PHP: socket_select

Description

int socket_select ( array &read, array &write, array &except, int tv_sec [, int tv_usec] )

socket_select() accepts arrays of sockets and waits for them to change status. Those coming with BSD sockets background will recognize that those socket resource arrays are in fact the so-called file descriptor sets. Three independent arrays of socket resources are watched.

The sockets listed in the read array will be watched to see if characters become available for reading (more precisely, to see if a read will not block - in particular, a socket resource is also ready on end-of-file, in which case a socket_read() will return a zero length string).

The sockets listed in the write array will be watched to see if a write will not block.

The sockets listed in the except array will be watched for exceptions.

Внимание

On exit, the arrays are modified to indicate which socket resource actually changed status.

You do not need to pass every array to socket_select(). You can leave it out and use an empty array or NULL instead. Also do not forget that those arrays are passed by reference and will be modified after socket_select() returns.

Пример 1. socket_select() example

<?php /* Prepare the read array */ $read = array($socket1, $socket2); $num_changed_sockets = socket_select($read, $write = NULL, $except = NULL, 0); if ($num_changed_sockets === false) { /* Error handling */ } else if ($num_changed_sockets > 0) { /* At least at one of the sockets something interesting happened */ } ?>

Замечание: Due a limitation in the current Zend Engine it is not possible to pass a constant modifier like NULL directly as a parameter to a function which expects this parameter to be passed by reference. Instead use a temporary variable or an expression with the leftmost member being a temporary variable:
Пример 2. Using NULL with socket_select()

<?php socket_select($r, $w, $e = NULL, 0); ?>

The tv_sec and tv_usec together form the timeout parameter. The timeout is an upper bound on the amount of time elapsed before socket_select() return. tv_sec may be zero , causing socket_select() to return immediately. This is useful for polling. If tv_sec is NULL (no timeout), socket_select() can block indefinitely.

On success socket_select() returns the number of socket resources contained in the modified arrays, which may be zero if the timeout expires before anything interesting happens. On error FALSE is returned. The error code can be retrieved with socket_last_error().

Замечание: Be sure to use the === operator when checking for an error. Since the socket_select() may return 0 the comparison with == would evaluate to TRUE:
Пример 3. Understanding socket_select()'s result

<?php if (false === socket_select($r, $w, $e = NULL, 0)) { echo "socket_select() failed, reason: " . socket_strerror(socket_last_error()) . "\n"; } ?>

Замечание: Be aware that some socket implementations need to be handled very carefully. A few basic rules:
You should always try to use socket_select() without timeout. Your program should have nothing to do if there is no data available. Code that depends on timeouts is not usually portable and difficult to debug.
No socket resource must be added to any set if you do not intend to check its result after the socket_select() call, and respond appropriately. After socket_select() returns, all socket resources in all arrays must be checked. Any socket resource that is available for writing must be written to, and any socket resource available for reading must be read from.
If you read/write to a socket returns in the arrays be aware that they do not necessarily read/write the full amount of data you have requested. Be prepared to even only be able to read/write a single byte.
It's common to most socket implementations that the only exception caught with the except array is out-of-bound data received on a socket.

socket_select

Whosawhatsis at that google email thingy
19-Apr-2006 07:35


Another solution to the problem of keys not being preserved is to have an additional array for looking up sockets that uses their resource identifiers as keys. This can be obtained using array_flip() in some cases, but is particularly useful if each socket is associated with an object. In this case, you can make the object's constructor add a pointer to itself to the lookup array with its socket resource identifier as a key and use the following code to execute a read method for the object associated with each socket returned by socket_select():



<?php

socket_select($reads, $writes, $excepts, 0);



foreach ($sockets as $socket) {

    $lookuparray[$socket]->read();

}

?>

ludvig dot ericson at gmail dot com
01-Nov-2005 05:35


Regarding the comment below, No, it does not, it's a system call and I believe it's rather hard to preserve keys.



Additionally, socket_select should be used like it was a user-inputted array, that you don't know what you sent in to.



<?php

$reads = $clients;

$reads[] = $server;



socket_select($reads);



foreach ($reads as $read) {

    /* do some stuff */

}

?>

crimson at NOSPAMtechnologist dot com
30-Sep-2005 04:15


Note that the resulting arrays do NOT maintain keys (PHP 4.3.2) after being run through this function:



Before:

Array

(

    [Client_Socket] => Resource id #6

    [Server_Socket] => Resource id #9

)



After:

Array

(

    [0] => Resource id #6

    [1] => Resource id #9

)



It would have been nice to have the keys stay to figure out which stream you need to receive from, but you'll have to use some fancy foreach loop to figure out which sockets to check.

vardhan ( at ) rogers ( dot ) com
28-Aug-2005 07:46


A simple PHP script using socket_select() to manage multiple connections.

connect using "telnet localhost 9050". it broadcasts your messages that you send through telnet to other users connected to the server -- sort of like a chat script



#!/usr/local/bin/php

<?php



    $port = 9050;

    

    // create a streaming socket, of type TCP/IP

    $sock = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);

    

    // set the option to reuse the port

    socket_set_option($sock, SOL_SOCKET, SO_REUSEADDR, 1);

    

    // "bind" the socket to the address to "localhost", on port $port

    // so this means that all connections on this port are now our resposibility to send/recv data, disconnect, etc..

    socket_bind($sock, 0, $port);

    

    // start listen for connections

    socket_listen($sock);



    // create a list of all the clients that will be connected to us..

