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Last updated: Thu, 21 Aug 2003

XCII. Semaphore, Shared Memory and IPC Functions


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.

Table 1. Limits of Shared Memory by the Unix OS

SHMMAXmax size of shared memory, normally 131072 bytes
SHMMINminimum 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.

Note: This extension is not available on Windows platforms.


No external libraries are needed to build this extension.


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.

Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

Table 2. Semaphore Configuration Options

For further details and definition of the PHP_INI_* constants see ini_set().

Resource Types

Predefined Constants

This extension has no constants defined.

Table of Contents
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

add a note add a note User Contributed Notes
Semaphore, Shared Memory and IPC Functions
hcuevas at galenicom dot com
23-Oct-2003 09:49
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.

Horaci Cuevas
david () php net
09-Jul-2003 03:45
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 09:50
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 */

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*/}

elran70 at hotmail dot com
23-Jun-2002 12:54
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";
        echo "Got semaphore $sem_id.\n";

    // Accuire semaphore
    if (! sem_acquire($sem_id))
        echo "Fail to aquire semaphore $sem_id.\n";
        echo "Success aquire semaphore $sem_id.\n";

    $shm_id =   shm_attach($SHMKEY, $MEMSIZE);
    if ($shm_id === false)
        echo "Fail to attach shared memory.\n";
        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 $shm_id.\n";
        shm_remove ($shm_id);
        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 $shm_id.\n";
        shm_remove ($shm_id);
        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";
        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";
        echo "Read var2=$var2.\n";

    // Release semaphore
    if (!sem_release($sem_id))
        echo "Fail to release $sem_id semaphore.\n";
        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";
        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";
        echo "Fail to remove $sem_id semaphore from SysV.\n";
    echo "End.\n";
joe at floodwerks dot com
02-May-2002 08:30
If only Sys V message queues were supported, I would be happy.
mikew at php dot net
01-Jun-2001 11:46
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 02:58
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.

 Last updated: Thu, 21 Aug 2003
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