Programming Perl, 4th Edition

select (ready file descriptors)

select RBITS, WBITS, EBITS, TIMEOUT

The four-argument select operator is totally unrelated to the previously described select operator. This operator is used to discover which (if any) of your file descriptors are ready to do input or output, or to report an exceptional condition. (This helps you avoid having to do polling.) It calls the select(2) syscall with the bit masks you’ve specified, which you can construct using fileno and vec, like this:

$rin = $win = $ein = "";
vec($rin, fileno(STDIN), 1)  = 1;
vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;

If you want to select on many filehandles, you might wish to write a subroutine:

sub fhbits {
    my @fhlist = @_;
    my $bits;
    for my $fh (@fhlist) {
        vec($bits, fileno($fh), 1) = 1;
    }
    return $bits;
}
$rin = fhbits(*STDIN, *TTY, *MYSOCK);

Notice we passed in the filehandles using their typeglobs, because passing them in as strings is a bad idea. If you are using autovivified filehandles, you don’t have to do this.

If you wish to use the same bit masks repeatedly (and it’s more efficient if you do), the usual idiom is:

($nfound, $timeleft) =
    select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

Or to block until any file descriptor becomes ready:

$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

As you ...