+=encoding utf8
+
=head1 NAME
-perlopentut - tutorial on opening things in Perl
+perlopentut - simple recipes for opening files and pipes 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 currently 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:
+Whenever you do I/O on a file in Perl, you do so through what in Perl is
+called a B<filehandle>. A filehandle is an internal name for an external
+file. It is the job of the C<open> function to make the association
+between the internal name and the external name, and it is the job
+of the C<close> function to break that associations.
- $ myprogram file1 file2 file3
+For your convenience, Perl sets up a few special filehandles that are
+already open when you run. These include C<STDIN>, C<STDOUT>, C<STDERR>,
+and C<ARGV>. Since those are pre-opened, you can use them right away
+without having to go to the trouble of opening them yourself:
-Can have all its files opened and processed one at a time
-using a construct no more complex than:
+ print STDERR "This is a debugging message.\n";
- while (<>) {
- # do something with $_
- }
+ print STDOUT "Please enter something: ";
+ $response = <STDIN> // die "how come no input?";
+ print STDOUT "Thank you!\n";
-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.
+ while (<ARGV>) { ... }
-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.
+As you see from those examples, C<STDOUT> and C<STDERR> are output
+handles, and C<STDIN> and C<ARGV> are input handles. Those are
+in all capital letters because they are reserved to Perl, much
+like the C<@ARGV> array and the C<%ENV> hash are. Their external
+associations were set up by your shell.
- use Getopt::Std;
+For eveyrthing else, you will need to open it on your own. Although there
+are many other variants, the most common way to call Perl's open() function
+is with three arguments and one return value:
- # -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
- getopts("vDo:");
+C< I<OK> = open(I<HANDLE>, I<MODE>, I<PATHNAME>)>
- # -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
- getopts("vDo:", \%args);
+Where:
-Or the standard Getopt::Long module to permit named arguments:
+=over
- use Getopt::Long;
- GetOptions( "verbose" => \$verbose, # --verbose
- "Debug" => \$debug, # --Debug
- "output=s" => \$output );
- # --output=somestring or --output somestring
+=item I<OK>
-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.
+will be some defined value if the open succeeds, but
+C<undef> if it fails;
- @ARGV = grep { -f && -T } @ARGV;
+=item I<HANDLE>
-If you're using the B<-n> or B<-p> command-line options, you
-should put changes to @ARGV in a C<BEGIN{}> block.
+should be an undefined scalar variable to be filled in by the
+C<open> function if it succeeds;
-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:
+=item I<MODE>
- $pwdinfo = `domainname` =~ /^(\(none\))?$/
- ? '< /etc/passwd'
- : 'ypcat passwd |';
+is the access mode and the encoding format to open the file with;
- open(PWD, $pwdinfo)
- or die "can't open $pwdinfo: $!";
+=item I<PATHNAME>
-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:
+is the external name of the file you want opened.
- $ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
+=back
-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.
+Most of the complexity of the C<open> function lies in the many
+possible values that the I<MODE> parameter can take on.
-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 last thing before we show you how to open files: opening
+files does not (usually) automatically lock them in Perl. See
+L<perlfaq5> for how to lock.
-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>:
+=head1 Opening Text Files
- @ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
+=head2 Opening Text Files for Reading
-Or, if you have the I<GET> program installed from LWP,
-you can fetch URLs before processing them:
+If you want to read from a text file, first open it in
+read-only mode like this:
- @ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
+ my $filename = "/some/path/to/a/textfile/goes/here";
+ my $encoding = ":encoding(UTF-8)";
+ my $handle = undef; # this will be filled in on success
-It's not for nothing that this is called magic C<E<lt>ARGVE<gt>>.
-Pretty nifty, eh?
+ open($handle, "< $encoding", $filename)
+ || die "$0: can't open $filename for reading: $!\n";
-=head1 Open E<agrave> la C
+As with the shell, in Perl the C<< "<" >> is used to open the file in
+read-only mode. If it succeeds, Perl allocates a brand new filehandle for
+you and fills in your previously undefined C<$handle> argument with a
+reference to that handle.
-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.
+Now you may use functions like C<readline>, C<read>, C<getc>, and
+C<sysread> on that handle. Probably the most common input function
+is the one that looks like an operator:
-C<sysopen> takes 3 (or 4) arguments.
+ $line = readline($handle);
+ $line = <$handle>; # same thing
- sysopen HANDLE, PATH, FLAGS, [MASK]
+Because the C<readline> function returns C<undef> at end of file or
+upon error, you will sometimes see it used this way:
-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. (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 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.
