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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, print, or say without a filehandle will 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.)

Whilst you can use select to temporarily "capture" the output of print like this:

{
    my $old_handle = select $new_handle;

    # This goes to $new_handle:
    print "ok 1\n";
    ...

    select $old_handle;
}

you might find it easier to localize the typeglob instead:

{
    local *STDOUT = $new_handle;

    print "ok 1\n";
    ...
}

The two are not exactly equivalent, but the latter might be clearer and will restore STDOUT if the wrapped code dies. The difference is that in the former, the original STDOUT can still be accessed by explicitly using it in a print statement (as print STDOUT ...), whereas in the latter the meaning of the STDOUT handle itself has temporarily been changed.

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.