select
select - reset default output or do I/O multiplexing
select FILEHANDLE
select
select RBITS,WBITS,EBITS,TIMEOUT
Returns the currently selected filehandle. Sets the current default
filehandle for output, if FILEHANDLE is supplied. 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:
<![CDATA[
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
]]>
FILEHANDLE may be an expression whose value gives the name of the
actual filehandle. Thus:
<![CDATA[
$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:
<![CDATA[
use FileHandle;
STDERR->autoflush(1);
]]>
This calls the select(2) system call with the bitmasks specified, which
can be constructed using
fileno()
and
vec()
, along these lines:
<![CDATA[
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
]]>
If you want to select on many filehandles you might wish to write a
subroutine:
<![CDATA[
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
]]>
The usual idiom is:
<![CDATA[
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
]]>
or to block until something becomes ready:
<![CDATA[
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
]]>
Any of the bitmasks 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 250-microsecond sleep this way:
<![CDATA[
select(undef, undef, undef, 0.25);
]]>
WARNING: Do not attempt to mix buffered I/O (like
read()
or <FH>)
with
select()
. You have to use
sysread()
instead.