Perl Programming Documentation
Download Perl
Explore

Manual

- Overview
- Tutorials
- FAQs
- History / Changes
- License

Reference

- Language
- Functions
- Operators
- Special Variables
- Pragmas
- Utilities
- Internals
- Platform Specific

Modules

- A • B • C • D • E
- F • G • H • I • L
- M • N • O • P • S
- T • U • X

Tools

- Preferences

# select

Perl 5 version 16.0 documentation
Go to top
Show recent pages
Recently read

# 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:

          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);

  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:

          $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 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;    }    $rin = fhbits(*STDIN, *TTY, *MYSOCK);

  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 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 <FH>) 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.