PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 shell way also has 2- and 3-argument forms, which have different semantics for handling the filename. The
choice is yours.
Open a la shell
Perl's "open" function was designed to mimic the way command-line redirection in the shell works. Here are
some basic examples from the shell:
$ myprogram file1 file2 file3
$ myprogram < inputfile
$ myprogram > outputfile
$ myprogram >> outputfile
$ myprogram | otherprogram
$ otherprogram | myprogram
And here are some more advanced examples:
$ otherprogram | myprogram f1 - f2
$ otherprogram 2>&1 | myprogram -
$ myprogram <&3
$ myprogram >&4
Programmers accustomed to constructs like those above can take comfort in learning that Perl directly supports
these familiar constructs using virtually the same syntax as the shell.
Simple Opens
The "open" function takes two arguments: the first is a filehandle, and the second is a single string
comprising both what to open and how to open it. "open" returns true when it works, and when it fails,
returns a false value and sets the special variable $! to reflect the system error. If the filehandle was
previously opened, it will be implicitly closed first.
For example:
open(INFO, "datafile") || die("can't open datafile: $!");
open(INFO, "< datafile") || die("can't open datafile: $!");
open(RESULTS,"> runstats") || die("can't open runstats: $!");
open(LOG, ">> logfile ") || die("can't open logfile: $!");
If you prefer the low-punctuation version, you could write that this way:
open INFO, "< datafile" or die "can't open datafile: $!";
open RESULTS,"> runstats" or die "can't open runstats: $!";
open LOG, ">> logfile " or die "can't open logfile: $!";
A few things to notice. First, the leading "<" is optional. If omitted, Perl assumes that you want to open
the file for reading.
Note also that the first example uses the "||" logical operator, and the second uses "or", which has lower
precedence. Using "||" in the latter examples would effectively mean
open INFO, ( "< datafile" || die "can't open datafile: $!" );
$filename = <INFO>; # oops, \n still there
open(EXTRA, "< $filename") || die "can't open $filename: $!";
This is not a bug, but a feature. Because "open" mimics the shell in its style of using redirection arrows to
specify how to open the file, it also does so with respect to extra whitespace around the filename itself as
well. For accessing files with naughty names, see "Dispelling the Dweomer".
There is also a 3-argument version of "open", which lets you put the special redirection characters into their
own argument:
open( INFO, ">", $datafile ) || die "Can't create $datafile: $!";
In this case, the filename to open is the actual string in $datafile, so you don't have to worry about
$datafile containing characters that might influence the open mode, or whitespace at the beginning of the
filename that would be absorbed in the 2-argument version. Also, any reduction of unnecessary string
interpolation is a good thing.
Indirect Filehandles
"open"'s first argument can be a reference to a filehandle. As of perl 5.6.0, if the argument is
uninitialized, Perl will automatically create a filehandle and put a reference to it in the first argument,
like so:
open( my $in, $infile ) or die "Couldn't read $infile: $!";
while ( <$in> ) {
# do something with $_
}
close $in;
Indirect filehandles make namespace management easier. Since filehandles are global to the current package,
two subroutines trying to open "INFILE" will clash. With two functions opening indirect filehandles like "my
$infile", there's no clash and no need to worry about future conflicts.
Another convenient behavior is that an indirect filehandle automatically closes when there are no more
references to it:
sub firstline {
open( my $in, shift ) && return scalar <$in>;
# no close() required
}
Indirect filehandles also make it easy to pass filehandles to and return filehandles from subroutines:
for my $file ( qw(this.conf that.conf) ) {
my $fin = open_or_throw('<', $file);
process_conf( $fin );
# no close() needed
}
use Carp;
sub open_or_throw {
my ($mode, $filename) = @_;
open my $h, $mode, $filename
or croak "Could not open '$filename': $!";
return $h;
print PRINTER "stuff\n";
close(PRINTER) || die "can't close lpr: $!";
If the trailing character is a pipe, you start up a new command and open a read-only filehandle leading out of
that command. This lets whatever that command writes to its standard output show up on your handle for
reading. For example:
open(NET, "netstat -i -n |") || die "can't fork netstat: $!";
while (<NET>) { } # do something with input
close(NET) || die "can't close netstat: $!";
What happens if you try to open a pipe to or from a non-existent command? If possible, Perl will detect the
failure and set $! as usual. But if the command contains special shell characters, such as ">" or "*", called
'metacharacters', Perl does not execute the command directly. Instead, Perl runs the shell, which then tries
to run the command. This means that it's the shell that gets the error indication. In such a case, the
"open" call will only indicate failure if Perl can't even run the shell. See "How can I capture STDERR from
an external command?" in perlfaq8 to see how to cope with this. There's also an explanation in perlipc.
