perlopentut - tutorial on opening things in Perl
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.
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 [Toc] [Back]
The "open" function takes two arguments: the first is a
filehandle, and the second is a single string comprising
both what to open and how to open it. "open" returns true
when it works, and when it fails, returns a false value
and sets the special variable $! to reflect the system
error. If the filehandle was previously opened, it will
be implicitly closed first.
For example:
open(INFO, "datafile") || die("can't open
datafile: $!");
open(INFO, "< datafile") || die("can't open
datafile: $!");
open(RESULTS,"> runstats") || die("can't open runstats: $!");
open(LOG, ">> logfile ") || die("can't open logfile: $!");
If you prefer the low-punctuation version, you could write
that this way:
open INFO, "< datafile" or die "can't open
datafile: $!";
open RESULTS,"> runstats" or die "can't open runstats: $!";
open LOG, ">> logfile " or die "can't open logfile: $!";
A few things to notice. First, the leading less-than is
optional. If omitted, Perl assumes that you want to open
the file for reading.
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: $!" );
which is definitely not what you want.
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 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, still there
open(EXTRA, "< $filename") || die "can't open $filename: $!";
This is not a bug, but a feature. Because "open" mimics
the shell in its style of using redirection arrows to
specify how to open the file, it also does so with respect
to extra white space around the filename itself as well.
For accessing files with naughty names, see "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 it goes out of scope or when you
undefine it:
sub firstline {
open( my $in, shift ) && return scalar <$in>;
# no close() required
}
Pipe Opens [Toc] [Back]
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 "stuff0;
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 nonexistent
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
The Minus File [Toc] [Back]
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.
Mixing Reads and Writes [Toc] [Back]
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, "+> 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 cooes to the rescue. The following
command takes all the o, C++, or yacc source or header
files and changes all theig foo's to bar's, leaving the
old version in the origina' filename with a ".orig" tacked
on the end: *
.
$ perl -i.orig -pe 's/[
C
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" => erbose, # --verbose
"Debug" => ebug, # --Debug
"output=s" => utput );
# --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:
$pwdinfo = `domainname` =~ /^(~ne)?$/
? '< /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#^744
It's not for nothing that this is called magic "<ARGV>".
Pretty nifty, eh?
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 white space. If it's there, it's part of the
path. The FLAGS argument contains one or more values
derived from the Fcntl module that have been or'd together
using the bitwise "|" operator. The final argument, the
MASK, is optional; if present, it is combined with the
user's current umask for the creation mode of the file.
You should usually omit this.
Although the traditional values of read-only, write-only,
and read-write are 0, 1, and 2 respectively, this is known
not to hold true on some systems. Instead, it's best to
load in the appropriate constants first from the Fcntl
module, which supplies the following standard flags:
O_RDONLY Read only
O_WRONLY Write only
O_RDWR Read and write
O_CREAT Create the file if it doesn't exist
O_EXCL Fail if the file already exists
O_APPEND Append to the file
O_TRUNC Truncate the file
O_NONBLOCK Non-blocking access
Less common flags that are sometimes available on some
operating systems include "O_BINARY", "O_TEXT",
"O_SHLOCK", "O_EXLOCK", "O_DEFER", "O_SYNC", "O_ASYNC",
"O_DSYNC", "O_RSYNC", "O_NOCTTY", "O_NDELAY" and "O_LARGEFILE".
Consult your open(2) manpage or its local equivalent
for details. (Note: starting from Perl release 5.6
the "O_LARGEFILE" flag, if available, is automatically
added to the sysopen() flags because large files are the
default.)
Here's how to use "sysopen" to emulate the simple "open"
calls we had before. We'll omit the "|| die $!" checks
for clarity, but make sure you always check the return
values in real code. These aren't quite the same, since
"open" will trim leading and trailing white space, but
you'll get the idea.
To open a file for reading:
open(FH, "< $path");
sysopen(FH, $path, O_RDONLY);
To open a file for writing, creating a new file if needed
or else truncating an old file:
open(FH, "> $path");
sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
To open a file for appending, creating one if necessary:
open(FH, ">> $path");
sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
To open a file for update, where the file must already
exist:
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
And here are things you can do with "sysopen" that you
cannot do with a regular "open". As you'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 [Toc] [Back]
If you omit the MASK argument to "sysopen", Perl uses the
octal value 0666. The normal MASK to use for executables
and directories should be 0777, and for anything else,
0666.
Why so permissive? Well, it isn't really. The MASK will
be modified by your process's current "umask". A umask is
a number representing disabled permissions bits; that is,
bits that will not be turned on in the created files' permissions
field.
For example, if your "umask" were 027, then the 020 part
would disable the group from writing, and the 007 part
would disable others from reading, writing, or executing.
Under these conditions, passing "sysopen" 0666 would create
a file with mode 0640, since "0666 & ~027" is 0640.
You should seldom use the MASK argument to "sysopen()".
That takes away the user's freedom to choose what permission
new files will have. Denying choice is almost always
a bad thing. One exception would be for cases where sensitive
or private data is being stored, such as with mail
folders, cookie files, and internal temporary files.
