CXXXII. Semaphore, Shared Memory and IPC Functions

Введение

This module provides wrappers for the System V IPC family of functions. It includes semaphores, shared memory and inter-process messaging (IPC).

Semaphores may be used to provide exclusive access to resources on the current machine, or to limit the number of processes that may simultaneously use a resource.

This module provides also shared memory functions using System V shared memory. Shared memory may be used to provide access to global variables. Different httpd-daemons and even other programs (such as Perl, C, ...) are able to access this data to provide a global data-exchange. Remember, that shared memory is NOT safe against simultaneous access. Use semaphores for synchronization.

Таблица 1. Limits of Shared Memory by the Unix OS

SHMMAX	max size of shared memory, normally 131072 bytes
SHMMIN	minimum size of shared memory, normally 1 byte
SHMMNI	max amount of shared memory segments on a system, normally 100
SHMSEG	max amount of shared memory segments per process, normally 6

The messaging functions may be used to send and receive messages to/from other processes. They provide a simple and effective means of exchanging data between processes, without the need for setting up an alternative using Unix domain sockets.

Замечание: Для Windows-платформ это расширение недоступно.

Требования

Эти функции всегда доступны.

Установка

Support for this functions are not enabled by default. To enable System V semaphore support compile PHP with the option --enable-sysvsem. To enable the System V shared memory support compile PHP with the option --enable-sysvshm. To enable the System V messages support compile PHP with the option --enable-sysvmsg.

Настройка во время выполнения

Поведение этих функций зависит от установок в php.ini.

Таблица 2. Semaphore Configuration Options

Name	Default	Changeable	Changelog
sysvmsg.value	"42"	PHP_INI_ALL
sysvmsg.string	"foobar"	PHP_INI_ALL

Для подробного описания констант PHP_INI_*, обратитесь к документации функции ini_set().

Типы ресурсов

Предопределенные константы

Перечисленные ниже константы определены данным расширением и могут быть доступны только в том случае, если PHP был собран с поддержкой этого расширения или же в том случае, если данное расширение подгружается во время выполнения.

Таблица 3. System V message constants

Constant	Type
MSG_IPC_NOWAIT	integer
MSG_NOERROR	integer
MSG_EXCEPT	integer

Содержание
ftok -- Convert a pathname and a project identifier to a System V IPC key
msg_get_queue -- Create or attach to a message queue
msg_receive -- Receive a message from a message queue
msg_remove_queue -- Destroy a message queue
msg_send -- Send a message to a message queue
msg_set_queue -- Set information in the message queue data structure
msg_stat_queue -- Returns information from the message queue data structure
sem_acquire -- Acquire a semaphore
sem_get -- Get a semaphore id
sem_release -- Release a semaphore
sem_remove -- Remove a semaphore
shm_attach -- Creates or open a shared memory segment
shm_detach -- Disconnects from shared memory segment
shm_get_var -- Returns a variable from shared memory
shm_put_var -- Inserts or updates a variable in shared memory
shm_remove_var -- Removes a variable from shared memory
shm_remove -- Removes shared memory from Unix systems

Semaphore, Shared Memory and IPC Functions

chrissavery at removeme dot gmail dot com
02-Jun-2006 01:18


I was confused by two things that caused strange behaviour in my use of semaphores with php scripts running under apache.



Often enough page requests will end up being filled by the same process as other simultaneous requests. So semaphores will block when you may not have expected.



Also note that sem_remove() will remove it for all processes, not just the calling one. So you have to be sure that the last process running removes the semaphore and none before. I thought there was some failures occurring when my child processes were dropping out with errors.



So you can't just use get, acquire, release, remove in one script that will be hit by a web user. (1) They may end up in the same process and will wait on the other, and (2) the first one to finish will destroy the semaphore for others.



I left out the remove call, and it works ok, but I still wonder if the semaphore is removed by php when the last script that did a get finishes? Also creating a child process to do the work using proc_open works to ensure seperate processes but to be careful you would want to limit the number somehow as well.

