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7.2.39 IPC::Open2 - Open a Process for Both Reading and Writing

use IPC::Open2;

# with named filehandles
$pid = open2(\*RDR, \*WTR, $cmd_with_args);
$pid = open2(\*RDR, \*WTR, $cmd, "


", "


", ...);
# with object-oriented handles
use FileHandle;
my($rdr, $wtr) = (FileHandle->new, FileHandle->new);
$pid = open2($rdr, $wtr, $cmd_with_args);

The open2() function forks a child process to execute the specified command. The first two arguments represent filehandles, one way or another. They can be FileHandle objects, or they can be references to typeglobs, which can either be explicitly named as above, or generated by the Symbol package, as in the example below. Whichever you choose, they represent handles through which your program can read from the command's standard output and write to the command's standard input, respectively. open2() differs from Perl's built-in open function in that it allows your program to communicate in both directions with the child process.

open2() returns the process ID of the child process. On failure it reports a fatal error.

Here's a simple use of open2() by which you can give the program user interactive access to the bc (1) command. ( bc is an arbitrary-precision arithmetic package.) In this case we use the Symbol module to produce "anonymous" symbols:

use IPC::Open2;
use Symbol;

$WTR = gensym();  # get a reference to a typeglob
$RDR = gensym();  # and another one

$pid = open2($RDR, $WTR, 'bc');

while (<STDIN>) {            # read commands from user
     print $WTR $_;          # write a command to bc(1)
     $line = <$RDR>;         # read the output of bc(1)
     print STDOUT "$line";   # send the output to the user

open2() establishes unbuffered output for $WTR . However, it cannot control buffering of output from the designated command. Therefore, be sure to heed the following warning.

WARNING: It is extremely easy for your program to hang while waiting to read the next line of output from the command. In the example just shown, bc is known to read and write one line at a time, so it is safe. But utilities like sort (1) that read their entire input stream before offering any output will cause a deadlock when used in the manner we have illustrated. You might do something like this instead:

$pid = open2($RDR, $WTR, 'sort');

while (<STDIN>) {
     print $WTR $_;
close($WTR);    # finish sending all output to sort(1)

while (<$RDR>) {     # now read the output of sort(1)
     print STDOUT "$_";

More generally, you may have to use select to determine which file descriptors are ready to read, and then sysread for the actual reading. See also

The IPC::open3 module shows an alternative that handles STDERR as well.

Previous: 7.2.38 integer - Do Arithmetic in Integer Instead of Double Programming Perl Next: 7.2.40 IPC::Open3 - Open a Process for Reading, Writing, and Error Handling
7.2.38 integer - Do Arithmetic in Integer Instead of Double Book Index 7.2.40 IPC::Open3 - Open a Process for Reading, Writing, and Error Handling