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select FILEHANDLE
select

Returns the currently selected filehandle. If FILEHANDLE is supplied, sets the new current default filehandle for output. This has two effects: first, a write or a print without a filehandle default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel.

For example, to set the top-of-form format for more than one output channel, you might do the following:

select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:

my $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:

STDERR->autoflush(1);

(Prior to Perl version 5.14, you have to use IO::Handle; explicitly first.)

Portability issues: "select" in perlport.

select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) syscall with the bit masks specified, which can be constructed using fileno and vec, along these lines:

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

If you want to select on many filehandles, you may wish to write a subroutine like this:

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

The usual idiom is:

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

or to block until something becomes ready just do this

my $nfound =
  select(my $rout = $rin, my $wout = $win, my $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);

Note that whether select gets restarted after signals (say, SIGALRM) is implementation-dependent. See also perlport for notes on the portability of select.

On error, select behaves just like select(2): it returns -1 and sets $!.

On some Unixes, select(2) may report a socket file descriptor as "ready for reading" even when no data is available, and thus any subsequent read would block. This can be avoided if you always use O_NONBLOCK on the socket. See select(2) and fcntl(2) for further details.

The standard IO::Select module provides a user-friendlier interface to select, mostly because it does all the bit-mask work for you.

WARNING: One should not attempt to mix buffered I/O (like read or readline) with select, except as permitted by POSIX, and even then only on POSIX systems. You have to use sysread instead.

Portability issues: "select" in perlport.