Denis Gerasimov
02-Nov-2005 04:19


Many (most?) developers use Win32 platform for PHP Web applications development while production servers mostly run Unix/Linux OS. Below is the stub code I use to make it possible to write scripts on Win32 that use semaphores:



<?php



if (substr(PHP_OS, 0, 3) == 'WIN') {  // if Windows OS detected



    function ftok($pathname, $proj)

    {

        if (empty($pathname) || !file_exists($pathname)) {  // an error occured

            return -1;

        }

        

        $pathname = $pathname . (string) $proj;

        $key = array();

        while (sizeof($key) < strlen($pathname)) {

            $key[] = ord(substr($pathname, sizeof($key), 1));

        }

        

        return dechex(array_sum($key));

    }

    

    function sem_acquire($sem_identifier)

    {

        return true;

    }



    function sem_get($key, $max_acquire = null, $perm = null, $auto_release = null)

    {

        return true;

    }

    

    function sem_release($sem_identifier)

    {

        return true;

    }

    

    function sem_remove($sem_identifier)

    {

        return true;

    }

    

}



?>



Of course, there is no way to test semaphores until you have no Unix/Linux test server.

p4nzer at yahoo dot com
01-Nov-2005 05:55


Actually, the way to lock a semaphore from C code appears to be:

<?

    struct sembuf semptr[2];



    if( (semid = semget(SEM_KEY, PHP_SEM_NEED_NUMBER, 0666 | IPC_CREAT)) < 0 ) {

        perror("semget");

        return 1;

    }



    semptr[0].sem_num = 0;

    semptr[0].sem_op = -1; 

    semptr[0].sem_flg = SEM_UNDO;

    semptr[1].sem_num = 1;

    semptr[1].sem_op =  1; 

    semptr[1].sem_flg = SEM_UNDO;



    if( semop(semid, &semptr[0], 2) < 0 ) {

    perror("semop");

    }

?>



PS: Disregard the <? and ?>, it's just for pretty printing... this is C code!

04-Feb-2004 08:25


Here is an article/tutorial on the subject:

http://www.php-mag.net/itr/online_artikel/psecom,id,448,nodeid,114

hcuevas at galenicom dot com
23-Oct-2003 07:49


Don't use semaphores to serialize access to an undefined number of resources. There is no way (yet) to know before locking if a semaphore is already locked, thus not being able to fully release the semaphore and occupying a semaphore resource for an undefined time. 



A possible solution is to build a shared mem pool and store there the current number of locks for a semaphore id. 



Cheers,

Horaci Cuevas

david () php net
09-Jul-2003 01:45


Here is a quick utility to dump the contents of a shm_ format memory segment:



http://www.davidc.net/php/shm/

Roman Laptev <tmp at laptev dot org>
02-Apr-2003 06:50


If you going to work with semaphore, which was created by some external program, you can try the following code for this program (C example):



#define SVSEM_MODE (SEM_R | SEM_A | SEM_R>>3 | SEM_R>>6) /* 0644 */

#define PHP_SEM_NEED_NUMBER 3



/*.......*/

int semid, semflag = SVSEM_MODE | IPC_CREAT | IPC_EXCL;

struct sembuf semptr;

union semun semopts;

/*.......*/

if( (semid = semget(sempath, PHP_SEM_NEED_NUMBER, semflag)) >= 0 ) {

  semopts.val = 1; /* initial value for sem */

  if( semctl( semid, 0, SETVAL, semopts) < 0 ) {/*error*/}

  if( semctl( semid, 1, SETVAL, semopts) < 0 ) {/*error*/}

  /* PHP wanna zero for its own semget at third sem.

