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fcntl may also refer to the module: Fcntl
fcntl FILEHANDLE,FUNCTION,SCALAR

Implements the fcntl(2) function. You'll probably have to say

use Fcntl;

first to get the correct constant definitions. Argument processing and value returned work just like ioctl below. For example:

use Fcntl;
my $flags = fcntl($filehandle, F_GETFL, 0)
    or die "Can't fcntl F_GETFL: $!";

You don't have to check for defined on the return from fcntl. Like ioctl, it maps a 0 return from the system call into "0 but true" in Perl. This string is true in boolean context and 0 in numeric context. It is also exempt from the normal Argument "..." isn't numeric warnings on improper numeric conversions.

Note that fcntl raises an exception if used on a machine that doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) manpage to learn what functions are available on your system.

Here's an example of setting a filehandle named $REMOTE to be non-blocking at the system level. You'll have to negotiate $| on your own, though.

use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

my $flags = fcntl($REMOTE, F_GETFL, 0)
    or die "Can't get flags for the socket: $!\n";

fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
    or die "Can't set flags for the socket: $!\n";

Portability issues: "fcntl" in perlport.