    // add the listening socket to this list

    $clients = array($sock);

    

    while (true) {

        // create a copy, so $clients doesn't get modified by socket_select()

        $read = $clients;

        

        // get a list of all the clients that have data to be read from

        // if there are no clients with data, go to next iteration

        if (socket_select($read, $write = NULL, $except = NULL, 0) < 1)

            continue;

        

        // check if there is a client trying to connect

        if (in_array($sock, $read)) {

            // accept the client, and add him to the $clients array

            $clients[] = $newsock = socket_accept($sock);

            

            // send the client a welcome message

            socket_write($newsock, "no noobs, but ill make an exception :)\n".

            "There are ".(count($clients) - 1)." client(s) connected to the server\n");

            

            socket_getpeername($newsock, $ip);

            echo "New client connected: {$ip}\n";

            

            // remove the listening socket from the clients-with-data array

            $key = array_search($sock, $read);

            unset($read[$key]);

        }

        

        // loop through all the clients that have data to read from

        foreach ($read as $read_sock) {

            // read until newline or 1024 bytes

            // socket_read while show errors when the client is disconnected, so silence the error messages

            $data = @socket_read($read_sock, 1024, PHP_NORMAL_READ);

            

            // check if the client is disconnected

            if ($data === false) {

                // remove client for $clients array

                $key = array_search($read_sock, $clients);

                unset($clients[$key]);

                echo "client disconnected.\n";

                // continue to the next client to read from, if any

                continue;

            }

            

            // trim off the trailing/beginning white spaces

            $data = trim($data);

            

            // check if there is any data after trimming off the spaces

            if (!empty($data)) {

            

                // send this to all the clients in the $clients array (except the first one, which is a listening socket)

                foreach ($clients as $send_sock) {

                

                    // if its the listening sock or the client that we got the message from, go to the next one in the list

                    if ($send_sock == $sock || $send_sock == $read_sock)

                        continue;

                    

                    // write the message to the client -- add a newline character to the end of the message

                    socket_write($send_sock, $data."\n");

                    

                } // end of broadcast foreach

                

            }

            

        } // end of reading foreach

    }



    // close the listening socket

    socket_close($sock);

?>

drenintell
01-May-2005 01:13


The continuation of my my previous post on 28-Apr-2005 10:19 at

http://ca3.php.net/manual/en/function.socket-select.php



Here it is: (Link is broken into 2 parts)



'http://gtkphp.org/php_socket_select_hangs

_explanation_and_solution.html'

Richard Neill
16-Sep-2004 09:37


It is probably a bad idea to watch an array of sockets for input with socket_select, and then socket_read() using PHP_NORMAL_READ.



Although this seems desirable, you can end up with a permanently blocked program, if someone sends you malformed input which is missing a trailing \n \r. Guess how I found that out.

calimero dot NOSPAM at NOSPAM dot creatixnet dot com
14-Feb-2003 04:36


Please note that the timeout parameter has important side-effects on the CPU usage of your script.



Setting the timeout to 0 will make your CPU looping without any time to have some rest and handle other running processes on your system, causing the system load to increase heavily while your script is running.



Personnaly, I use a value of 15 ms for this parameter. this ensures a good listening frequency while letting your system load clear.



Example :

$read = array($ListeningSocket);

$num_changed_sockets = socket_select($read, $write = NULL, $except = NULL, 0, 10);



Hope this helps.

daveb at optusnet dot com dot au
09-Sep-2002 09:27


If you haven't done any network programming before, PHP's socket_select() might appear a bit strange to you. I've written a simple php "partyline" script to demonstrate the multi-socket use of select'ing at http://dave.dapond.com/socketselect.php.txt

julian dot haupt at gmx dot de
09-Jul-2002 10:15


hello,


i just made a class which acts similiar to Perl's IO::Select in order to make socket selecting very easy





your script should look something like that:





<?php


$server = new Server;


$client = new Client;





for (;;) {


  foreach ($select->can_read(0) as $socket) {





    if ($socket == $client->socket) {


      // New Client Socket


      $select->add(socket_accept($client->socket));


    }


    else {


      //there's something to read on $socket


    }


  }


}


?>





you should of course implement some routines to detect broken sockets and remove them from the select object.





you can also do output buffering and check in the main-loop for sockets that are ready to write





<?php


class select {


  var $sockets;





  function select($sockets) {





    $this->sockets = array();





    foreach ($sockets as $socket) {


      $this->add($socket);


    }


  }





  function add($add_socket) {


    array_push($this->sockets,$add_socket);


  }





  function remove($remove_socket) {


    $sockets = array();





    foreach ($this->sockets as $socket) {


      if($remove_socket != $socket)


        $sockets[] = $socket;


    }





    $this->sockets = $sockets;


  }





  function can_read($timeout) {


    $read = $this->sockets;


    socket_select($read,$write = NULL,$except = NULL,$timeout);


    return $read;


  }





  function can_write($timeout) {


    $write = $this->sockets;


    socket_select($read = NULL,$write,$except = NULL,$timeout);


    return $write;


  }


}


?>