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 2 3 4 | select(REPORT1);$^ = 'report1_top';select(REPORT2);$^ = '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 2 | 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:
1 2 3 4 | $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:
1 2 3 4 5 6 7 8 9 | 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:
1 2 | ($nfound,$timeleft) = 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 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.
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