If you would like to open a bidirectional pipe, the IPC::Open2 library will handle this for you. Check out
"Bidirectional Communication with Another Process" in perlipc
perl-5.6.x introduced a version of piped open that executes a process based on its command line arguments
without relying on the shell. (Similar to the "system(@LIST)" notation.) This is safer and faster than
executing a single argument pipe-command, but does not allow special shell constructs. (It is also not
supported on Microsoft Windows, Mac OS Classic or RISC OS.)
Here's an example of "open '-|'", which prints a random Unix fortune cookie as uppercase:
my $collection = shift(@ARGV);
open my $fortune, '-|', 'fortune', $collection
or die "Could not find fortune - $!";
while (<$fortune>)
{
print uc($_);
}
close($fortune);
And this "open '|-'" pipes into lpr:
open my $printer, '|-', 'lpr', '-Plp1'
or die "can't run lpr: $!";
print {$printer} "stuff\n";
close($printer)
or die "can't close lpr: $!";
The Minus File
Again following the lead of the standard shell utilities, Perl's "open" function treats a file whose name is a
single minus, "-", in a special way. If you open minus for reading, it really means to access the standard
input. If you open minus for writing, it really means to access the standard output.
If minus can be used as the default input or default output, what happens if you open a pipe into or out of
minus? What's the default command it would run? The same script as you're currently running! This is
actually a stealth "fork" hidden inside an "open" call. See "Safe Pipe Opens" in perlipc for details.
open(LOGFILE, "+>> /var/log/applog")
|| die "can't open /var/log/applog: $!";
The first one won't create a new file, and the second one will always clobber an old one. The third one will
create a new file if necessary and not clobber an old one, and it will allow you to read at any point in the
file, but all writes will always go to the end. In short, the first case is substantially more common than
the second and third cases, which are almost always wrong. (If you know C, the plus in Perl's "open" is
historically derived from the one in C's fopen(3S), which it ultimately calls.)
In fact, when it comes to updating a file, unless you're working on a binary file as in the WTMP case above,
you probably don't want to use this approach for updating. Instead, Perl's -i flag comes to the rescue. The
following command takes all the C, C++, or yacc source or header files and changes all their foo's to bar's,
leaving the old version in the original filename with a ".orig" tacked on the end:
$ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]
This is a short cut for some renaming games that are really the best way to update textfiles. See the second
question in perlfaq5 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 );
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 files named "-" (and so on) in your directory, they won't be processed
as literal files by "open". You'll need to pass them as "./-", much as you would for the rm program, or you
could use "sysopen" as described below.
One of the more interesting applications is to change files of a certain name into pipes. For example, to
autoprocess gzipped or compressed files by decompressing them with gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
Or, if you have the GET program installed from LWP, you can fetch URLs before processing them:
@ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
It's not for nothing that this is called magic "<ARGV>". Pretty nifty, eh?
Open a la C
If you want the convenience of the shell, then Perl's "open" is definitely the way to go. On the other hand,
if you want finer precision than C's simplistic fopen(3S) provides you should look to Perl's "sysopen", which
is a direct hook into the open(2) system call. That does mean it's a bit more involved, but that's the price
of precision.
"sysopen" takes 3 (or 4) arguments.
sysopen HANDLE, PATH, FLAGS, [MASK]
The HANDLE argument is a filehandle just as with "open". The PATH is a literal path, one that doesn't pay
attention to any greater-thans or less-thans or pipes or minuses, nor ignore whitespace. If it's there, it's
part of the path. The FLAGS argument contains one or more values derived from the Fcntl module that have been
or'd together using the bitwise "|" operator. The final argument, the MASK, is optional; if present, it is
combined with the user's current umask for the creation mode of the file. You should usually omit this.
