7.21.1. Problem

7.21.2. Solution

$size = pack("L", 0);
ioctl(FH, $FIONREAD, $size)     or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);

# $size bytes can be read

Make sure the input filehandle is unbuffered (because you've used an I/O layer like :unix on it), or use only sysread.

7.21.3. Discussion

The Perl ioctl function is a direct interface to the operating system's ioctl(2) system call. If your system doesn't have the FIONREAD request or the ioctl(2) call, you can't use this recipe. FIONREAD and the other ioctl(2) requests are numeric values normally found lurking in C include files.

Perl's h2ph tool tries to convert C include files to Perl code, which can be require d. FIONREAD ends up defined as a function in the sys/ioctl.ph file:

require "sys/ioctl.ph";

$size = pack("L", 0);
ioctl(FH, FIONREAD( ), $size)    or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);

If h2ph wasn't installed or doesn't work for you, you can manually grep the include files:

% grep FIONREAD /usr/include/*/*
/usr/include/asm/ioctls.h:#define FIONREAD      0x541B

If you install Inline::C from CPAN, you can write a C subroutine to obtain the constant for you:

use Inline C;

$FIONREAD = get_FIONREAD( );
# ...

_ _END_ _
_ _C_ _
#include <sys/ioctl.h>

int get_FIONREAD( ) {
  return FIONREAD;
}

If all else fails, write a small C program using the editor of champions:

% cat > fionread.c
#include <sys/ioctl.h>
main( ) {
    printf("%#08x\n", FIONREAD);
}
^D
% cc -o fionread fionread.c
% ./fionread
0x4004667f

Then hardcode it, leaving porting as an exercise to your successor.

$FIONREAD = 0x4004667f;         # XXX: opsys dependent

$size = pack("L", 0);
ioctl(FH, $FIONREAD, $size)     or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);

FIONREAD requires a filehandle connected to a stream, which means sockets, pipes, and tty devices all work, but regular files don't.

7.21.4. See Also