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Manual Reference Pages  -  IO::SELECT (3)

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IO::Select - OO interface to the select system call



    use IO::Select;

    $s = IO::Select->new();


    @ready = $s->can_read($timeout);

    @ready = IO::Select->new(@handles)->can_read(0);


The IO::Select package implements an object approach to the system select function call. It allows the user to see what IO handles, see IO::Handle, are ready for reading, writing or have an exception pending.


new ( [ HANDLES ] ) The constructor creates a new object and optionally initialises it with a set of handles.


add ( HANDLES ) Add the list of handles to the IO::Select object. It is these values that will be returned when an event occurs. IO::Select keeps these values in a cache which is indexed by the fileno of the handle, so if more than one handle with the same fileno is specified then only the last one is cached.

Each handle can be an IO::Handle object, an integer or an array reference where the first element is an IO::Handle or an integer.

remove ( HANDLES ) Remove all the given handles from the object. This method also works by the fileno of the handles. So the exact handles that were added need not be passed, just handles that have an equivalent fileno
exists ( HANDLE ) Returns a true value (actually the handle itself) if it is present. Returns undef otherwise.
handles Return an array of all registered handles.
can_read ( [ TIMEOUT ] ) Return an array of handles that are ready for reading. TIMEOUT is the maximum amount of time to wait before returning an empty list, in seconds, possibly fractional. If TIMEOUT is not given and any handles are registered then the call will block.
can_write ( [ TIMEOUT ] ) Same as can_read except check for handles that can be written to.
has_exception ( [ TIMEOUT ] ) Same as can_read except check for handles that have an exception condition, for example pending out-of-band data.
count () Returns the number of handles that the object will check for when one of the can_ methods is called or the object is passed to the select static method.
bits() Return the bit string suitable as argument to the core select() call.
select ( READ, WRITE, EXCEPTION [, TIMEOUT ] ) select is a static method, that is you call it with the package name like new. READ, WRITE and EXCEPTION are either undef or IO::Select objects. TIMEOUT is optional and has the same effect as for the core select call.

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 error an empty list is returned.


Here is a short example which shows how IO::Select could be used to write a server which communicates with several sockets while also listening for more connections on a listen socket

    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;
            else {
                # Process socket

                # Maybe we have finished with the socket


Graham Barr. Currently maintained by the Perl Porters. Please report all bugs to <>.


Copyright (c) 1997-8 Graham Barr <>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
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perl v5.22.1 IO::SELECT (3) 2015-10-17

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