PHP - Manual: sem_acquire

2024-11-14

sem_acquire

(PHP 4, PHP 5, PHP 7, PHP 8)

sem_acquire — Acquire a semaphore

说明

sem_acquire(SysvSemaphore $semaphore, bool $non_blocking = false): bool

sem_acquire() by default blocks (if necessary) until the semaphore can be acquired. A process attempting to acquire a semaphore which it has already acquired will block forever if acquiring the semaphore would cause its maximum number of semaphore to be exceeded.

After processing a request, any semaphores acquired by the process but not explicitly released will be released automatically and a warning will be generated.

参数

semaphore: semaphore is a semaphore obtained from sem_get().
non_blocking: Specifies if the process shouldn't wait for the semaphore to be acquired. If set to true, the call will return false immediately if a semaphore cannot be immediately acquired.

返回值

成功时返回 true，或者在失败时返回 false。

更新日志

版本	说明
8.0.0	`semaphore` expects a SysvSemaphore instance now; previously, a resource was expected.

参见

sem_get() - Get a semaphore id
sem_release() - Release a semaphore

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User Contributed Notes 7 notes

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gladd at trash dot eris dot qinetiq dot com ¶

18 years ago


Just to clarify what is meant by "process" above:

On the Apache webserver, many PHP requests will be executed within the same process space because it is multithreaded. However, any semaphores got and acquired by a script and not released and removed will still be automatically cleaned up by the PHP interpreter each time the script terminates.

Remove any trash before emailing!

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Pinky ¶

10 years ago


sem_acquire() is blocking, meaning that subsequent calls with the same semaphore will block indefinitely until the semaphore is released. This ensures serialization, but it is not very practical if all you want to do is check if you should proceed or not. Unfortunately, PHP does not yet support any method of querying the state of a semaphore in a non-blocking manner.

It may seem possible to put together such a mechanism by hand, using shared memory (shm_ functions). However, be warned that it is not trivial and ultimately non-productive. You cannot, for example, simply pick a shared mem var, store the semaphore key and query it. Such an operation would be non-transactional and non-atomic ie. it is possible for two or more parallel processes to manage to read "not locked" from the shared mem var before one of them manages to mark it "locked". You would have to use a (blocking) semaphore to serialize access to the shared mem var, thus recreating the very problem you are trying to solve.

In other words, if non-blocking queries are crucial to you, you need to either request that this issue be solved by the PHP designers, or pick another mechanism to do your locking, one that already has this feature.

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Sander Backus ¶

8 years ago


Note that when you reset $sem_identifier the semaphore won't block anymore!

This code does NOT work: 
    $key     = ftok(__FILE__,'m');
    $a        = sem_get($key);
    sem_acquire($a);
    $a = false;

while this one does: 

    $key     = ftok(__FILE__,'m');
    $a        = sem_get($key);
    sem_acquire($a);
    //$a = false;

So: use unique var names for your identifier!

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arjuna w ¶

10 years ago


In my tests sem_acquire() is 150 times faster than flock()

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Ben ¶

1 year ago


I recently had to use semaphores to avoid running an update of a database concurrently.

For testing it I used two browser windows (chrome) on the same Desktop.

In this case semaphores did not work as expected.
Also flock on a file did not work as expected.

Running the script from command line did work as expected.

As soon as I used two different browsers, one on my desktop another within a virutal machine, it works like expected.

Hope that helps anybody avoiding strange debugging sessions on this topic...

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-3

anatoliy at ukhvanovy dot name ¶

7 years ago


If you need non-blocking semaphores, here is an example how you may implement it. Use a shared memory variable to mark whether or not a lock exists and then use a semaphore around operations against that variable. I'll call my shared variable as 'token'.

