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 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.
The open function takes two arguments: the first is a filehandle, and the second is a single string comprising both what to open and how to open it. open returns true when it works, and when it fails, returns a false value and sets the special variable $! to reflect the system error. If the filehandle was previously opened, it will be implicitly closed first.
For example:
open(INFO, "datafile") || die("can't open datafile: $!"); open(INFO, "< datafile") || die("can't open datafile: $!"); open(RESULTS,"> runstats") || die("can't open runstats: $!"); open(LOG, ">> logfile ") || die("can't open logfile: $!");
If you prefer the low-punctuation version, you could write that this way:
open INFO, "< datafile" or die "can't open datafile: $!"; open RESULTS,"> runstats" or die "can't open runstats: $!"; open LOG, ">> logfile " or die "can't open logfile: $!";
A few things to notice. First, the leading less-than is optional. If omitted, Perl assumes that you want to open the file for reading.
The other important thing to notice is that, just as in the shell, any white space before or after the filename is ignored. This is good, because you wouldn't want these to do different things:
open INFO, "<datafile" open INFO, "< datafile" open INFO, "< datafile"
Ignoring surround whitespace also helps for when you read a filename in from a different file, and forget to trim it before opening:
$filename = <INFO>; # oops, \n still there open(EXTRA, "< $filename") || die "can't open $filename: $!";
This is not a bug, but a feature. Because open mimics the shell in its style of using redirection arrows to specify how to open the file, it also does so with respect to extra white space around the filename itself as well. For accessing files with naughty names, see Dispelling the Dweomer.
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, C
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:
What happens if you try to open a pipe to or from a non-existent command?
In most systems, such an open will not return an error. That's because in the traditional fork/exec model, running the other program happens only in the forked child process,
which means that the failed exec can't be reflected in the return value of open. Only a failed fork shows up there. See Why doesn't open() return an error when a pipe open fails? to see how to cope with this. There's also an explanation in the perlipc manpage.
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
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 current running! This is actually a
stealth fork hidden inside an open call. See Safe Pipe Opens for details.
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.
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
In fact, when it comes to updating a file, unless you're working on a binary file as in the
WTMP case above, you probably don't want to use this approach for updating. Instead, Perl's
-i flag comes to the rescue. The following command takes all the
C,
C++, or yacc source or header files and changes all their foo's to bar's, leaving the old version in the original file name with a ``.orig'' tacked on the end:
This is a short cut for some renaming games that are really the best way to
update textfiles. See the second question in
the perlfaq5 manpage for more details.
One of the most common uses for open is one you never even notice. When you process the
ARGV filehandle using
Can have all its files opened and processed one at a time using a construct
no more complex than:
If
You are welcome to pre-process your
Or the standard Getopt::Long module to permit named arguments:
Another reason for preprocessing arguments is to make an empty argument
list default to all files:
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.
If you're using the -n or -p command-line options, you should put changes to
Remember that a normal open has special properties, in that it might call
This sort of thing also comes into play in filter processing. Because
That program will read from the file f1, the process cmd1, standard input (tmpfile in this case), the f2 file, the cmd2 command, and finally the f3 file.
Yes, this also means that if you have a file named ``-'' (and so on) in
your directory, that they won't be processed as literal files by open. You'll need to pass them as ``./-'' much as you would for the rm program. Or you could use sysopen as described below.
One of the more interesting applications is to change files of a certain
name into pipes. For example, to autoprocess gzipped or compressed files by
decompressing them with gzip:
Or, if you have the GET program installed from
LWP, you can fetch URLs before processing them:
It's not for nothing that this is called magic
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
sysopen takes 3 (or 4) arguments.
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:
Less common flags that are sometimes available on some operating systems
include
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:
To open a file for writing, creating a new file if needed or else
truncating an old file:
To open a file for appending, creating one if necessary:
To open a file for update, where the file must already exist:
And here are things you can do with sysopen that you cannot do with a regular open. As you see, it's just a matter of controlling the flags in the third
argument.
To open a file for writing, creating a new file which must not previously
exist:
To open a file for appending, where that file must already exist:
To open a file for update, creating a new file if necessary:
To open a file for update, where that file must not already exist:
To open a file without blocking, creating one if necessary:
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
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.
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,
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:
This way if
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
It can be easier (and certainly will be faster) just to use real
filehandles though:
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
If you're using magic
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
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
But be warned that users will not appreciate being unable to use ``-'' to
mean standard input, per the standard convention.
You've probably noticed how Perl's warn and die functions can produce messages like:
That's because you opened a filehandle
FH, and had read in seven records from it. But what
was the name of the file, not the handle?
If you aren't running with
Since you're using the pathname of the file as its handle, you'll get
warnings more like
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.
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.
One clever move with
STDOUT is to explicitly close it when you're done with
the program.
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.
And then these can be read directly or passed on to subprocesses. This
makes it look as though the program were initially invoked with those
redirections from the command line.
