CXXIX. 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.

表格 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.

注: 本扩展模块在 Windows 平台下不可用。




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.


这些函数的行为受 php.ini 的影响。

表格 2. Semaphore Configuration Options

有关 PHP_INI_* 常量进一步的细节与定义参见附录 G


以下常量由本扩展模块定义,因此只有在本扩展模块被编译到 PHP 中,或者在运行时被动态加载后才有效。

表格 3. System V message constants


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
chrissavery at removeme dot gmail dot com
03-Jun-2006 04:18
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 08:19
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:


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));

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


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 09:55
Actually, the way to lock a semaphore from C code appears to be:
   struct sembuf semptr

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

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;

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

PS: Disregard the <? and ?>, it's just for pretty printing... this is C code!
hcuevas at galenicom dot com
23-Oct-2003 10:49
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 04:45
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 10:50
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 01:54
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";
mikew at php dot net
01-Jun-2001 12:46
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.