FAQ 5.32 How can I tell whether there's a character waiting on a filehandle?

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5.32: How can I tell whether there's a character waiting on a filehandle?

The very first thing you should do is look into getting the
Term::ReadKey extension from CPAN. As we mentioned earlier, it now even
has limited support for non-portable (read: not open systems, closed,
proprietary, not POSIX, not Unix, etc.) systems.

You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
It's very system dependent. Here's one solution that works on BSD
systems:

sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}

If you want to find out how many characters are waiting, there's also
the FIONREAD ioctl call to be looked at. The *h2ph* tool that comes with
Perl 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 *grep* the
include files by hand:

% 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.c
% ./fionread
0x4004667f

And 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, meaning that
sockets, pipes, and tty devices work, but *not* files.



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