XCVII. Semaphore, Shared Memory and IPC Functions

�vod

Toto roz��en� poskytuje rozhran� k rodin� IPC funkc� Systemu V. To zahrnuje semafory, sd�lenou pam� a meziprocesov� zpr�vy (IPC).

Semafory se daj� pou��vat k poskytov�n� exkluzivn�ho p��stupu k prost�edk�m na dan�m syst�mu, nebo k omezen� po�tu proces�, kter� mohou sou�asn� pou��vat ur�it� prost�edek.

Toto roz��en� tak� poskytuje funkce pro pr�ci se sd�lenou pam�t� vyu��vaj�c� System V sd�lenou pam�. Sd�len� pm�t se d� pou��vat k poskytov�n� p��stupu ke glob�ln�m prom�nn�m. R�zn� httpd-daemoni a dokonce i jin� programy (nap�. Perl, C, ...) mohou k t�mto dat�m p�istupovat, a vytvo�it tak glob�ln� v�m�nu dat. Pamatujte, �e sd�len� pam� nen� chr�n�na proti simult�n�m p��stup�m. K synchronizaci pou�ijte semafory.

Tabulka 1. Omezen� sd�len� pam�ti syst�mem Unix

SHMMAX	max. velikost sd�len� pam�ti, norm�ln� 131072 byt�
SHMMIN	min. velikost sd�len� pam�ti, norm�lne 1 byte
SHMMNI	max. po�et segment� sd�len� pam�ti, norm�ln� 100
SHMSEG	max. po�et segment� sd�len� pam�ti na proces, norm�ln� 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.

Pozn�mka: Toto roz��en� nen� k dispozici na platform�ch Windows.

Po�adavky

Tyto funkce jsou k dispozici jako sou��st standardn�ho modulu, kter� je v�dy dostupn�.

Instalace

Podpora t�chto funkc� nen� ve v�choz�m nastaven� zapnuta. Pro zapnut� podpory semafor� Systemu V zkompilujte PHP s volbou --enable-sysvsem. Pro zapnut� podpory sd�len� pam�ti Systemu V zkompilujte PHP s volbou --enable-sysvshm. Pro zapnut� podpory zpr�v Systemu V zkompilujte PHP s volbou --enable-sysvmsg.

Konfigurace b�hu

Chov�n� t�chto funkc� je ovlivn�no nastaven�m parametr� v php.ini.

Tabulka 2. Konfigura�n� volby pro Semaphore

N�zev	V�choz� hodnota	Lze zm�nit
sysvmsg.value	"42"	PHP_INI_ALL
sysvmsg.string	"foobar"	PHP_INI_ALL

Pro bli�� podrobnosti a definici PHP_INI_* konstant viz ini_set().

Typy prost�edk�

P�eddefinovan� konstanty

Toto roz��en� nem� definov�ny ��dn� konstanty.

Obsah
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 -- Z�skat semafor
sem_get -- Z�skat id semaforu
sem_release -- Uvolnit semafor
sem_remove -- Remove a semaphore
shm_attach -- Vytvo�it nebo otev��t segment sd�len� pam�ti
shm_detach -- Odpojit se od segmentu sd�len� pam�ti
shm_get_var -- Vr�tit prom�nnou ze sd�len� pam�ti
shm_put_var -- Vlo�it nebo modifikovat prom�nnou do sd�len� pam�ti
shm_remove_var -- Odstranit prom�nnou ze sd�len� pam�ti
shm_remove -- Odstranit sd�lenou pam� ze syst�mu

add a note User Contributed Notes
Semaphore, Shared Memory and IPC Functions

05-Feb-2004 04:25


Here is an article/tutorial on the subject:

hcuevas at galenicom dot com
23-Oct-2003 02:49


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 08:45


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

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


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 05:54


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
01-Jun-2001 04:46


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
21-Sep-2000 06:58


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.

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