   * look at ./PHP_SOURCE_PATH/ext/sysvsem/sysvsem.c

   */

  semopts.val = 0;

  if( semctl( semid, 2, SETVAL, semopts) < 0 ) {/*error*/}

}

else if(errno == EEXIST) { /* connect only  */

  if( (semid = semget(sempath, PHP_SEM_NEED_NUMBER, SVSEM_MODE | IPC_CREAT)) < 0 ) {/*error*/}

}

else {/*error*/}



/*.......*/

/* If you want acquire the sem */

  semptr.sem_num = 0;

  semptr.sem_op = -1; /* lock it */

  semptr.sem_flg = SEM_UNDO;

  while( semop(semid, &semptr, 1) < 0 ) {/*error*/}

/*.......*/



Thanks,

Roma

elran70 at hotmail dot com
22-Jun-2002 10:54


Samlpe code for using most of the functions here:



    $MEMSIZE    =   512;//  size of shared memory to allocate

    $SEMKEY     =   1;  //  Semaphore key

    $SHMKEY     =   2;  //  Shared memory key



    echo "Start.\n";

    // Get semaphore

    $sem_id = sem_get($SEMKEY, 1);

    if ($sem_id === false)

    {

        echo "Fail to get semaphore";

        exit;

    }

    else

        echo "Got semaphore $sem_id.\n";



    // Accuire semaphore

    if (! sem_acquire($sem_id))

    {

        echo "Fail to aquire semaphore $sem_id.\n";

        sem_remove($sem_id);

        exit;

    }

    else

        echo "Success aquire semaphore $sem_id.\n";



    $shm_id =   shm_attach($SHMKEY, $MEMSIZE);

    if ($shm_id === false)

    {

        echo "Fail to attach shared memory.\n";

        sem_remove($sem_id);

        exit;

    }

    else

        echo "Success to attach shared memory : $shm_id.\n";



    // Write variable 1

    if (!shm_put_var($shm_id, 1, "Variable 1"))

    {

        echo "Fail to put var 1 on shared memory $shm_id.\n";

        sem_remove($sem_id);

        shm_remove ($shm_id);

        exit;

    }

    else

        echo "Write var1 to shared memory.\n";



    // Write variable 2

    if (!shm_put_var($shm_id, 2, "Variable 2"))

    {

        echo "Fail to put var 2 on shared memory $shm_id.\n";

        sem_remove($sem_id);

        shm_remove ($shm_id);

        exit;

    }

    else

        echo "Write var2 to shared memory.\n";



    // Read variable 1

    $var1   =   shm_get_var ($shm_id, 1);

    if ($var1 === false)

    {

        echo "Fail to retrive Var 1 from Shared memory $shm_id, return value=$var1.\n";

    }

    else

        echo "Read var1=$var1.\n";



    // Read variable 1

    $var2   =   shm_get_var ($shm_id, 2);

    if ($var1 === false)

    {

        echo "Fail to retrive Var 2 from Shared memory $shm_id, return value=$var2.\n";

    }

    else

        echo "Read var2=$var2.\n";



    // Release semaphore

    if (!sem_release($sem_id))

        echo "Fail to release $sem_id semaphore.\n";

    else

        echo "Semaphore $sem_id released.\n";



    // remove shared memory segmant from SysV

    if (shm_remove ($shm_id))

        echo "Shared memory successfully removed from SysV.\n";

    else

        echo "Fail to remove $shm_id shared memory from SysV.\n";



    // Remove semaphore

    if (sem_remove($sem_id))

        echo "semaphore removed successfully from SysV.\n";

    else

        echo "Fail to remove $sem_id semaphore from SysV.\n";

    echo "End.\n";

mikew at php dot net
31-May-2001 09:46


As for security,   please look at the perm argument to shm_get.   Shared Memory blocks has the same permission semantics as unix user/group/other file permissions.   As long as your webserver is running as a user that no other users can script to..  and as long as the permissions are set to 600,  you should be fine and have no security concerns.

php at stolt dot de
20-Sep-2000 11:58


The integer keys for sem_get() and shm_attach() have to be systemwide unique. There is no method to ensure that no other process on the system will use your specific key (security! and possible malfunction). Also shared memory is very seldom used there are possibilities for conflicts! To see the used id's you can use the program 'ipcs' (at least under SuseLinux;) ). Thanks Christian C.