Although the traditional values of read-only, write-only, and read-write are 0, 1, and 2 respectively, this is
known not to hold true on some systems. Instead, it's best to load in the appropriate constants first from
the Fcntl module, which supplies the following standard flags:
"O_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 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 whitespace, but you'll get the idea.
To open a file for reading:
open(FH, "< $path");
sysopen(FH, $path, O_RDONLY);
To open a file for writing, creating a new file if needed or else truncating an old file:
open(FH, "> $path");
sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
To open a file for appending, creating one if necessary:
open(FH, ">> $path");
sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
To open a file for update, where the file must already exist:
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
And here are things you can do with "sysopen" that you cannot do with a regular "open". As you'll see, it's
just a matter of controlling the flags in the third argument.
To open a file for writing, creating a new file which must not previously exist:
sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
To open a file for appending, where that file must already exist:
sysopen(FH, $path, O_WRONLY | O_APPEND);
To open a file for update, creating a new file if necessary:
sysopen(FH, $path, O_RDWR | O_CREAT);
To open a file for update, where that file must not already exist:
sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
To open a file without blocking, creating one if necessary:
sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
Permissions a la mode
If you omit the MASK argument to "sysopen", Perl uses the octal value 0666. The normal MASK to use for
executables and directories should be 0777, and for anything else, 0666.
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
separator, and disallows ASCII NULs within a valid filename. Most systems follow these conventions, including
all POSIX systems as well as proprietary Microsoft systems. The only vaguely popular system that doesn't work
this way is the "Classic" Macintosh system, which uses a colon where the rest of us use a slash. Maybe
"sysopen" isn't such a bad idea after all.
If you want to use "<ARGV>" processing in a totally boring and non-magical way, you could do this first:
# "Sam sat on the ground and put his head in his hands.
# 'I wish I had never come here, and I don't want to see
# no more magic,' he said, and fell silent."
for (@ARGV) {
s#^([^./])#./$1#;
$_ .= "\0";
}
while (<>) {
# now process $_
}
But be warned that users will not appreciate being unable to use "-" to mean standard input, per the standard
convention.
Paths as Opens
You've probably noticed how Perl's "warn" and "die" functions can produce messages like:
Some warning at scriptname line 29, <FH> line 7.
That's because you opened a filehandle FH, and had read in seven records from it. But what was the name of
the file, rather than the handle?
If you aren't running with "strict refs", or if you've turned them off temporarily, then all you have to do is
this:
open($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
open(STDIN, "< datafile")
|| die "can't open datafile: $!";
open(STDOUT, "> output")
|| die "can't open output: $!";
And then these can be accessed directly or passed on to subprocesses. This makes it look as though the
program were initially invoked with those redirections from the command line.
It's probably more interesting to connect these to pipes. For example:
$pager = $ENV{PAGER} || "(less || more)";
open(STDOUT, "| $pager")
|| die "can't fork a pager: $!";
This makes it appear as though your program were called with its stdout already piped into your pager. You
can also use this kind of thing in conjunction with an implicit fork to yourself. You might do this if you
would rather handle the post processing in your own program, just in a different process:
head(100);
while (<>) {
print;
}
sub head {
my $lines = shift || 20;
return if $pid = open(STDOUT, "|-"); # return if parent
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) {
last if --$lines < 0;
print;
}
exit;
}
This technique can be applied to repeatedly push as many filters on your output stream as you wish.
Other I/O Issues
These topics aren't really arguments related to "open" or "sysopen", but they do affect what you do with your
open files.
Opening Non-File Files
When is a file not a file? Well, you could say when it exists but isn't a plain file. We'll check whether
it's a symbolic link first, just in case.
if (-l $file || ! -f _) {
print "$file is not a plain file\n";
}
What other kinds of files are there than, well, files? Directories, symbolic links, named pipes, Unix-domain
sockets, and block and character devices. Those are all files, too--just not plain files. This isn't the
same issue as being a text file. Not all text files are plain files. Not all plain files are text files.
That's why there are separate "-f" and "-T" file tests.
