shm_attach

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muytoloco at yahoo dot com dot br
24-Feb-2006 01:33


If one process make a shm_attach to one inexistent memory area, this area will be created under the same priviliegies of the script running user. If another process will try to create or acces the same area, runnig by other user with different privileges of the first user, an error will occur.

rch at todo dot com dot uy
28-Mar-2005 08:17


Cecil, the key of a var is an integer (not the name ). You can put multiples vars in the same share.



#!/usr/local/bin/php -q

<?PHP



$SHM_KEY = ftok(__FILE__, chr( 4 ) ); 



$data =  shm_attach($SHM_KEY, 1024, 0666);



$test1 = array("hello","world","1","2","3");

$test2 = array("hello","world","4","5","6");

$test3 = array("hello","world","7","8","9");



shm_put_var($data, 1, $test1);

shm_put_var($data, 2,$test2);

shm_put_var($data, 3,$test3);



print_r(shm_get_var($data, 1));

print_r(shm_get_var($data, 2));

print_r(shm_get_var($data, 3));



shm_detach($data);

?>

andreyKEINSPAM at php dot net
25-Apr-2004 12:31


As far as I see from the sources of ext/sysvshm, it's not needed to  do  arithmetic (bit) OR (|) of "perm" with  IPC_CREAT (and IPC_EXCL). The extension will do it for you. It tries to attach to the memory segment and if the try did not succeed it tries to attach but with flags set to user_flag | IPC_CREAT | IPC_EXCL.

The exact code (shm_flag is the third param to the function) :

if ((shm_id = shmget(shm_key, 0, 0)) < 0) {

      if (shm_size < sizeof(sysvshm_chunk_head)) {

            php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed for key 0x%x: memorysize too small", shm_key);

            efree(shm_list_ptr);

            RETURN_FALSE;

      }

      if ((shm_id = shmget(shm_key, shm_size, shm_flag | IPC_CREAT | IPC_EXCL)) < 0) {

            php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed for key 0x%x: %s", shm_key, strerror(errno));

             efree(shm_list_ptr);

             RETURN_FALSE;

     }

}

Cecil
18-Mar-2004 04:37


Here is an example of how to use one shared memory block to store multiple variables or arrays.. unfortunetly in order to store more than ONE variable, you have to use sem_get() multiple times.. same goes for shm_attach(), shm_put_var() and shm_get_var()



#!/usr/local/bin/php -q

<?PHP

// test.php



$SHM_KEY = ftok(__FILE__,'A');



$shmid  = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);

$shmid2 = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);

$shmid3 = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);



$data =  shm_attach($shmid, 1024);

$data2 = shm_attach($shmid2, 1024);

$data3 = shm_attach($shmid3, 1024);



$test = array("hello","world","1","2","3");

$test2 = array("hello","world","4","5","6");

$test3 = array("hello","world","7","8","9");



shm_put_var($data,$inmem,$test);

shm_put_var($data2,$inmem2,$test2);

shm_put_var($data3,$inmem3,$test3);



print_r(shm_get_var($data,$inmem));

print_r(shm_get_var($data2,$inmem2));

print_r(shm_get_var($data3,$inmem3));



shm_detach($data);

shm_detach($data2);

shm_detach($data2);

?>



to REALLY test it.. create a second script like so and run it..



#!/usr/local/bin/php -q

<?PHP

// test2.php



$SHM_KEY = ftok(__FILE__,'A');



$shmid  = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);

$shmid2 = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);

$shmid3 = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);



$data =  shm_attach($shmid, 1024);

$data2 = shm_attach($shmid2, 1024);

$data3 = shm_attach($shmid3, 1024);



print_r(shm_get_var($data,$inmem));

print_r(shm_get_var($data2,$inmem2));

print_r(shm_get_var($data3,$inmem3));



shm_detach($data);

shm_detach($data2);

shm_detach($data2);

?>



As you can see, test2.php doesn't insert anything into shared memory.. yet it pulls out 3 totally different arrays already stored..



Hope that helps.. took me a bit to get it right.. everyone seems to have their own idea of how shm should be used. lol.



BTW, not sure how the ftok works to be honest, cause I didn't change the __FILE__ to match the file path of test.php or anything.. I would think that the file path out have to be the exact same to work correctly.. oh well, it worked as-is! haha..



- Cecil

php at cytrax dot de
19-Jan-2002 12:36


The memsize needed for shared memory (tested on linux system) can be calculated with:



For each varialbe you want to store: 24 Bytes

+ serialized var-size (see below) alligned by 4 Bytes

+ 16 Bytes



For a update of a var with the same key, the memory for the old var will be needed extra.

koester at x-itec dot de
22-Mar-2001 04:41


4194304 means 4 MB and NOT 4 GB for shared memory on FreeBSD. You can increase the shared memory at runtime if you like.

lew at persiankitty dot com
25-Jan-2001 08:30


Finding out shared memory kernel settings in FreeBSD:





sys1# sysctl -a | grep -i SHM





kern.ipc.shmmax: 4194304


kern.ipc.shmmin: 1


kern.ipc.shmmni: 96


kern.ipc.shmseg: 64


kern.ipc.shmall: 1024


kern.ipc.shm_use_phys: 0





Shows us that we can allocate a total of 4GB (wow) of shared memory, etc...

webmaster at mail dot communityconnect dot com
11-Sep-1999 04:50


With Sun Solaris 2.x, the MAXIMUM shared memory value allowed is 1,048,576.  The maximum allowed value can be determined using the command /usr/sbin/sysdef.  On Linux, there does not seem to be any system enforced maximum size.  To change the maximum allowed size on Solaris 2.x, use set shmsys:shminfo_shmmax=[new value].

h dot raaf at i-k-c dot net
25-Mar-1999 09:40


Notice that 'int key' for shared-memory is shared with the keys used for semaphores. So you get in trouble when you use the same key value for semaphores and shared memory!

eric at superstats dot com
13-Feb-1999 11:04


Objects are stored serialized in shm_put_var, so to find a value for memsize, you can use strlen(serialize($object_to_store_in_shm)).

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shm_attach

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