It's probably more interesting to connect these to pipes. For example:
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:
This technique can be applied to repeatedly push as many filters on your
output stream as you wish.
These topics aren't really arguments related to open or sysopen, but they do affect what you do with your open 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.
What other kinds of files are there than, well, files? Directories,
symbolic links, named pipes, Unix-domain sockets, and block and character
devices. Those are all files, too--just not plain files. This isn't the same issue as being a text file. Not all text files
are plain files. Not all plain files are textfiles. That's why there are
separate -f
and -T file tests.
To open a directory, you should use the opendir function, then process it with readdir, carefully restoring the directory name if necessary:
If you want to process directories recursively, it's better to use the
File::Find module. For example, this prints out all files recursively, add
adds a slash to their names if the file is a directory.
This finds all bogus symbolic links beneath a particular directory:
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:
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. Unix-domain sockets are rather different beasts as well; they're
described in Unix-Domain TCP Clients and Servers.
When it comes to opening devices, it can be easy and it can tricky. We'll
assume that if you're opening up a block device, you know what you're
doing. The character devices are more interesting. These are typically used
for modems, mice, and some kinds of printers. This is described in How do I read and write the serial port?
It's often enough to open them carefully:
With descriptors that you haven't opened using sysopen, such as a socket, you can set them to be non-blocking using fcntl:
Rather than losing yourself in a morass of twisting, turning ioctls, all dissimilar, if you're going to manipulate ttys, it's best to make calls out to the
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 for that. Here's an example. Once you have it, you can use
FH as a bidirectional filehandle.
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:
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:
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 binmodeing on the handle.
Now you can use read and print on that handle without worrying about the system non-standard
I/O library breaking your data. It's not a pretty picture, but then, legacy systems seldom are.
CP/M will be with us until the end of days, and after.
On systems with exotic
I/O systems, it turns out that, astonishingly enough, even unbuffered
I/O using
sysread and syswrite might do sneaky data mutilation behind your back.
Depending on the vicissitudes of your runtime system, even these calls may
need binmode or
In a multitasking environment, you may need to be careful not to collide with other processes who want to do
I/O on the same files as others are working on. You'll often need shared or exclusive locks on files for reading and writing respectively. You might just pretend that only exclusive locks exist.
Never use the existence of a file
Perl's most portable locking interface is via the flock function, whose simplicity is emulated on systems that don't directly
support it, such as SysV or WindowsNT. The underlying semantics may affect
how it all works, so you should learn how flock is implemented on your system's port of Perl.
File locking does not lock out another process that would like to do
I/O.
A file lock only locks out others trying to get a lock, not processes trying to do
I/O. Because locks are advisory, if one process uses locking and another doesn't, all bets are off.
By default, the flock call will block until a lock is granted.
A request for a shared lock will be granted as soon as there is no exclusive locker.
A request for a exclusive lock will be granted as soon as there is no locker of any kind. Locks are on file descriptors, not file names. You can't lock a file until you open it, and you can't hold on to a lock once the file has been closed.
Here's how to get a blocking shared lock on a file, typically used for
reading:
You can get a non-blocking lock by using
This can be useful for producing more user-friendly behaviour by warning if
you're going to be blocking:
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
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:
The open and sysopen function in
Copyright 1998 Tom Christiansen.
When included as part of the Standard Version of Perl, or as part of its
complete documentation whether printed or otherwise, this work may be
distributed only under the terms of Perl's Artistic License. Any
distribution of this file or derivatives thereof outside of that package
require that special arrangements be made with copyright holder.
Irrespective of its distribution, all code examples in these files are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit.
A simple comment in the code giving credit would be courteous but is not required.
First release: Sat Jan 9 08:09:11
MST 1999
open(PRINTER, "| lpr -Plp1") || die "cannot fork: $!";
print PRINTER "stuff\n";
close(PRINTER) || die "can't close lpr: $!";
open(NET, "netstat -i -n |") || die "cannot fork: $!";
while (<NET>) { } # do something with input
close(NET) || die "can't close netstat: $!";
The Minus File
Mixing Reads and Writes
open(WTMP, "+< /usr/adm/wtmp")
|| die "can't open /usr/adm/wtmp: $!";
open(SCREEN, "+> /tmp/lkscreen")
|| die "can't open /tmp/lkscreen: $!";
open(LOGFILE, "+>> /tmp/applog"
|| die "can't open /tmp/applog: $!";
fopen(3S),
which
it ultimately calls.)
$ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]
Filters
<ARGV>
, Perl actually does an implicit open on each file in
@ARGV. Thus a program called like this:
$ myprogram file1 file2 file3
while (<>) {
# do something with $_
}
@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.
@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);
use Getopt::Long;
GetOptions( "verbose" => \$verbose, # --verbose
"Debug" => \$debug, # --Debug
"output=s" => \$output );
# --output=somestring or --output somestring
@ARGV = glob("*") unless @ARGV;
@ARGV = grep { -f && -T } @ARGV;
@ARGV
in a BEGIN{} block.
