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# fcntl

Perl 5 version 16.0 documentation
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# fcntl

Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage

- 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;    fcntl($filehandle, F_GETFL, $packed_return_buffer)        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.

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

  Portability issues: fcntl in perlport.