[PHP 4, PHP 5, PHP 7, PHP 8]
sem_get — Get a semaphore id
Description
sem_get[
int $key,
int $max_acquire = 1,
int $permissions =
0666,
bool $auto_release = true
]: SysvSemaphore|false
A second call to sem_get[] for the same key will return a different semaphore identifier, but both identifiers access the same underlying semaphore.
If key is
0, a new private semaphore is created for each call to sem_get[].
Parameters
key max_acquire The number of processes that can acquire the semaphore simultaneously is set to max_acquire.
permissionsThe semaphore permissions. Actually this value is set only if the process finds it is the only process currently attached to the semaphore.
auto_releaseSpecifies if the semaphore should be automatically released on request shutdown.
Return Values
Returns a positive semaphore identifier on success, or false on error.
Changelog
| 8.0.0 | On success, this function returns a SysvSemaphore instance now; previously, a resource was returned. |
| 8.0.0 | The type of auto_release has been changed from int to bool.
|
Notes
Warning
When using sem_get[] to access a semaphore created outside PHP, note that the semaphore must have been created as a set of 3 semaphores [for example, by specifying 3 as the nsems parameter when calling the C semget[] function], otherwise PHP will be unable to access the semaphore.
See Also
- sem_acquire[] - Acquire a semaphore
- sem_release[] - Release a semaphore
- ftok[] - Convert a pathname and a project identifier to a System V IPC key
soger ¶
11 years ago
Actually it looks like the semaphore is automatically released not on request shutdown but when the variable you store it's resource ID is freed. That is a very big difference.
kakkau at grr dot la ¶
6 years ago
It is possible to create an "infinite" amount of semaphores when setting $key = 0.
Run sem_get multiple times
php > sem_get[0,0];
and check the output of
$ ipcs -s
------ Semaphore Arrays --------
key semid owner perms nsems
0x00000000 1277952 user 666 3
0x00000000 1310721 user 666 3
As you can see there were multiple semaphores set up with key 0.
For any other integer sem_get works as expected. It returns another resource id pointing to the semaphore previously created and does not create another semaphore.
kakkau at grr dot la ¶
6 years ago
For those that encounter strange behavior in using sem_acquire[] on resources generated by sem_get[]. Have a look at sem_get[]'s 4th parameter auto_release. It allows multiple acquisitions through reassignments to resource variables.
./multi.acquire.php