SDO_DAS_Relational::executeQuery
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();
?>
CXXXI. Funzioni per i semafori, la memoria condivisa ed IPC
Introduzione
Questo modulo fornisce le funzioni relative all'IPC di System V. Queste includono semafori, memoria condivisa e messaggi tra i processi (IPC).
I semafori possono essere utilizzati per fornire un accesso esclusivo alle risorse sulla macchina corrente, oppure per limitare il numero di processi che possono utilizzare simultaneamente una risorsa.
Questo modulo fornisce anche le funzioni per la memoria condivisa a partire dalla gestione della memoria condivisa di System V. La memoria condivisa può essere utilizzata per fornire l'accesso a variabili globali. Differenti demoni httpd e anche altri programmi (tipo Perl, C, ...) sono in grado di accedere a questi dati creando uno scambio di dati globale. Si ricordi che la memoria condivisa non è garantita nei confronti di accessi simultanei. Si utilizzino i semafori per la sincronizzazione.
Tabella 1. Limiti della memoria condivisa posti da UNIX
| SHMMAX | dimensione massima della memoria condivisa, solitamente 131072 bytes |
| SHMMIN | dimensione minima della memoria condivisa, solitamente 1 byte |
| SHMMNI | massimo ammontare dei segmenti di memoria condivisa sul sistema, solitamente 100 |
| SHMSEG | numero massimo di segmenti di memoria condivisa per processo, solitamente 6 |
Le funzioni relative ai messaggi possono essere usate per inviare e ricevere messaggi da/per altri processi. Esse permettono un semplice ed efficace metodo di interscambio dati tra i processi, senza dovere ricorrere ad alternative quali i socket nel dominio Unix.
Nota: Questo modulo non è disponibile su piattaforme Windows.
Requisiti
Non sono necessarie librerie esterne per utilizzare questo modulo.
Installazione
Di default non viene abilitato il supporto per queste funzioni. Per abilitare il supporto dei semafori di System V, compilare il PHP con l'opzione --enable-sysvsem. Per abilitare il supporto della memoria condivisa, compilare il PHP con l'opzione --enable-sysvshm. Per abilitare il supporto dei messaggi, compilare il PHP con l'opzione --enable-sysvmsg.
Configurazione di Runtime
Il comportamento di queste funzioni è influenzato dalle impostazioni di php.ini.
Tabella 2. Opzioni per la configurazione dei Semafori
| Nome | Default | Modificabile | Variazioni |
|---|---|---|---|
| sysvmsg.value | "42" | PHP_INI_ALL | |
| sysvmsg.string | "foobar" | PHP_INI_ALL |
Costanti predefinite
Queste costanti sono definite da questa estensione e sono disponibili solo se l'estensione è stata compilata nel PHP o se è stata caricata dinamicamente a runtime.
- Sommario
- ftok -- Converte il percorso e un identificatore di progetto in un chiave IPC di System V
- msg_get_queue -- Crea, o si collega ad una coda di messaggi
- msg_receive -- Riceve un messaggio da una coda
- msg_remove_queue -- Distrugge una coda di messaggi
- msg_send -- Invia un messaggio ad una coda di messaggi
- msg_set_queue -- Valorizza le informazioni nella struttura dati della coda dei messaggi
- msg_stat_queue -- Restituisce informazioni dalla struttura dati della coda
- sem_acquire -- Acquisisce un semaforo
- sem_get -- Ottiene l'id di un semaforo
- sem_release -- Rilascia un semaforo
- sem_remove -- Rimuove un semaforo
- shm_attach -- Crea oppure apre un segmento di memoria condivisa
- shm_detach -- Disconnette da un segmento di memoria condivisa
- shm_get_var -- Restituisce una variabile dalla memoria condivisa
- shm_put_var -- Inserisce o aggiorna una variabile nella memoria condivisa
- shm_remove_var -- Rimuove una variabile dalla memoria condivisa
- shm_remove -- Rimuove un segmento di memoria condivisa dal sistema Unix
add a note
User Contributed NotesFunzioni per i semafori, la memoria condivisa ed IPC
mr dot smaon at gmail dot com
25-Jan-2007 09:08
25-Jan-2007 09:08
hekkwan at gmail dot com
14-Nov-2006 06:15
14-Nov-2006 06:15
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 !
chrissavery at removeme dot gmail dot com
02-Jun-2006 01:18
02-Jun-2006 01:18
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.
Denis Gerasimov
02-Nov-2005 04:19
02-Nov-2005 04:19
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
01-Nov-2005 05:55
01-Nov-2005 05:55
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!
04-Feb-2004 08:25
Here is an article/tutorial on the subject:
http://www.php-mag.net/itr/online_artikel/psecom,id,448,nodeid,114
hcuevas at galenicom dot com
23-Oct-2003 07:49
23-Oct-2003 07:49
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
david () php net
09-Jul-2003 01:45
09-Jul-2003 01:45
Here is a quick utility to dump the contents of a shm_ format memory segment:
http://www.davidc.net/php/shm/
Roman Laptev <tmp at laptev dot org>
02-Apr-2003 06:50
02-Apr-2003 06:50
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
elran70 at hotmail dot com
22-Jun-2002 10:54
22-Jun-2002 10:54
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";
mikew at php dot net
31-May-2001 09:46
31-May-2001 09:46
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.
php at stolt dot de
20-Sep-2000 11:58
20-Sep-2000 11:58
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.