IO::Select

Section: Perl Programmers Reference Guide (3perl)
Updated: 2019-04-20
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NAME

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

SYNOPSIS

    use IO::Select;

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

    $s->add(\*STDIN);
    $s->add($some_handle);

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

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

DESCRIPTION

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.

CONSTRUCTOR

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

METHODS

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 (with $! unchanged), in seconds, possibly fractional. If "TIMEOUT" is not given and any handles are registered then the call will block indefinitely. Upon error, an empty list is returned, with $! set to indicate the error. To distinguish between timeout and error, set $! to zero before calling this method, and check it after an empty list is returned.
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.
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 $! unchanged. Upon error, an empty list is returned, with $! set to indicate the error. To distinguish between timeout and error, set $! to zero before calling this method, and check it after an empty list is returned.

EXAMPLE

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;
                $sel->add($new);
            }
            else {
                # Process socket

                # Maybe we have finished with the socket
                $sel->remove($fh);
                $fh->close;
            }
        }
    }

AUTHOR

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