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

    NAME

         perlopentut - tutorial on opening things in Perl
    
    
    

    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.
    
    
    

    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 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".
    
         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 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 Why doesn't
         open() return an error when a pipe open fails? entry 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
         Bidirectional Communication with Another Process entry in
         the perlipc manpage
    
         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 the Safe Pipe Opens entry
         in the perlipc manpage 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, "+> /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 -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 the perlfaq5 manpage for more details.
    
         Filters
    
         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 -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'
                             : '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?
    
    
    

    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, 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.  (Note: starting from Perl release 5.6 the
         O_LARGEFILE flag, if available, is automatically added to
         the sysopen() flags because large files are the the
         default.)
    
         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);
    
    
         Permissions A la mode
    
         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.
    
    
    

    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 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.
    
         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 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.
    
         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,
         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> line 7.
    
    
         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 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.
    
    
    

    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 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 Named Pipes entry in the perlipc manpage.
         Unix-domain sockets are rather different beasts as well;
         they're described in the Unix-Domain TCP Clients and Servers
         entry 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
         How do I read and write the serial port? entry 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
         `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 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
         Sockets: Client/Server Communication entry 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.linpro.no/lwp/');
    
    
         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 `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 `binmode'ing 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.
    
         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 `-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: $!";
    
    
    
    

    SEE ALSO

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

    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.
    
    
    

    HISTORY

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


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