sem_get()

(PHP 4, PHP 5, PHP 7)

Get a semaphore id

说明

sem_get() returns an id that can be used to access the System V semaphore with the given$key.

A second call to sem_get() for the same key will return a different semaphore identifier, but both identifiers access the same underlying semaphore.

参数

The number of processes that can acquire the semaphore simultaneously is set to$max_acquire.

The semaphore permissions. Actually this value is set only if the process finds it is the only process currently attached to the semaphore.

Specifies if the semaphore should be automatically released on request shutdown.

返回值

Returns a positive semaphore identifier on success, or FALSE on error.

注释

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 thensemsparameter when calling the Csemget()function), otherwise PHP will be unable to access the semaphore.

参见

• sem_acquire()Acquire a semaphore
• sem_release()Release a semaphore
• ftok()Convert a pathname and a project identifier to a System V IPC key

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.

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.

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 

Be aware that there is no way to ensure that you have exclusive access to a lock, despite setting max_acquire=1.
In example,

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