@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";
}
}
Opening Named Pipes
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 "Named Pipes" in perlipc. Unix-
domain sockets are rather different beasts as well; they're described in "Unix-Domain TCP Clients and Servers"
in perlipc.
When it comes to opening devices, it can be easy and it can be tricky. We'll assume that if you're opening up
a block device, you know what you're doing. The character devices are more interesting. These are typically
used for modems, mice, and some kinds of printers. This is described in "How do I read and write the serial
port?" in perlfaq8 It's often enough to open them carefully:
sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
# (O_NOCTTY no longer needed on POSIX systems)
or die "can't open /dev/ttyS1: $!";
open(TTYOUT, "+>&TTYIN")
or die "can't dup TTYIN: $!";
$ofh = select(TTYOUT); $| = 1; select($ofh);
print TTYOUT "+++at\015";
$answer = <TTYIN>;
With descriptors that you haven't opened using "sysopen", such as sockets, you can set them to be non-blocking
using "fcntl":
use Fcntl;
my $old_flags = fcntl($handle, F_GETFL, 0)
or die "can't get flags: $!";
fcntl($handle, F_SETFL, $old_flags | O_NONBLOCK)
or die "can't set non blocking: $!";
Rather than losing yourself in a morass of twisting, turning "ioctl"s, all dissimilar, if you're going to
manipulate ttys, it's best to make calls out to the stty(1) program if you have it, or else use the portable
POSIX interface. To figure this all out, you'll need to read the termios(3) manpage, which describes the
POSIX interface to tty devices, and then POSIX, which describes Perl's interface to POSIX. There are also
interface, but it's still easy to get the contents of a document:
use LWP::Simple;
$doc = get('http://www.cpan.org/');
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" on the handle.
sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
|| die "can't open records.data: $!";
Now you can use "read" and "print" on that handle without worrying about the non-standard system I/O library
breaking your data. It's not a pretty picture, but then, legacy systems seldom are. CP/M will be with us
until the end of days, and after.
On systems with exotic I/O systems, it turns out that, astonishingly enough, even unbuffered I/O using
"sysread" and "syswrite" might do sneaky data mutilation behind your back.
while (sysread(WHENCE, $buf, 1024)) {
syswrite(WHITHER, $buf, length($buf));
}
Depending on the vicissitudes of your runtime system, even these calls may need "binmode" or "O_BINARY" first.
Systems known to be free of such difficulties include Unix, the Mac OS, Plan 9, and Inferno.
File Locking
In a multitasking environment, you may need to be careful not to collide with other processes who want to do
I/O on the same files as you are working on. You'll often need shared or exclusive locks on files for reading
and writing respectively. You might just pretend that only exclusive locks exist.
Never use the existence of a file "-e $file" as a locking indication, because there is a race condition
between the test for the existence of the file and its creation. It's possible for another process to create
a file in the slice of time between your existence check and your attempt to create the file. Atomicity is
critical.
Perl's most portable locking interface is via the "flock" function, whose simplicity is emulated on systems
that don't directly support it such as SysV or Windows. The underlying semantics may affect how it all works,
so you should learn how "flock" is implemented on your system's port of Perl.
File locking does not lock out another process that would like to do I/O. A file lock only locks out others
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))
the layers being used:
· The three-(or more)-argument form of "open" is being used and the second argument contains something else
in addition to the usual '<', '>', '>>', '|' and their variants, for example:
open(my $fh, "<:crlf", $fn);
· The two-argument form of "binmode" is being used, for example
binmode($fh, ":encoding(utf16)");
For more detailed discussion about PerlIO see PerlIO; for more detailed discussion about Unicode and I/O see
perluniintro.
SEE ALSO
The "open" and "sysopen" functions in perlfunc(1); the system open(2), dup(2), fopen(3), and fdopen(3)
manpages; the POSIX documentation.
AUTHOR and COPYRIGHT
Copyright 1998 Tom Christiansen.
This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in these files are hereby placed into the public domain.
You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit.
A simple comment in the code giving credit would be courteous but is not required.
HISTORY
First release: Sat Jan 9 08:09:11 MST 1999
perl v5.16.3 2013-03-04 PERLOPENTUT(1)
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