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 [Toc] [Back]
Perl is more of a DWIMmer language than something like
Java--where DWIM is an acronym for "do what I mean". But
this principle sometimes leads to more hidden magic than
one knows what to do with. In this way, Perl is also
filled with dweomer, an obscure word meaning an enchantment.
Sometimes, Perl's DWIMmer is just too much like
dweomer for comfort.
If magic "open" is a bit too magical for you, you don't
have to turn to "sysopen". To open a file with arbitrary
weird characters in it, it's necessary to protect any
leading and trailing whitespace. Leading whitespace is
protected by inserting a "./" in front of a filename that
starts with whitespace. Trailing whitespace is protected
by appending an ASCII NUL byte (" ") at the end of the
string.
$file =~ s#^(#./$1#;
open(FH, "< $file ") || die "can't open $file: $!";
This assumes, of course, that your system considers dot
the current working directory, slash the directory separator,
and disallows ASCII NULs within a valid filename.
Most systems follow these conventions, including all POSIX
systems as well as proprietary Microsoft systems. The
only vaguely popular system that doesn't work this way is
the proprietary Macintosh system, which uses a colon where
the rest of us use a slash. Maybe "sysopen" isn't such a
bad idea after all.
If you want to use "<ARGV>" processing in a totally boring
and non-magical way, you could do this first:
# "Sam sat on the ground and put his head in his
hands.
# 'I wish I had never come here, and I don't want to
see
# no more magic,' he said, and fell silent."
for (@ARGV) {
s#^([^./])#./$1#;
$_ .= " ";
}
while (<>) {
# now process $_
}
But be warned that users will not appreciate being unable
to use "-" to mean standard input, per the standard convention.
Paths as Opens [Toc] [Back]
You've probably noticed how Perl's "warn" and "die" functions
can produce messages like:
Some warning at scriptname line 29, <FH> line 7.
That's because you opened a filehandle FH, and had read in
seven records from it. But what was the name of the file,
rather than the handle?
If you aren't running with "strict refs", or if you've
turned them off temporarily, then all you have to do is
this:
open($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 [Toc] [Back]
Remember how we said that Perl's open took two arguments?
That was a passive prevarication. You see, it can also
take just one argument. If and only if the variable is a
global variable, not a lexical, you can pass "open" just
one argument, the filehandle, and it will get the path
from the global scalar variable of the same name.
$FILE = "/etc/motd";
open FILE or die "can't open $FILE: $!";
while (<FILE>) {
# whatever
}
Why is this here? Someone has to cater to the hysterical
porpoises. It's something that's been in Perl since the
very beginning, if not before.
Playing with STDIN and STDOUT [Toc] [Back]
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
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 [Toc] [Back]
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 file0;
}
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.
To open a directory, you should use the "opendir" function,
then process it with "readdir", carefully restoring
the directory name if necessary:
opendir(DIR, $dirname) or die "can't opendir $dirname:
$!";
while (defined($file = readdir(DIR))) {
# do something with "$dirname/$file"
}
closedir(DIR);
If you want to process directories recursively, it's better
to use the File::Find module. For example, this
prints out all files recursively and adds a slash to their
names if the file is a directory.
@ARGV = qw(.) unless @ARGV;
use File::Find;
find sub { print $File::Find::name, -d && '/', "0 },
@ARGV;
This finds all bogus symbolic links beneath a particular
directory:
find sub { print "$File::Find::name0 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 $whither0;
} else {
print "$file points nowhere: $!0;
}
}
Opening Named Pipes [Toc] [Back]
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 15";
$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 some high-level modules on
CPAN that can help you with these games. Check out
Term::ReadKey and Term::ReadLine.
Opening Sockets [Toc] [Back]
What else can you open? To open a connection using sockets,
you won't use one of Perl's two open functions. See
"Sockets: 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 [Toc] [Back]
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 [Toc] [Back]
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 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, typically
used for reading:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename") or die "can't open filename:
$!";
flock(FH, LOCK_SH) or die "can't lock filename:
$!";
# now read from FH
You can get a non-blocking lock by using "LOCK_NB".
flock(FH, LOCK_SH | LOCK_NB)
or die "can't lock filename: $!";
This can be useful for producing more user-friendly
behaviour by warning if you're going to be blocking:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename") or die "can't open filename:
$!";
unless (flock(FH, LOCK_SH | LOCK_NB)) {
$| = 1;
print "Waiting for lock...";
flock(FH, LOCK_SH) or die "can't lock filename:
$!";
print "got it.0
}
# 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, "0
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 [Toc] [Back]
In Perl 5.8.0 a new I/O framework called "PerlIO" was
introduced. 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 being used:
o 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", $fn);
o The two-argument form of "binmode" is being used, for
example
binmode($fh, ":encoding(utf16)");
For more detailed discussion about PerlIO see PerlIO; for
more detailed discussion about Unicode and I/O see perluniintro.
The "open" and "sysopen" functions in perlfunc(1); the
system open(2), dup(2), fopen(3), and fdopen(3) manpages;
the POSIX documentation.
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.
First release: Sat Jan 9 08:09:11 MST 1999
perl v5.8.5 2002-11-06 20 [ Back ] |