<?php
echo '<pre>';

$resourceSemaphore = sem_get(7);
$tokenSemaphore = sem_get(8);
$tokenValue = shm_attach(9, 100);

function myEcho($v) {
    echo microtime() . ' ' . $v . "\n";
}

// sem_remove($resourceSemaphore);
// sem_remove($tokenSemaphore);
// exit();

function try_lock() {
    global $resourceSemaphore, $tokenSemaphore, $tokenValue;
    myEcho('begin try_lock()');
    myEcho('acquire token semaphore');
    sem_acquire($tokenSemaphore);
    myEcho('    token semaphore acquired');
    $tmp = shm_get_var($tokenValue, 6);
    myEcho('    token value: ' . var_export($tmp, true));
    $exit = $tmp;
    if (!$exit) {
        $tmp = shm_put_var($tokenValue, 6, true);
        $tmp = shm_get_var($tokenValue, 6);
        myEcho('    token new value: ' . var_export($tmp, true));
    }
    myEcho('release token semaphore');
    sem_release($tokenSemaphore);
    if ($exit) return false;
    myEcho('acquire resource semaphore');
    sem_acquire($resourceSemaphore);
    myEcho('    resource semaphore acquired');
    return true;
}

function release() {
    global $resourceSemaphore, $tokenSemaphore, $tokenValue;
    myEcho('release resource semaphore');
    sem_release($resourceSemaphore);
    myEcho('acquire token semaphore');
    sem_acquire($tokenSemaphore);
    myEcho('    token semaphore acquired');
    $tmp = shm_get_var($tokenValue, 6);
    myEcho('    token value: ' . var_export($tmp, true));
    $tmp = shm_put_var($tokenValue, 6, false);
    $tmp = shm_get_var($tokenValue, 6);
    myEcho('    token new value: ' . var_export($tmp, true));
    myEcho('release token semaphore');
    sem_release($tokenSemaphore);
}

for ($triesLeft = 5; $triesLeft > 0 && !try_lock(); $triesLeft--) {
    myEcho('failed to acquire resource');
    myEcho('wait for 1 sec');
    sleep(1);
    myEcho('try again');
}
myEcho('    access the resource for 4 sec');
//paste here your code, accessing your resource
sleep(4);
release();
myEcho('the end');
?>
When I execute this script in two parallel instances, I get the following output:
-------(first instance)----------------------------------------
... 482 begin try_lock()
... 482 acquire token semaphore
... 482     token semaphore acquired
... 482     token value: false
... 482     token new value: true
... 482 release token semaphore
... 482 acquire resource semaphore
... 482     resource semaphore acquired
... 482     access the resource for 4 sec
... 486 release resource semaphore
... 486 acquire token semaphore
... 486     token semaphore acquired
... 486     token value: true
... 486     token new value: false
... 486 release token semaphore
... 486 the end
-------(second instance)----------------------------------------
... 485 begin try_lock()
... 485 acquire token semaphore
... 485     token semaphore acquired
... 485     token value: true
... 485 release token semaphore
... 485 failed to acquire resource
... 485 wait for 1 sec
... 
... 486 wait for 1 sec
... 487 try again
... 487 begin try_lock()
... 487 acquire token semaphore
... 487     token semaphore acquired
... 487     token value: false
... 487     token new value: true
... 487 release token semaphore
... 487 acquire resource semaphore
... 487     resource semaphore acquired
... 487     access the resource for 4 sec
... 491 release resource semaphore
... 491 acquire token semaphore
... 491     token semaphore acquired
... 491     token value: true
... 491     token new value: false
... 491 release token semaphore
... 491 the end

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-3

anatoliy at ukhvanovy dot name ¶

7 years ago


Unfortunately, PHP does not currently support non-blocking semaphores.

If something like this is necessary you can utilize semaphores together with shared memory to create your own non-blocking lock mechanisms.

Use a shared memory variable to mark whether or not a lock exists and then use a semaphore around operations against that variable.

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官方地址：https://www.php.net/manual/en/function.sem-acquire.php

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sem_acquire

说明

参数

返回值

更新日志

参见

User Contributed Notes 7 notes