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:
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:
STDERR->autoflush(1);
(Prior to Perl version 5.14, you have to use IO::Handle;
explicitly first.)
Portability issues: "select" in perlport.
This calls the select(2) syscall with the bit masks specified, which can be constructed using fileno
and vec
, along these lines:
my $rin = my $win = my $ein = '';
vec($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;
}
my $rin = fhbits(\*STDIN, $tty, $mysock);
The usual idiom is:
my ($nfound, $timeleft) =
select(my $rout = $rin, my $wout = $win, my $eout = $ein,
$timeout);
or to block until something becomes ready just do this
my $nfound =
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:
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.