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select

Perl 5 version 26.1 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 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:

    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.     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:

    1.     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:

    1.     my $rin = my $win = my $ein = '';
    2.     vec($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 = @_;
    3.         my $bits = "";
    4.         for my $fh (@fhlist) {
    5.             vec($bits, fileno($fh), 1) = 1;
    6.         }
    7.         return $bits;
    8.     }
    9.     my $rin = fhbits(\*STDIN, $tty, $mysock);

    The usual idiom is:

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

    or to block until something becomes ready just do this

    1.  my $nfound =
    2.    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:

    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 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.