sem_get

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

sem_getObtener 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 10 notes

up
10
soger
12 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.
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4
kakkau at grr dot la
7 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.
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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().
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1
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.
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1
Michael Z.
12 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.
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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/
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0
ein at anti-logic dot com
16 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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0
joeldg AT listbid.com
20 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);
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0
joeldg at listbid.com
20 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);
?>
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-2
cyrus dot drive at gmail dot com
15 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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