Perl Cookbook

Determining the Number of Bytes to Read

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 in 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 ...