Fonctions Sémaphore

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";

I have been trying to get a php console script and a C application to use a common semaphore for a while. I just got it working, so I thought Id paste the code here incase anyone needs to do this, however, this is not the place for long code examplesI used c code from the php implementation to set up the semaphore set and then mimic the way the php interpreter implements a mutext type locking scheme, using a common semop call.One has to do the process in the same way as its done in the php implementation, otherwise you run the risk of the php interpreter resetting the semaphore set for you.The basic idea is.1) sem_get - use a three semaphore set1.1) increment the first sem1.2) check the usage count (sem 3), if only one, set the max_attach using sem 2 for mutex behaviour to sem 3 2) decrement sem 13) for locking / unlocking use the first semaphore, but always call the above from your c-code.If you want a copy of my code, email me and I'll happily send it to you !

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.

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.

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

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