raw-id: //depot/cfgperl@2611

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+=head1 NAME
+
+perlopentut - tutorial on opening things in Perl
+
+=head1 DESCRIPTION
+
+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.
+
+=head1 Open E<agrave> la shell
+
+Perl's C<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.
+
+=head2 Simple Opens
+
+The C<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.  C<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 C<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 L</"Dispelling
+the Dweomer">.
+
+=head2 Pipe Opens
+
+In C, when you want to open a file using the standard I/O library,
+you use the C<fopen> function, but when opening a pipe, you use the
+C<popen> function.  But in the shell, you just use a different redirection
+character.  That's also the case for Perl.  The C<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 C<open> will not return an error. That's
+because in the traditional C<fork>/C<exec> model, running the other
+program happens only in the forked child process, which means that
+the failed C<exec> can't be reflected in the return value of C<open>.
+Only a failed C<fork> shows up there.  See L<perlfaq8/"Why doesn't open()
+return an error when a pipe open fails?"> to see how to cope with this.
+There's also an explanation in L<perlipc>.
+
+If you would like to open a bidirectional pipe, the IPC::Open2
+library will handle this for you.  Check out L<perlipc/"Bidirectional
+Communication with Another Process">
+
+=head2 The Minus File
+
+Again following the lead of the standard shell utilities, Perl's
+C<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 C<fork> hidden inside an C<open> call.  See L<perlipc/"Safe Pipe
+Opens"> for details.
+
+=head2 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, "+> /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 C<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 B<-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 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 
+L<perlfaq5> for more details.
+
+=head2 Filters 
+
+One of the most common uses for C<open> is one you never
+even notice.  When you process the ARGV filehandle using
+C<E<lt>ARGVE<gt>>, 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 C<E<lt>ARGVE<gt>> 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 B<-n> or B<-p> command-line options, you
+should put changes to @ARGV in a C<BEGIN{}> block.
+
+Remember that a normal C<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
+C<E<lt>ARGVE<gt>> processing employs the normal, shell-style Perl C<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 F<f1>, the process F<cmd1>, standard
+input (F<tmpfile> in this case), the F<f2> file, the F<cmd2> command,
+and finally the F<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 C<open>.
+You'll need to pass them as "./-" much as you would for the I<rm> program.
+Or you could use C<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 I<gzip>:
+
+    @ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_  } @ARGV;
+
+Or, if you have the I<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 C<E<lt>ARGVE<gt>>.
+Pretty nifty, eh?
+
+=head1 Open E<agrave> la C
+
+If you want the convenience of the shell, then Perl's C<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
+C<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.
+
+C<sysopen> takes 3 (or 4) arguments.
+
+    sysopen HANDLE, PATH, FLAGS, [MASK]
+
+The HANDLE argument is a filehandle just as with C<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 C<O_BINARY>, C<O_TEXT>, C<O_SHLOCK>, C<O_EXLOCK>, C<O_DEFER>,
+C<O_SYNC>, C<O_ASYNC>, C<O_DSYNC>, C<O_RSYNC>, C<O_NOCTTY>, C<O_NDELAY>
+and C<O_LARGEFILE>.  Consult your open(2) manpage or its local equivalent
+for details.
+
+Here's how to use C<sysopen> to emulate the simple C<open> calls we had
+before.  We'll omit the C<|| die $!> checks for clarity, but make sure
+you always check the return values in real code.  These aren't quite
+the same, since C<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 C<sysopen> that you cannot do with
+a regular C<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);
+
+=head2 Permissions E<agrave> la mode
+
+If you omit the MASK argument to C<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 C<umask>.  A umask is a number representing
+I<disabled> permissions bits; that is, bits that will not be turned on
+in the created files' permissions field.
+
+For example, if your C<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
+C<sysopen> 0666 would create a file with mode 0640, since C<0666 &~ 027>
+is 0640.
+
+You should seldom use the MASK argument to C<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.
+
+=head1 Obscure Open Tricks
+
+=head2 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, C<2E<gt>&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: $!";
+
+If you're using magic C<E<lt>ARGVE<gt>>, you could even pass in as a
+command line argument in @ARGV something like C<"E<lt>&=$MHCONTEXTFD">,
+but we've never seen anyone actually do this.
+
+=head2 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 I<dweomer>, an obscure word meaning an enchantment.
+Sometimes, Perl's DWIMmer is just too much like dweomer for comfort.
+
+If magic C<open> is a bit too magical for you, you don't have to turn
+to C<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 C<"./"> in front of a
+filename that starts with whitespace.  Trailing whitespace is protected
+by appending an ASCII NUL byte (C<"\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 C<sysopen> isn't such a bad idea after all.
+
+If you want to use C<E<lt>ARGVE<gt>> 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 $_
+    } 
+
+But be warned that users will not appreciate being unable to use "-"
+to mean standard input, per the standard convention.
+
+=head2 Paths as Opens
+
+You've probably noticed how Perl's C<warn> and C<die> functions can
+produce messages like:
+
+    Some warning at scriptname line 29, <FH> chunk 7.
+
+That's because you opened a filehandle FH, and had read in seven records
+from it.  But what was the name of the file, not the handle?
+
+If you aren't running with C<strict refs>, or if you've turn them off
+temporarily, then all you have to do is this:
+
+    open($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.
+
+=head2 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 C<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.
+
+=head2 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 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.
+
+=head1 Other I/O Issues
+
+These topics aren't really arguments related to C<open> or C<sysopen>,
+but they do affect what you do with your open files.
+
+=head2 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 I<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 C<-f>
+and C<-T> file tests.
+
+To open a directory, you should use the C<opendir> function, then
+process it with C<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 I<what> it points to, then
+C<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 L<perlipc/"Named Pipes">.
+Unix-domain sockets are rather different beasts as well; they're
+described in L<perlipc/"Unix-Domain TCP Clients and Servers">.
+
+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 L<perlfaq8/"How do I read and write the serial port?">
+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 C<sysopen>, such as a
+socket, you can set them to be non-blocking using C<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 C<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 L<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.
+
+What else can you open?  To open a connection using sockets, you won't use
+one of Perl's two open functions.  See L<perlipc/"Sockets: Client/Server
+Communication"> 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/');
+
+=head2 Binary Files
+
+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 C<binmode> function on the appropriate
+handles before doing regular I/O on them:
+
+    binmode(STDIN);
+    binmode(STDOUT);
+    while (<STDIN>) { print } 
+
+Passing C<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 C<binmode>ing on the handle.
+
+    sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
+        || die "can't open records.data: $!";
+
+Now you can use C<read> and C<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 C<sysread> and C<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 C<binmode> or C<O_BINARY> first.  Systems known to be free of
+such difficulties include Unix, the Mac OS, Plan9, and Inferno.
+
+=head2 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 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 C<-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 C<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 C<flock> is implemented on your
+system's port of Perl.
+
+File locking I<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 C<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 C<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 C<sysopen> the file so it can be locked before it gets
+emptied.  You can get a nonblocking version using C<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: $!";
+
+=head1 SEE ALSO 
+
+The C<open> and C<sysopen> function in perlfunc(1);
+the standard open(2), dup(2), fopen(3), and fdopen(3) manpages;
+the POSIX documentation.
+
+=head1 AUTHOR and COPYRIGHT
+
+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.
+
+=head1 HISTORY
+
+First release: Sat Jan  9 08:09:11 MST 1999