select - reset default output or do I/O multiplexing

- NAME
- SYNOPSIS
- DESCRIPTION

# NAME

select - reset default output or do I/O multiplexing

# SYNOPSIS

select FILEHANDLE

select

select RBITS,WBITS,EBITS,TIMEOUT

# DESCRIPTION

Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a `write` or a `print` without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have 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:

        $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);

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

On error, `select` behaves like the `select(2)` system call : it returns -1 and sets `$!`.

Note: on some Unixes, the `select(2)` system call may report a socket file descriptor as ``ready for reading'', when actually no data is available, thus a subsequent read blocks. It can be avoided using always the O_NONBLOCK flag on the socket. See `select(2)` and `fcntl(2)` for further details.

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.

select - reset default output or do I/O multiplexing