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Perl 5 version 18.1 documentation
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    Implements the fcntl(2) function. You'll probably have to say

    1. use Fcntl;

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

    1. use Fcntl;
    2. fcntl($filehandle, F_GETFL, $packed_return_buffer)
    3. 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 -w 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.

    1. use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
    2. $flags = fcntl(REMOTE, F_GETFL, 0)
    3. or die "Can't get flags for the socket: $!\n";
    4. $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
    5. or die "Can't set flags for the socket: $!\n";

    Portability issues: fcntl in perlport.