sem_get

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

sem_get — Obtener el id de un semáforo

Descripción

sem_get(
    int $key,
    int $max_acquire = 1,
    int $perm = 0666,
    int $auto_release = 1
): resource

sem_get() devuelve un id que se puede usar para acceder al semáforo de System V con la clave dada por key.

Una segunda llamada a sem_get() para la misma clave devolverá un identificador de semáforo diferente, pero ambos identificadores accederán al mismo semáforo subyacente.

Parámetros

key
max_acquire: El número de procesos que puede adquirir el semáforo simultáneamente está establecido por max_acquire.
perm: Los permisos del semáforo. En realidad este valor se establece sólo si el proceso que lo encuentra es el único proceso actualmente adjunto al semáforo.
auto_release: Especifica si el semáforo debería ser liberado automáticamente al cierre de la petición.

Valores devueltos

Devuelve un identificador de semáforo positivo en caso de éxito, o false en caso de error.

Notas

Advertencia

Al usar sem_get() para acceder a un semáforo creado fuera de PHP, se ha de observar que el semáforo debe haber sido creado como un conjunto de 3 semáforos (por ejemplo, especificando 3 en el parámetro nsems, al llamar a la función semget() de C), de lo contrario, PHP no podrá acceder al semáforo.

Ver también

sem_acquire() - Adquirir un semáforo
sem_release() - Liberar un semáforo
ftok() - Convertir un nombre de ruta y un identificador de proyecto a una clave IPC de System V

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User Contributed Notes 9 notes

down

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.

down

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.

down

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.

down

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().

down

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.

down

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/

down

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);
?>

down

-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);

down

-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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