+ $line = <$handle>;
+ if (defined $line) {
+ # do something with $line
+ }
+ else {
+ # $line is not valid, so skip it
+ }
-To open a file for writing, creating a new file which must not previously
-exist:
+You can also just quickly C<die> on an undefined value this way:
- sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
+ $line = <$handle> // die "no input found";
-To open a file for appending, where that file must already exist:
+However, if hitting EOF is an expected and normal event, you
+would not to exit just because you ran out of input. Instead,
+you probably just want to exit an input loop. Immediately
+afterwards you can then test to see if there was an actual
+error that caused the loop to terminate, and act accordingly:
- sysopen(FH, $path, O_WRONLY | O_APPEND);
+ while (<$handle>) {
+ # do something with data in $_
+ }
+ if ($!) {
+ die "unexpected error while reading from $filename: $!";
+ }
-To open a file for update, creating a new file if necessary:
+B<A Note on Encodings>: Having to specify the text encoding every time
+might seem a bit of a bother. To set up a default encoding for C<open> so
+that you don't have to supply it each time, you can use the C<open> pragma:
- sysopen(FH, $path, O_RDWR | O_CREAT);
+ use open qw< :encoding(UTF-8) >;
-To open a file for update, where that file must not already exist:
+Once you've done that, you can safely omit the encoding part of the
+open mode:
- sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
+ open($handle, "<", $filename)
+ || die "$0: can't open $filename for reading: $!\n";
-To open a file without blocking, creating one if necessary:
+But never use the bare C<< "<" >> without having set up a default encoding
+first. Otherwise, Perl cannot know which of the many, many, many possible
+flavors of text file you have, and Perl will have no idea how to correctly
+map the data in your file into actual characters it can work with. Other
+common encoding formats including C<"ASCII">, C<"ISO-8859-1">,
+C<"ISO-8859-15">, C<"Windows-1252">, C<"MacRoman">, and even C<"UTF-16LE">.
+See L<perlunitut> for more about encodings.
- sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
+=head2 Opening Text Files for Writing
-=head2 Permissions E<agrave> la mode
+On the other hand, you want to write to a file, you first have to decide
+what to do about any existing contents. You have two basic choices here:
+to preserve or to clobber.
-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.
+If you want to preserve any existing contents, then you want to open the
+file in append mode. As in the shell, in Perl you use C<<< ">>" >>> to
+open an existing file in append mode, and creates the file if it does not
+already exist.
-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.
+ my $handle = undef;
+ my $filename = "/some/path/to/a/textfile/goes/here";
+ my $encoding = ":encoding(UTF-8)";
-For example, if your C<umask> were 027, then the 020 part would
-disable the group from writing, and the 007 part would disable others
-from reading, writing, or executing. Under these conditions, passing
-C<sysopen> 0666 would create a file with mode 0640, since C<0666 &~ 027>
-is 0640.
+ open($handle, ">> $encoding", $filename)
+ || die "$0: can't open $filename for appending: $!\n";
-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
+Now you can write to that filehandle using any of C<print>, C<printf>,
+C<say>, C<write>, or C<syswrite>.
-=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.
+The file does not have to exist just to open it in append mode. If the
+file did not previously exist, then the append-mode open creates it for
+you. But if the file does previously exist, its contents are safe from
+harm because you will be adding your new text past the end of the old text.
- open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
- open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!";
+On the other hand, sometimes you want to clobber whatever might already be
+there. To empty out a file before you start writing to it, you can open it
+in write-only mode:
-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:
+ my $handle = undef;
+ my $filename = "/some/path/to/a/textfile/goes/here";
+ my $encoding = ":encoding(UTF-8)";
- somefunction("&main::LOGFILE");
+ open($handle, "> $encoding", $filename)
+ || die "$0: can't open $filename in write-open mode: $!\n";
-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.
+Here again Perl works just like the shell in that the C<< ">" >> clobbers
+an existing file.
-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:
+As with the append mode, when you open a file in write-only mode,
+you can now write to that filehandle using any of C<print>, C<printf>,
+C<say>, C<write>, or C<syswrite>.
- use IO::Socket;
- $handle = IO::Socket::INET->new("www.perl.com:80");
- $fd = $handle->fileno;
- somefunction("&$fd"); # not an indirect function call
+What about read-write mode? You should probably pretend it doesn't exist,
+because opening text files in read-write mode is unlikely to do what you
+would like. See L<perlfaq5> for details.
-It can be easier (and certainly will be faster) just to use real
-filehandles though:
+=head1 Opening Binary Files
- 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 file to be opened contains binary data instead of text characters,
+then the C<MODE> argument to C<open> is a little different. Instead of
+specifying the encoding, you tell Perl that your data are in raw bytes.
