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.\"
.IX Title "IO::Select 3perl"
.TH IO::Select 3perl "2023-11-25" "perl v5.36.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
IO::Select \- OO interface to the select system call
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use IO::Select;
\&
\&    $s = IO::Select\->new();
\&
\&    $s\->add(\e*STDIN);
\&    $s\->add($some_handle);
\&
\&    @ready = $s\->can_read($timeout);
\&
\&    @ready = IO::Select\->new(@handles)\->can_read(0);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \f(CW\*(C`IO::Select\*(C'\fR package implements an object approach to the system \f(CW\*(C`select\*(C'\fR
function call. It allows the user to see what \s-1IO\s0 handles, see IO::Handle,
are ready for reading, writing or have an exception pending.
.SH "CONSTRUCTOR"
.IX Header "CONSTRUCTOR"
.IP "new ( [ \s-1HANDLES\s0 ] )" 4
.IX Item "new ( [ HANDLES ] )"
The constructor creates a new object and optionally initialises it with a set
of handles.
.SH "METHODS"
.IX Header "METHODS"
.IP "add ( \s-1HANDLES\s0 )" 4
.IX Item "add ( HANDLES )"
Add the list of handles to the \f(CW\*(C`IO::Select\*(C'\fR object. It is these values that
will be returned when an event occurs. \f(CW\*(C`IO::Select\*(C'\fR keeps these values in a
cache which is indexed by the \f(CW\*(C`fileno\*(C'\fR of the handle, so if more than one
handle with the same \f(CW\*(C`fileno\*(C'\fR is specified then only the last one is cached.
.Sp
Each handle can be an \f(CW\*(C`IO::Handle\*(C'\fR object, an integer or an array
reference where the first element is an \f(CW\*(C`IO::Handle\*(C'\fR or an integer.
.IP "remove ( \s-1HANDLES\s0 )" 4
.IX Item "remove ( HANDLES )"
Remove all the given handles from the object. This method also works
by the \f(CW\*(C`fileno\*(C'\fR of the handles. So the exact handles that were added
need not be passed, just handles that have an equivalent \f(CW\*(C`fileno\*(C'\fR
.IP "exists ( \s-1HANDLE\s0 )" 4
.IX Item "exists ( HANDLE )"
Returns a true value (actually the handle itself) if it is present.
Returns undef otherwise.
.IP "handles" 4
.IX Item "handles"
Return an array of all registered handles.
.IP "can_read ( [ \s-1TIMEOUT\s0 ] )" 4
.IX Item "can_read ( [ TIMEOUT ] )"
Return an array of handles that are ready for reading.  \f(CW\*(C`TIMEOUT\*(C'\fR is the
maximum amount of time to wait before returning an empty list (with \f(CW$!\fR
unchanged), in seconds, possibly fractional.  If \f(CW\*(C`TIMEOUT\*(C'\fR is not given
and any handles are registered then the call will block indefinitely.
Upon error, an empty list is returned, with \f(CW$!\fR set to indicate the
error.  To distinguish between timeout and error, set \f(CW$!\fR to zero
before calling this method, and check it after an empty list is returned.
.IP "can_write ( [ \s-1TIMEOUT\s0 ] )" 4
.IX Item "can_write ( [ TIMEOUT ] )"
Same as \f(CW\*(C`can_read\*(C'\fR except check for handles that can be written to.
.IP "has_exception ( [ \s-1TIMEOUT\s0 ] )" 4
.IX Item "has_exception ( [ TIMEOUT ] )"
Same as \f(CW\*(C`can_read\*(C'\fR except check for handles that have an exception
condition, for example pending out-of-band data.
.IP "count ()" 4
.IX Item "count ()"
Returns the number of handles that the object will check for when
one of the \f(CW\*(C`can_\*(C'\fR methods is called or the object is passed to
the \f(CW\*(C`select\*(C'\fR static method.
.IP "\fBbits()\fR" 4
.IX Item "bits()"
Return the bit string suitable as argument to the core \fBselect()\fR call.
.IP "select ( \s-1READ, WRITE, EXCEPTION\s0 [, \s-1TIMEOUT\s0 ] )" 4
.IX Item "select ( READ, WRITE, EXCEPTION [, TIMEOUT ] )"
\&\f(CW\*(C`select\*(C'\fR is a static method, that is you call it with the package name
like \f(CW\*(C`new\*(C'\fR. \f(CW\*(C`READ\*(C'\fR, \f(CW\*(C`WRITE\*(C'\fR and \f(CW\*(C`EXCEPTION\*(C'\fR are either \f(CW\*(C`undef\*(C'\fR or
\&\f(CW\*(C`IO::Select\*(C'\fR objects. \f(CW\*(C`TIMEOUT\*(C'\fR is optional and has the same effect as
for the core select call.
.Sp
If at least one handle is ready for the specified kind of operation,
the result will be an array of 3 elements, each a reference to an array
which will hold the handles that are ready for reading, writing and
have exceptions respectively.  Upon timeout, an empty list is returned,
with \f(CW$!\fR unchanged.  Upon error, an empty list is returned, with \f(CW$!\fR
set to indicate the error.  To distinguish between timeout and error,
set \f(CW$!\fR to zero before calling this method, and check it after an
empty list is returned.
.SH "EXAMPLE"
.IX Header "EXAMPLE"
Here is a short example which shows how \f(CW\*(C`IO::Select\*(C'\fR could be used
to write a server which communicates with several sockets while also
listening for more connections on a listen socket
.PP
.Vb 2
\&    use IO::Select;
\&    use IO::Socket;
\&
\&    $lsn = IO::Socket::INET\->new(Listen => 1, LocalPort => 8080);
\&    $sel = IO::Select\->new( $lsn );
\&
\&    while(@ready = $sel\->can_read) {
\&        foreach $fh (@ready) {
\&            if($fh == $lsn) {
\&                # Create a new socket
\&                $new = $lsn\->accept;
\&                $sel\->add($new);
\&            }
\&            else {
\&                # Process socket
\&
\&                # Maybe we have finished with the socket
\&                $sel\->remove($fh);
\&                $fh\->close;
\&            }
\&        }
\&    }
.Ve
.SH "AUTHOR"
.IX Header "AUTHOR"
Graham Barr. Currently maintained by the Perl Porters.  Please report all
bugs at <https://github.com/Perl/perl5/issues>.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 1997\-8 Graham Barr <gbarr@pobox.com>. All rights reserved.
This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.