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select

Perl 5 version 12.2 documentation
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select

Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage
  • 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 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:

    1.     select(REPORT1);
    2.     $^ = 'report1_top';
    3.     select(REPORT2);
    4.     $^ = 'report2_top';

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

    1.     $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:

    1.     use IO::Handle;
    2.     STDERR->autoflush(1);
  • 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:

    1.     $rin = $win = $ein = '';
    2.     vec($rin,fileno(STDIN),1) = 1;
    3.     vec($win,fileno(STDOUT),1) = 1;
    4.     $ein = $rin | $win;

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

    1.     sub fhbits {
    2.         my(@fhlist) = split(' ',$_[0]);
    3.         my($bits);
    4.         for (@fhlist) {
    5.             vec($bits,fileno($_),1) = 1;
    6.         }
    7.         $bits;
    8.     }
    9.     $rin = fhbits('STDIN TTY SOCK');

    The usual idiom is:

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

    or to block until something becomes ready just do this

    1.     $nfound = select($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:

    1.     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 like select(2): it returns -1 and sets $! .

    On some Unixes, select(2) may report a socket file descriptor as "ready for reading" when no data is available, and thus a subsequent read blocks. This can be avoided if you always use O_NONBLOCK 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.