-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:
+ my $filename = "/some/path/to/a/binary/file/goes/here";
+ my $encoding = ":raw :bytes"
+ my $handle = undef; # this will be filled in on success
- $fd = $ENV{"MHCONTEXTFD"};
- open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!";
+And then open as before, choosing C<<< "<" >>>, C<<< ">>" >>>, or
+C<<< ">" >>> as needed:
-If you're using magic C<E<lt>ARGVE<gt>>, you could even pass in as a
-command line argument in @ARGV something like C<"E<lt>&=$MHCONTEXTFD">,
-but we've never seen anyone actually do this.
+ open($handle, "< $encoding", $filename)
+ || die "$0: can't open $filename for reading: $!\n";
-=head2 Dispelling the Dweomer
+ open($handle, ">> $encoding", $filename)
+ || die "$0: can't open $filename for appending: $!\n";
-Perl is more of a DWIMmer language than something like Java--where DWIM
-is an acronym for "do what I mean". But this principle sometimes leads
-to more hidden magic than one knows what to do with. In this way, Perl
-is also filled with I<dweomer>, an obscure word meaning an enchantment.
-Sometimes, Perl's DWIMmer is just too much like dweomer for comfort.
+ open($handle, "> $encoding", $filename)
+ || die "$0: can't open $filename in write-open mode: $!\n";
-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.
+Alternately, you can change to binary mode on an existing handle this way:
- $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 $_
- }
-
-But be warned that users will not appreciate being unable to use "-"
-to mean standard input, per the standard convention.
-
-=head2 Paths as Opens
-
-You've probably noticed how Perl's C<warn> and C<die> functions can
-produce messages like:
-
- Some warning at scriptname line 29, <FH> line 7.
+ binmode($handle) || die "cannot binmode handle";
-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> line 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: $!" }
+This is especially handy for the handles that Perl has already opened for you.
-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.
+ binmode(STDIN) || die "cannot binmode STDIN";
+ binmode(STDOUT) || die "cannot binmode STDOUT";
-You don't have to accept the STDIN and STDOUT you were given. You are
-welcome to reopen them if you'd like.
+You can also pass C<binmode> an explicit encoding to change it on the fly.
+This isn't exactly "binary" mode, but we still use C<binmode> to do it:
- 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.
+ binmode(STDIN, ":encoding(MacRoman)") || die "cannot binmode STDIN";
+ binmode(STDOUT, ":encoding(UTF-8)") || die "cannot binmode STDOUT";
-It's probably more interesting to connect these to pipes. For example:
+Once you have your binary file properly opened in the right mode, you can
+use all the same Perl I/O functions as you used on text files. However,
+you may wish to use the fixed-size C<read> instead of the variable-sized
+C<readline> for your input.
- $pager = $ENV{PAGER} || "(less || more)";
- open(STDOUT, "| $pager")
- || die "can't fork a pager: $!";
+Here's an example of how to copy a binary file:
-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:
+ my $BUFSIZ = 64 * (2 ** 10);
+ my $name_in = "/some/input/file";
+ my $name_out = "/some/output/flie";
- head(100);
- while (<>) {
- print;
- }
+ my($in_fh, $out_fh, $buffer);
- 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;
- }
+ open($in_fh, "<", $name_in) || die "$0: cannot open $name_in for reading: $!";
+ open($out_fh, ">", $name_out) || die "$0: cannot open $name_out for writing: $!";
-This technique can be applied to repeatedly push as many filters on your
-output stream as you wish.
+ for my $fh ($in_fh, $out_fh) {
+ binmode($fh) || die "binmode failed";
+ }
-=head1 Other I/O Issues
+ while (read($in_fh, $buffer, $BUFSIZ)) {
+ unless (print $out_fh $buffer) {
+ die "couldn't write to $name_out: $!";
+ }
+ }
-These topics aren't really arguments related to C<open> or C<sysopen>,
-but they do affect what you do with your open files.
+ close($in_fh) || die "couldn't close $name_in: $!";
+ close($out_fh) || die "couldn't close $name_out: $!";
-=head2 Opening Non-File Files
+=head1 Opening Pipes
-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.
+To be announced.
- if (-l $file || ! -f _) {
- print "$file is not a plain file\n";
- }
+=head1 Low-level File Opens via sysopen
-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 be announced. Or deleted.
-To open a directory, you should use the C<opendir> function, then
-process it with C<readdir>, carefully restoring the directory
-name if necessary:
+=head1 SEE ALSO
- 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.linpro.no/lwp/');
-
-=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.
+To be announced.
=head1 AUTHOR and COPYRIGHT
-Copyright 1998 Tom Christiansen.
+To be announced.
-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.
+=head1 HISTORY
-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.
+To be announced.
-=head1 HISTORY
-First release: Sat Jan 9 08:09:11 MST 1999
https://perl5.git.perl.org / perl5.git / blobdiff