select(REPORT1); $^ = 'report1_top'; select(REPORT2); $^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:
use IO::Handle; STDERR->autoflush(1);
This calls the
select(2)
system call with
the bit masks specified, which can be constructed using fileno() and vec(), along these lines:
$rin = $win = $ein = ''; vec($rin,fileno(STDIN),1) = 1; vec($win,fileno(STDOUT),1) = 1; $ein = $rin | $win;
If you want to select on many filehandles you might wish to write a subroutine:
sub fhbits { my(@fhlist) = split(' ',$_[0]); my($bits); for (@fhlist) { vec($bits,fileno($_),1) = 1; } $bits; } $rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) = select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in $timeleft
, so calling
select()
in scalar context
just returns $nfound
.
Any of the bit masks can also be undef. The timeout, if specified, is in
seconds, which may be fractional. Note: not all implementations are capable
of returning the$timeleft
. If not, they always return
$timeleft
equal to the supplied $timeout
.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
WARNING: One should not attempt to mix buffered I/O (like read() or <FH>) with select(), except as permitted by POSIX, and even then only on POSIX systems. You have to use sysread() instead.