Semaphore Functions

ftok — Convert a pathname and a project identifier to a System V IPC key
msg_get_queue — Create or attach to a message queue
msg_queue_exists — Check whether a message queue exists
msg_receive — Receive a message from a message queue
msg_remove_queue — Destroy a message queue
msg_send — Send a message to a message queue
msg_set_queue — Set information in the message queue data structure
msg_stat_queue — Returns information from the message queue data structure
sem_acquire — Acquire a semaphore
sem_get — Get a semaphore id
sem_release — Release a semaphore
sem_remove — Remove a semaphore
shm_attach — Creates or open a shared memory segment
shm_detach — Disconnects from shared memory segment
shm_get_var — Returns a variable from shared memory
shm_has_var — Check whether a specific entry exists
shm_put_var — Inserts or updates a variable in shared memory
shm_remove_var — Removes a variable from shared memory
shm_remove — Removes shared memory from Unix systems

User Contributed Notes

13 years ago


Samlpe code for using most of the functions here:

    $MEMSIZE    =   512;//  size of shared memory to allocate
    $SEMKEY     =   1;  //  Semaphore key
    $SHMKEY     =   2;  //  Shared memory key

    echo "Start.\n";
    // Get semaphore
    $sem_id = sem_get($SEMKEY, 1);
    if ($sem_id === false)
    {
        echo "Fail to get semaphore";
        exit;
    }
    else
        echo "Got semaphore $sem_id.\n";

    // Accuire semaphore
    if (! sem_acquire($sem_id))
    {
        echo "Fail to aquire semaphore $sem_id.\n";
        sem_remove($sem_id);
        exit;
    }
    else
        echo "Success aquire semaphore $sem_id.\n";

    $shm_id =   shm_attach($SHMKEY, $MEMSIZE);
    if ($shm_id === false)
    {
        echo "Fail to attach shared memory.\n";
        sem_remove($sem_id);
        exit;
    }
    else
        echo "Success to attach shared memory : $shm_id.\n";

    // Write variable 1
    if (!shm_put_var($shm_id, 1, "Variable 1"))
    {
        echo "Fail to put var 1 on shared memory $shm_id.\n";
        sem_remove($sem_id);
        shm_remove ($shm_id);
        exit;
    }
    else
        echo "Write var1 to shared memory.\n";

    // Write variable 2
    if (!shm_put_var($shm_id, 2, "Variable 2"))
    {
        echo "Fail to put var 2 on shared memory $shm_id.\n";
        sem_remove($sem_id);
        shm_remove ($shm_id);
        exit;
    }
    else
        echo "Write var2 to shared memory.\n";

    // Read variable 1
    $var1   =   shm_get_var ($shm_id, 1);
    if ($var1 === false)
    {
        echo "Fail to retrive Var 1 from Shared memory $shm_id, return value=$var1.\n";
    }
    else
        echo "Read var1=$var1.\n";

    // Read variable 1
    $var2   =   shm_get_var ($shm_id, 2);
    if ($var1 === false)
    {
        echo "Fail to retrive Var 2 from Shared memory $shm_id, return value=$var2.\n";
    }
    else
        echo "Read var2=$var2.\n";

    // Release semaphore
    if (!sem_release($sem_id))
        echo "Fail to release $sem_id semaphore.\n";
    else
        echo "Semaphore $sem_id released.\n";

    // remove shared memory segmant from SysV
    if (shm_remove ($shm_id))
        echo "Shared memory successfully removed from SysV.\n";
    else
        echo "Fail to remove $shm_id shared memory from SysV.\n";

    // Remove semaphore
    if (sem_remove($sem_id))
        echo "semaphore removed successfully from SysV.\n";
    else
        echo "Fail to remove $sem_id semaphore from SysV.\n";
    echo "End.\n";

php at stolt dot de

15 years ago


The integer keys for sem_get() and shm_attach() have to be systemwide unique. There is no method to ensure that no other process on the system will use your specific key (security! and possible malfunction). Also shared memory is very seldom used there are possibilities for conflicts! To see the used id's you can use the program 'ipcs' (at least under SuseLinux;) ). Thanks Christian C.

HansRich

1 year ago


For shared memory needs, these functions are greatly outdated and inefficient.
They don't provide read/write locking and they use a single linked list for storing the variables, which means they have an average complexity of O(n/2) where n is the number of variables in the segment. 