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: $!";
<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
@ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
@ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
<ARGV>
. Pretty nifty, eh?
Open à la C
fopen(3S)
provides, then you should look
to Perl's
sysopen, which is a direct hook into the open(2)
system call. That
does mean it's a bit more involved, but that's the price of precision.
sysopen HANDLE, PATH, FLAGS, [MASK]
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
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.
open(FH, "< $path");
sysopen(FH, $path, O_RDONLY);
open(FH, "> $path");
sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
open(FH, ">> $path");
sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
sysopen(FH, $path, O_WRONLY | O_APPEND);
sysopen(FH, $path, O_RDWR | O_CREAT);
sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
Permissions à la mode
0666 &~ 027
is 0640.
Obscure Open Tricks
Re-Opening Files (dups)
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: $!";
somefunction("&main::LOGFILE");
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.
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
use IO::Socket;
local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
die "can't connect" unless defined(fileno(REMOTE));
somefunction("&main::REMOTE");
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: $!";
<ARGV>
, you could even pass in as a command line argument in @ARGV
something like "<&=$MHCONTEXTFD"
, but we've never seen anyone actually do this.
Dispelling the Dweomer
"./"
in front of a filename that starts with whitespace. Trailing whitespace is protected by appending an
ASCII
NUL byte (
"\0"
) at the end off the string.
$file =~ s#^(\s)#./$1#;
open(FH, "< $file\0") || die "can't open $file: $!";
<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 $_
}
Paths as Opens
Some warning at scriptname line 29, <FH> chunk 7.
strict refs
, or if you've turn them off temporarily, then all you have to do is this:
open($path, "< $path") || die "can't open $path: $!";
while (<$path>) {
# whatever
}
Some warning at scriptname line 29, </etc/motd> chunk 7.
Single Argument Open
$FILE = "/etc/motd";
open FILE or die "can't open $FILE: $!";
while (<FILE>) {
# whatever
}
Playing with STDIN and STDOUT
END { close(STDOUT) || die "can't close stdout: $!" }
open(STDIN, "< datafile")
|| die "can't open datafile: $!";
open(STDOUT, "> output")
|| die "can't open output: $!";
$pager = $ENV{PAGER} || "(less || more)";
open(STDOUT, "| $pager")
|| die "can't fork a pager: $!";
head(100);
while (<>) {
print;
}
sub head {
my $lines = shift || 20;
return unless $pid = open(STDOUT, "|-");
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) {
print;
last if --$lines < 0;
}
exit;
}
Other I/O Issues
Opening Non-File Files
if (-l $file || ! -f _) {
print "$file is not a plain file\n";
}
opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
while (defined($file = readdir(DIR))) {
# do something with "$dirname/$file"
}
closedir(DIR);
@ARGV = qw(.) unless @ARGV;
use File::Find;
find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;
find sub { print "$File::Find::name\n" if -l && !-e }, $dir;
if (-l $file) {
if (defined($whither = readlink($file))) {
print "$file points to $whither\n";
} else {
print "$file points nowhere: $!\n";
}
}
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>;
use Fcntl;
fcntl(Connection, F_SETFL, O_NONBLOCK)
or die "can't set non blocking: $!";
stty(1)
program if you have it, or else use the portable
POSIX interface. To figure this all out, you'll need to read the termios(3)
manpage, which describes the
POSIX interface to tty devices, and then
the POSIX manpage, which describes Perl's interface to
POSIX. There are also some high-level modules on
CPAN that can help you with these games. Check out Term::ReadKey and Term::ReadLine.
use IO::Socket;
local *FH = IO::Socket::INET->new("www.perl.com:80");
use LWP::Simple;
$doc = get('http://www.sn.no/libwww-perl/');
Binary Files
binmode(STDIN);
binmode(STDOUT);
while (<STDIN>) { print }
sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
|| die "can't open records.data: $!";
while (sysread(WHENCE, $buf, 1024)) {
syswrite(WHITHER, $buf, length($buf));
}
O_BINARY
first. Systems known to be free of such difficulties include Unix, the Mac
OS, Plan9, and Inferno.
File Locking
-e $file
as a locking indication, because there is a race condition between the test
for the existence of the file and its creation. Atomicity is critical.
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
LOCK_NB
.
flock(FH, LOCK_SH | LOCK_NB)
or die "can't lock filename: $!";
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
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
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR | O_CREAT)
or die "can't open numfile: $!";
# autoflush FH
$ofh = select(FH); $| = 1; select ($ofh);
flock(FH, LOCK_EX)
or die "can't write-lock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0)
or die "can't rewind numfile : $!";
print FH $num+1, "\n"
or die "can't write numfile: $!";
truncate(FH, tell(FH))
or die "can't truncate numfile: $!";
close(FH)
or die "can't close numfile: $!";
SEE ALSO
perlfunc(1);
the standard open(2),
dup(2),
fopen(3),
and fdopen(3)
manpages; the
POSIX documentation.
AUTHOR and COPYRIGHT
HISTORY