Problem

You want to know how many bytes to read from a filehandle with read or sysread.

Solution

Use the FIONREAD ioctl call:

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

# $size bytes can be read

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 C include files.

Perl's h2ph tool tries to convert C include files to Perl code, which can be required. 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

Or 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
% ./fionread
0x4004667f

Then hard-code 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 work, but files don't.

If this is too much system programming for you, try to think outside the problem. Read from the filehandle in non-blocking mode (see Recipe 7.14). If you manage to read something, then that's how much was waiting to be read. If you couldn't read anything, there was nothing to be read.

See Also

Recipe 7.14; your system's ioctl (2) manpage; the ioctl function in perlfunc (1) and in Chapter 3 of Programming Perl