For a better alternative use APC Functions which implement proper read/write locking and use a hash table so they have good efficiency even for large amounts of variables:
http://php.net/manual/en/ref.apc.php

chrissavery at removeme dot gmail dot com

9 years ago


I was confused by two things that caused strange behaviour in my use of semaphores with php scripts running under apache.

Often enough page requests will end up being filled by the same process as other simultaneous requests. So semaphores will block when you may not have expected.

Also note that sem_remove() will remove it for all processes, not just the calling one. So you have to be sure that the last process running removes the semaphore and none before. I thought there was some failures occurring when my child processes were dropping out with errors.

So you can't just use get, acquire, release, remove in one script that will be hit by a web user. (1) They may end up in the same process and will wait on the other, and (2) the first one to finish will destroy the semaphore for others.

I left out the remove call, and it works ok, but I still wonder if the semaphore is removed by php when the last script that did a get finishes? Also creating a child process to do the work using proc_open works to ensure seperate processes but to be careful you would want to limit the number somehow as well.

mikew at php dot net

14 years ago


As for security,   please look at the perm argument to shm_get.   Shared Memory blocks has the same permission semantics as unix user/group/other file permissions.   As long as your webserver is running as a user that no other users can script to..  and as long as the permissions are set to 600,  you should be fine and have no security concerns.

piosaw79

4 years ago


I'm using this simple functions to mimic semaphores on Windows (with $max_acquire = 1):



<?php

if ( !function_exists('sem_get') ) {

    function sem_get($key) { return fopen(__FILE__.'.sem.'.$key, 'w+'); }

    function sem_acquire($sem_id) { return flock($sem_id, LOCK_EX); }

    function sem_release($sem_id) { return flock($sem_id, LOCK_UN); }

}

?>

hekkwan at gmail dot com

9 years ago


I have been trying to get a php console script and a C application to use a common semaphore for a while. I just got it working, so I thought Id paste the code here incase anyone needs to do this, however, this is not the place for long code examples

I used c code from the php implementation to set up the semaphore set and then mimic the way the php interpreter implements a mutext type locking scheme, using a common semop call.

One has to do the process in the same way as its done in the php implementation, otherwise you run the risk of the php interpreter resetting the semaphore set for you.

The basic idea is.
1) sem_get - use a three semaphore set
1.1) increment the first sem
1.2) check the usage count (sem 3), if only one, set the max_attach using sem 2 for mutex behaviour to sem 3 
2) decrement sem 1
3) for locking / unlocking use the first semaphore, but always call the above from your c-code.

If you want a copy of my code, email me and I'll happily send it to you !

Denis Gerasimov

10 years ago


Many (most?) developers use Win32 platform for PHP Web applications development while production servers mostly run Unix/Linux OS. Below is the stub code I use to make it possible to write scripts on Win32 that use semaphores:

<?php

if (substr(PHP_OS, 0, 3) == 'WIN') {  // if Windows OS detected

    function ftok($pathname, $proj)
    {
        if (empty($pathname) || !file_exists($pathname)) {  // an error occured
            return -1;
        }
        
        $pathname = $pathname . (string) $proj;
        $key = array();
        while (sizeof($key) < strlen($pathname)) {
            $key[] = ord(substr($pathname, sizeof($key), 1));
        }
        
        return dechex(array_sum($key));
    }
    
    function sem_acquire($sem_identifier)
    {
        return true;
    }

    function sem_get($key, $max_acquire = null, $perm = null, $auto_release = null)
    {
        return true;
    }
    
    function sem_release($sem_identifier)
    {
        return true;
    }
    
    function sem_remove($sem_identifier)
    {
        return true;
    }
    
}

?>

Of course, there is no way to test semaphores until you have no Unix/Linux test server.

p4nzer at yahoo dot com

10 years ago


Actually, the way to lock a semaphore from C code appears to be:
<?
    struct sembuf semptr[2];

    if( (semid = semget(SEM_KEY, PHP_SEM_NEED_NUMBER, 0666 | IPC_CREAT)) < 0 ) {
        perror("semget");
        return 1;
    }

    semptr[0].sem_num = 0;
    semptr[0].sem_op = -1; 
    semptr[0].sem_flg = SEM_UNDO;
    semptr[1].sem_num = 1;
    semptr[1].sem_op =  1; 
    semptr[1].sem_flg = SEM_UNDO;

    if( semop(semid, &semptr[0], 2) < 0 ) {
    perror("semop");
    }
?>

PS: Disregard the <? and ?>, it's just for pretty printing... this is C code!

hcuevas at galenicom dot com

12 years ago


Don't use semaphores to serialize access to an undefined number of resources. There is no way (yet) to know before locking if a semaphore is already locked, thus not being able to fully release the semaphore and occupying a semaphore resource for an undefined time. 

