sem_get

Michael Z. (2011-08-25 06:37:47)

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.

soger (2011-03-28 08:20:56)

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.

pail dot luo at gmail dot com (2009-02-26 18:40:05)

A very simple to introduce semaphore...

<?php
$SEMKey = "123456" ;

## Get Semaphore id
$seg = sem_get( $SEMKey, 2, 0666, -1) ;

if ( $argv[1]=="remove" ) {
    sem_remove($seg);
}

echo "Try to acquire ..."
sem_acquire($seg);
echo "Acquired...\n" ;

echo "Press Any Key to continue...\n";
$fh = fopen("php://stdin", "r");
$a = fgets( $fh);
fclose($fh);

sem_release($seg);
?>

cyrus dot drive at gmail dot com (2008-05-01 17:51:02)

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); }
    
}
?>

quickshiftin at gmail dot com (2007-07-30 20:55:35)

in regards to the last post, it looks like there is a mistake.
well, understandably, the documentation isnt quite clear on a point the post uses to rationalize its claim, allow me to explain.
the purpose of a semaphore is to manage access to a shared resource, so natrually it follows that if multiple attempts to access the same semaphore arent blocking (when expected to), then the api simply isnt being used properly.

the problem is that the documentation does not stipulate what will happen if the max_acquire paramter is varied upon successive invocations of the sem_get method.  so setting it to 100, then to 1 on 2 successive calls will have an undefined behavior.  if the value is kept constant however (and set to 1 for the example), you will find, as i did that the second attempt to acquire the semaphore will block.  NOTE: this does not work when using 
php -a

here is the revised sample code:
<?php
$fp1 = sem_get(fileinode('commonResource'), 1);

sem_acquire($fp1);
echo 'got mutex' . PHP_EOL;

$fp2 = sem_get(fileinode('commonResource'), 1);
sem_acquire($fp2);
echo 'got mutex again' . PHP_EOL;
?>

note there are 2 references; $fp1 and $fp2 and you will not see the second message because the script will block forever.

ein at anti-logic dot com (2007-06-26 04:21:47)

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.

neofutur (2006-08-28 12:49:06)

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/

joeldg AT listbid.com (2003-05-02 11:39:57)

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);

joeldg at listbid.com (2003-05-02 00:36:12)

<?
// 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);
?>