PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)NAMEperlopentut - 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 impli-
citly 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:
perl v5.8.8 2006-06-30 1
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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.
Note also that the first example uses the "||" logical
operator, and the second uses "or", which has lower pre-
cedence. Using "||" in the latter examples would effec-
tively mean
open INFO, ( "< datafile" || die "can't open datafile: $!" );
which is definitely not what you want.
The other important thing to notice is that, just as in the
shell, any whitespace 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 surrounding 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
whitespace around the filename itself as well. For access-
ing 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 argu-
ment:
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 con-
taining 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.
perl v5.8.8 2006-06-30 2
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 con-
flicts.
Another convenient behavior is that an indirect filehandle
automatically closes when it goes out of scope or when you
undefine it:
sub firstline {
open( my $in, shift ) && return scalar <$in>;
# no close() required
}
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 opens 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 "can't run lpr: $!";
print PRINTER "stuff\n";
close(PRINTER) || die "can't close lpr: $!";
If the trailing character is a pipe, you start up a new com-
mand 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
perl v5.8.8 2006-06-30 3
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 'metacharac-
ters', 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 indica-
tion. 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 per-
lipc.
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 per-
lipc
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 read-
ing, it really means to access the standard input. If you
open minus for writing, it really means to access the stan-
dard 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 per-
lipc 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.
perl v5.8.8 2006-06-30 4
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
open(WTMP, "+< /usr/adm/wtmp")
|| die "can't open /usr/adm/wtmp: $!";
open(SCREEN, "+> lkscreen")
|| die "can't open lkscreen: $!";
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 ver-
sion 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
perl v5.8.8 2006-06-30 5
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
fact, $ARGV, the currently open file during "<ARGV>" pro-
cessing, 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 argu-
ments:
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: $!";
perl v5.8.8 2006-06-30 6
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
This sort of thing also comes into play in filter process-
ing. 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 bit-
wise "|" 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
perl v5.8.8 2006-06-30 7
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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". Con-
sult 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:
perl v5.8.8 2006-06-30 8
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
And here are things you can do with "sysopen" that you can-
not 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.
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
perl v5.8.8 2006-06-30 9
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 descrip-
tor 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 han-
dle 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 argu-
ment, 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");
perl v5.8.8 2006-06-30 10
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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: $!";
If you're using magic "<ARGV>", you could even pass in as a
command line argument in @ARGV something like "<&=$MHCON-
TEXTFD", 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. Some-
times, Perl's DWIMmer is just too much like dweomer for com-
fort.
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 of 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 sys-
tems 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:
perl v5.8.8 2006-06-30 11
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
# "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 $_
}
But be warned that users will not appreciate being unable to
use "-" to mean standard input, per the standard convention.
Paths as Opens
You've probably noticed how Perl's "warn" and "die" func-
tions can produce messages like:
Some warning at scriptname line 29, <FH> line 7.
That's because you opened a filehandle FH, and had read in
seven records from it. But what was the name of the file,
rather than the handle?
If you aren't running with "strict refs", or if you've
turned them off temporarily, then all you have to do is
this:
open($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 argu-
ment, the filehandle, and it will get the path from the glo-
bal scalar variable of the same name.
perl v5.8.8 2006-06-30 12
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
$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 accessed directly or passed on to sub-
processes. 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:
perl v5.8.8 2006-06-30 13
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)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 sock-
ets, 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.
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);
perl v5.8.8 2006-06-30 14
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
If you want to process directories recursively, it's better
to use the File::Find module. For example, this prints out
all files recursively and 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";
}
}
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);
perl v5.8.8 2006-06-30 15
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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) man-
page, which describes the POSIX interface to tty devices,
and then 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.
Opening Sockets
What else can you open? To open a connection using sockets,
you won't use one of Perl's two open functions. See "Sock-
ets: Client/Server Communication" in perlipc 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
perl v5.8.8 2006-06-30 16
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 sys-
tems 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
perl v5.8.8 2006-06-30 17
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 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 an 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, typi-
cally 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
perl v5.8.8 2006-06-30 18
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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 dis-
suaded 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: $!";
IO Layers
In Perl 5.8.0 a new I/O framework called "PerlIO" was intro-
duced. This is a new "plumbing" for all the I/O happening in
Perl; for the most part everything will work just as it did,
but PerlIO also brought in some new features such as the
ability to think of I/O as "layers". One I/O layer may in
addition to just moving the data also do transformations on
the data. Such transformations may include compression and
decompression, encryption and decryption, and transforming
between various character encodings.
Full discussion about the features of PerlIO is out of scope
for this tutorial, but here is how to recognize the layers
perl v5.8.8 2006-06-30 19
PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)
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, "<:utf8", $fn);
+ The two-argument form of "binmode" is being used, for
example
binmode($fh, ":encoding(utf16)");
For more detailed discussion about PerlIO see PerlIO; for
more detailed discussion about Unicode and I/O see perluni-
intro.
SEE ALSO
The "open" and "sysopen" functions in perlfunc(1); the sys-
tem 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 pro-
grams for fun or for profit as you see fit. A simple com-
ment in the code giving credit would be courteous but is not
required.
HISTORY
First release: Sat Jan 9 08:09:11 MST 1999
perl v5.8.8 2006-06-30 20
