sem_get

    (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

    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.

    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.

    permissions

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

    auto_release

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

    Return Values

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

    Changelog

    Version Description
    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

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    User Contributed Notes 10 notes

    up
    3
    Dan East
    3 years ago
    Note that the default permissions parameter is octal! Thus the default of 0666 is NOT the same as 666, or 0x666.

    If you specify the permission as decimal 666 then you end up with permissions that prevent the semaphore from being read. The symptom is that you can only sem_get it once, and subsequent sem_get will fail (until you ipcrm or sem_remove it and delete it entirely).

    Thus these are all equivalent to the default:
    sem_get ( 123, 1, 0666)
    sem_get ( 123, 1, 438)
    sem_get ( 123, 1, 0x1b6)

    Most PHP developers (myself included) work with octal numbers so infrequently that the number 0666 can easily be mistaken as 666 or maybe 0x666.
    up
    12
    soger
    13 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.
    up
    6
    kakkau at grr dot la
    8 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.
    up
    1
    kakkau at grr dot la
    8 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
    <?php
    class Sem {
    private     $key = null;
    private     $res = null;
    public function     __construct() {
    $this->key = ftok(".",".");
    $this->set_res();
    $this->acquire();
    }
    public function     set_res() {
    // 4th parameter auto_released is 1 by default
    $this->res = sem_get($this->key, 1, 0600, 1);
    }
    public function     acquire() {
    echo     "acquired='".sem_acquire($this->res,true)."'\n";
    }
    }

    $s = new Sem();
    $s->set_res();
    $s->acquire();

    ?>

    $ php multi.acquire.php
    acquired='1'
    acquired='1'

    To avoid reacquiring by default set sem_get()'s parameter auto_release to 0 or check if your resource variable is already set, e.g. by using is_null().
    up
    1
    Michael Z.
    13 years ago
    Watch out when you use fileinode() to get a unique semaphore key (as suggested in some comment on this or a related function) in conjunction with version control software: It seems, for example, SVN will change the inode. Using such a file will leave you with your mutex not working reliably and your system's semaphore pool being filled until further attempts to get a semaphore will fail. Use ipcs and ipcrm commands from linux-util-ng (on most distros probably) to examine/fix related problems.
    up
    1
    neofutur
    17 years ago
    with gentoo php5 you will need to add the USE flag :
    sysvipc

    see :
    http://forums.gentoo.org/viewtopic-t-464175-highlight-semget+php.html

    and also :
    http://overlays.gentoo.org/proj/php/
    up
    1
    joeldg at listbid.com
    21 years ago
    <?
    // thanks to
    // http://www.ecst.csuchico.edu/~beej/guide/ipc/shmem.html
    $SHM_KEY = ftok("/home/joeldg/homeymail/shmtest.php", 'R');
    $shmid = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);
    $data = shm_attach($shmid, 1024);

    $data = "test";
    printf("shared contents: %s\n", $data);

    shm_detach($data);
    ?>
    up
    0
    joeldg AT listbid.com
    21 years ago
    Heh, actually the above comment I added is not technically correct, it was more of an idea to display the function.

    $SHM_KEY = ftok("/home/joeldg/homeymail/shmtest.php", 'R');
    $shmid = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);
    $data = shm_attach($shmid, 1024);
    // we now have our shm segment

    // lets place a variable in there
    shm_put_var ($data, $inmem, "test");
    // now lets get it back. we could be in a forked process and still have
    // access to this variable.
    printf("shared contents: %s\n", shm_get_var($data, $inmem));

    shm_detach($data);
    up
    -1
    ein at anti-logic dot com
    17 years ago
    Be aware that there is no way to ensure that you have exclusive access to a lock, despite setting max_acquire=1.

    In example,
    <?
    $fp = sem_get(fileinode('lock_file', 100);
    sem_acquire($fp);

    $fp2 = sem_get(fileinode('lock_file', 1);
    sem_acquire($fp2);
    ?>

    This will not block on the second sem_aquire. Therefore, if you have functions or processes that utilize shared locks (>1 max_acquire) you will still need to provide a seperate lock mechanism (ie flock) for write access, making the sem_ functions useless.

    Some more info, in flock, each reference to the lock file has it's own options (can be shared exclusive blocking non blocking etc), but apparently php's sem functions only support these options per semaphore, not per semaphore-reference.
    up
    -5
    cyrus dot drive at gmail dot com
    16 years ago
    Implementation of a read-write semaphore in PHP:

    <?php
    class rw_semaphore {

    const     READ_ACCESS = 0;
    const     WRITE_ACCESS = 1;

    /**
    * @access private
    * @var resource - mutex semaphore
    */
    private $mutex;

    /**
    * @access private
    * @var resource - read/write semaphore
    */
    private $resource;

    /**
    * @access private
    * @var int
    */
    private $writers = 0;

    /**
    * @access private
    * @var int
    */
    private $readers = 0;

    /**
    * Default constructor
    *
    * Initialize the read/write semaphore
    */
    public function __construct() {
    $mutex_key = ftok('/home/cyrus/development/php/sysvipc/rw_semaphore.php', 'm');
    $resource_key = ftok('/home/cyrus/development/php/sysvipc/rw_semaphore.php', 'r');
    $this->mutex = sem_get($mutex_key, 1);
    $this->resource = sem_get($resource_key, 1);
    }

    /**
    * Destructor
    *
    * Remove the read/write semaphore
    */
    public function __destruct() {
    sem_remove($this->mutex);
    sem_remove($this->resource);
    }

    /**
    * Request acess to the resource
    *
    * @param int $mode
    * @return void
    */
    private function request_access($access_type = self::READ_ACCESS) {
    if (    $access_type == self::WRITE_ACCESS) {
    sem_acquire($this->mutex);

    /* update the writers counter */
    $this->writers++;

    sem_release($this->mutex);
    sem_acquire($this->resource);
    } else {
    sem_acquire($this->mutex);
    if (    $this->writers > 0 || $this->readers == 0) {
    sem_release($this->mutex);
    sem_acquire($this->resource);
    sem_acquire($this->mutex);
    }
    /* update the readers counter */
    $this->readers++;

    sem_release($this->mutex);
    }
    }

    private function     request_release($access_type = self::READ_ACCESS) {
    if (    $access_type == self::WRITE_ACCESS) {
    sem_acquire($this->mutex);

    /* update the writers counter */
    $this->writers--;

    sem_release($this->mutex);
    sem_release($this->resource);
    } else {
    sem_acquire($this->mutex);

    /* update the readers counter */
    $this->readers--;

    if (    $this->readers == 0)
    sem_release($this->resource);

    sem_release($this->mutex);
    }
    }

    /**
    * Request read access to the resource
    *
    * @return void
    */
    public function read_access() { $this->request_access(self::READ_ACCESS); }

    /**
    * Release read access to the resource
    *
    * @return void
    */
    public function read_release() { $this->request_release(self::READ_ACCESS); }

    /**
    * Request write access to the resource
    *
    * @return void
    */
    public function write_access() { $this->request_access(self::WRITE_ACCESS); }

    /**
    * Release write access to the resource
    *
    * @return void
    */
    public function write_release() { $this->request_release(self::WRITE_ACCESS); }

    }
    ?>
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