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    sem_get

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

    sem_get — セマフォ ID を得る

    説明

    sem_get(
        int $key,
        int $max_acquire = 1,
        int $permissions = 0666,
        bool $auto_release = true
    ): SysvSemaphore|false

    sem_get() は、 System V セマフォを指定したキーでアクセスするために使用可能な ID を返します。

    同じキーで sem_get() を 2 度コールした場合、 別のセマフォ ID が返されます。 しかし、どちらの ID も同じそのセマフォをアクセスします。

    key に 0 を指定すると、 sem_get() が呼び出される度に新しい非公開のセマフォが作られます。

    パラメータ

    key

    max_acquire

    同時にセマフォを得ることが可能なプロセス数を max_acquire にセットします。

    permissions

    セマフォのパーミッション。 実際には、この値はプロセスが現在そのセマフォに付随している 唯一のプロセスであることがわかった場合にのみセットされます。

    auto_release

    リクエストの終了時に自動的にセマフォを開放するかどうかを指定します。

    戻り値

    成功した場合に正のセマフォ ID、エラー時には false を返します。

    変更履歴

    バージョン 説明
    8.0.0 成功した時、 この関数は SysvSemaphore クラスのインスタンスを返すようになりました。 これより前のバージョンでは、リソースが返されていました。
    8.0.0 auto_release の型が int から bool に変更されました

    注意

    警告

    PHP 以外のところで作られたセマフォに sem_get() でアクセスするときには、 3 つのセマフォのセットとして作ったものでなければいけないことに注意しましょう (たとえば、C の semget() 関数で作るなら、nsems に 3 を指定します)。 そうしないと、PHP からそのセマフォにアクセスできなくなります。

    参考

    • sem_acquire() - セマフォを得る
    • sem_release() - セマフォを解放する
    • ftok() - パス名とプロジェクト ID を、System V IPC キーに変換する

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    User Contributed Notes 9 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
    9 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
    18 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
    0
    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
    -1
    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
    -4
    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.
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