CIX. Semaphore, Shared Memory and IPC Functions

Wstęp

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.

Tabela 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.

Notatka: To rozszerzenie nie jest dostępne na platformie Windows.

Wymagania

Do zbudowania tego rozszerzenia nie są wymagane żadne zewnętrzne biblioteki.

Instalacja

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.

Konfiguracja czasu wykonywania

Na działanie tych funcji wpływają ustawienia zawarte w pliku php.ini.

Tabela 2. Semaphore Configuration Options

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

Szczegóły i definicje dotyczące stałych PHP_INI_* znajdują się w rozdziale Dodatek H.

Typy zasobów

Stałe predefinopwane

Poniższe stałe są zdefiniowane w tym rozszerzeniu i stają się dostępne, gdy rozszerzenie jest dokompilowane do PHP, lub załadowane dynamicznie przy starcie.

Tabela 3. System V message constants

Constant	Type
MSG_IPC_NOWAIT	integer
MSG_NOERROR	integer
MSG_EXCEPT	integer

Spis treści
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

User Contributed Notes


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.


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


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";