A possible solution is to build a shared mem pool and store there the current number of locks for a semaphore id. 

Cheers,
Horaci Cuevas

Roman Laptev <tmp at laptev dot org>

13 years ago


If you going to work with semaphore, which was created by some external program, you can try the following code for this program (C example):

#define SVSEM_MODE (SEM_R | SEM_A | SEM_R>>3 | SEM_R>>6) /* 0644 */
#define PHP_SEM_NEED_NUMBER 3

/*.......*/
int semid, semflag = SVSEM_MODE | IPC_CREAT | IPC_EXCL;
struct sembuf semptr;
union semun semopts;
/*.......*/
if( (semid = semget(sempath, PHP_SEM_NEED_NUMBER, semflag)) >= 0 ) {
  semopts.val = 1; /* initial value for sem */
  if( semctl( semid, 0, SETVAL, semopts) < 0 ) {/*error*/}
  if( semctl( semid, 1, SETVAL, semopts) < 0 ) {/*error*/}
  /* PHP wanna zero for its own semget at third sem.
   * look at ./PHP_SOURCE_PATH/ext/sysvsem/sysvsem.c
   */
  semopts.val = 0;
  if( semctl( semid, 2, SETVAL, semopts) < 0 ) {/*error*/}
}
else if(errno == EEXIST) { /* connect only  */
  if( (semid = semget(sempath, PHP_SEM_NEED_NUMBER, SVSEM_MODE | IPC_CREAT)) < 0 ) {/*error*/}
}
else {/*error*/}

/*.......*/
/* If you want acquire the sem */
  semptr.sem_num = 0;
  semptr.sem_op = -1; /* lock it */
  semptr.sem_flg = SEM_UNDO;
  while( semop(semid, &semptr, 1) < 0 ) {/*error*/}
/*.......*/

Thanks,
Roma

mr dot smaon at gmail dot com

9 years ago


Here is a simple Mutex class implementation, using semaphore on Linux and flock on windows.
Filename is optionnal, but you can provide it anyway. This way the file will be created on windows, not on linux as it is not needed.
It's fast written and certainly lacks error checking code.

<?php

    class Mutex
    {
        private $id;
        private $sem_id;
        private $is_acquired = false;
        private $is_windows = false;
        private $filename = '';
        private $filepointer;

        function __construct()
        {
            if(substr(PHP_OS, 0, 3) == 'WIN')
                $this->is_windows = true;
        }

        public function init($id, $filename = '')
        {
            $this->id = $id;

            if($this->is_windows)
            {
                if(empty($filename)){
                    print "no filename specified";
                    return false;
                }
                else
                    $this->filename = $filename;
            }
            else
            {
                if(!($this->sem_id = sem_get($this->id, 1))){
                    print "Error getting semaphore";
                    return false;
                }
            }

            return true;
        }

        public function acquire()
        {
            if($this->is_windows)
            {
                if(($this->filepointer = @fopen($this->filename, "w+")) == false)
                {
                    print "error opening mutex file<br>";
                    return false;
                }
                
                if(flock($this->filepointer, LOCK_EX) == false)
                {
                    print "error locking mutex file<br>";
                    return false;
                }
            }
            else
            {
                if (! sem_acquire($this->sem_id)){
                    print "error acquiring semaphore";
                    return false;
                }
            }

            $this->is_acquired = true;
            return true;
        }

        public function release()
        {
            if(!$this->is_acquired)
                return true;

            if($this->is_windows)
            {
                if(flock($this->filepointer, LOCK_UN) == false)
                {
                    print "error unlocking mutex file<br>";
                    return false;
                }

                fclose($this->filepointer);
            }
            else
            {
                if (! sem_release($this->sem_id)){
                    print "error releasing semaphore";
                    return false;
                }
            }

            $this->is_acquired = false;
            return true;
        }

        public function getId()
        {
            return $this->sem_id;
        }
    }

?>

Example use:

<?php

    $mutex = new Mutex();
    $mutex->init(1, "mutex_file.txt");
    $mutex->acquire();

    //Whatever you want single-threaded here...
    $mutex->release();

?>

Semaphore Functions

Table of Contents

User Contributed Notes