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shm_attach — Shared Memory Segment anlegen oder anbinden
shm_attach() liefert eine ID über die auf ein System V Shared Memory Segment mit dem gegebenen key zugegriffen werden kann. Der erste Zugriff auf ein noch nicht existierendes Segment legt dieses mit der Größe memsize und Zugriffsberechtigungen gemäß der in perm gesetzten Bits an.
Ein zweimaliger Aufruf der Funktion shm_attach() mit dem gleichen key liefert eine andere ID, über beide IDs wird aber trotzdem auf das gleiche Shared Memory Segment zugegriffen. memsize und perm werden in diesem Fall ignoriert.
Eine numerische Shared Memory Segment Id.
Die Größe des Shared Memory Segments in Bytes. Wird dieser Parameter nicht übergeben so wird der Wert aus sysvshm.init_mem in php.ini angenommen. Ist auch dieser nicht gesetzt ist der Vorgabewert 10000 Bytes. bytes.
Optionale Zugriffsrechte als Bitmaske, Default ist 0666.
Liefert ein Shared Memory Resource Handle.
In my log I have found string
"shm_attach(): failed for key 0x366f: No space left on device"
But have not found any suggestion for this at php.net and at google.
So question was how to make free memory used by shm_attach .
At first to view (Linux) how many segments allocated, use
:~# ipcs -mu
then
limits
:~# ipcs -ml
to remove segment use:
:~# ipcrm -m [shmid]
otherwise you can reboot server, or launch sh script based on commands above.
To avoid troubles with "No space left on device" ALWAYS use
shm_remove() & shm_detach() after call shm_attach().
A simple Sample to introduce SHM.
<?php
if ( sizeof($argv)<2 ) {
echo "Usage: $argv[0] send|recv|rem|dele ID [msg] \n\n" ;
echo " EX: $argv[0] send 1 \"This is no 1\" \n" ;
echo " $argv[0] recv 1 \n" ;
echo " $argv[0] rem 1 \n" ;
echo " $argv[0] dele \n" ;
exit;
}
// $SHMKey = ftok(__FILE__, "Z");
$SHMKey = "123456" ;
## Create/Open a shm
$seg = shm_attach( $SHMKey, 1024, 0666 ) ;
switch ( $argv[1] ) {
case "send":
shm_put_var($seg, $argv[2], $argv[3]);
echo "send msg done...\n" ;
break;
case "recv":
$data = shm_get_var($seg, $argv[2]);
echo $data . "\n" ;
break;
case "rem":
shm_remove_var($seg, $argv[2]);
break;
case "dele":
shm_remove($seg);
break;
case "dele2":
`/usr/bin/ipcrm -M 123456`;
break;
}
?>
Responding to jpeter1978 at yahoo dot com ... Assuming a character is typically two bytes, the size of a serialized variable (incl array or obj) is 2 * its strlen(). The 44 is 4 more than the 24 + 16 = 40 suggested by php at cytrax dot de plus 4 bytes for worst case 4-byte alignment. So this is probably a reliable formula if you are too lazy to align it using something similar to zeppelinux at comcast dot net as in:
($strlen($serialized_array_or_obj) /4 ) * 4 ) + 40;
The zeppelinux formula would use 20 (for 4-byte integer cpu) or 36 (for 64-bit or 8-byte cpu) for the ending constant so 40 or 44 is probably just achieving header padding but it certainly can't hurt.
<?php
//how to calculate the minimum $memsize required to store the variable $foo where $foo='foobar'.
// when shm_attach() is called for the first time, PHP writes a header to the beginning of the shared memory.
$shmHeaderSize = (PHP_INT_SIZE * 4) + 8;
// when shm_put_var() is called, the variable is serialized and a small header is placed in front of it before it is written to shared memory.
$shmVarSize = (((strlen(serialize($foo))+ (4 * PHP_INT_SIZE)) /4 ) * 4 ) + 4;
// now add the two together to get the total memory required. Of course, if you are storing more than one variable then you dont need to add $shmHeaderSize for each variable, only add it once.
$memsize = $shmHeaderSize + $shmVarSize;
//this will give you just enough memory to store the one variable using shm_put_var().
$shm_id = shm_attach ( $key, $memsize, 0666 ) ;
shm_put_var ( $shm_id , $variable_key , $foo );
any attempt to store another variable will result in a 'not enough memory' error.
Be aware that if you change the contents of $foo to a larger value and then you try to write it to shared memory again using shm_put_var(), then you will get a 'not enough memory' error. In this case, you will have to resize your shared memory segment and then write the new value.
If you are only storing variables that contain a single integer value, then you can avoid having to resize by always allocating the largest amount of memory that is required to store an int, which should be:
$shmIntVarSize = (((strlen(serialize(PHP_INT_MAX))+ (4 * PHP_INT_SIZE)) /4 ) * 4 ) + 4;
?>
Hi :)
I write small class for build bright message between my application.
<?
class Bright_Message
{
var $bright;
var $SHM_KEY;
var $my_pid;
function Bright_Message($SHM_KEY=null)
{
$this->my_pid = getmypid();//Get own pid
if (is_null($SHM_KEY)) $this->SHM_KEY = '123123123';
$this->bright = shm_attach($this->SHM_KEY, 1024, 0666);
$this->one_instance();
}
function get_msg($id,$remove=true)
{
if(@$msg=shm_get_var($this->bright,$id))
{
if ($remove) @shm_remove_var($this->bright,$id);
return $msg;
} else return false;
}
function snd_msg($id,$msg)
{
@shm_put_var($this->bright,$id,$msg);
return true;
}
function one_instance()
{
$SHM_PID = $this->get_msg(1,false);
if((strpos(exec('ps p'.$SHM_PID),$_SERVER['SCRIPT_FILENAME'])) === false)
$this->snd_msg(1,$this->my_pid); else
{
echo "This program exists on pid: $SHM_PID\r\n\r\n";
exit;
}
}
}
?>
send.php:
<?
include "bridge_message.class.php";
$shm = new Bright_Message();
$shm->snd_msg(2,'this is my simple message');
?>
receive.php:
<?
include "bridge_message.class.php";
$shm = new Bright_Message();
$msg = get_msg(2);
echo print_r($msg,1);
?>
I tried all the suggestions above for getting the object size (in bytes) for $memsize, but they didn't work universally for the two types of objects I tried (string and array of strings).
After doing some googling and experimenting, I've found the following magic formula:
$memsize = ( strlen( serialize( $object ) ) + 44 ) * 2;
I found this in someone else's code, so I can't explain it.
As a follow-up to my last post regarding shm_attach and its limit capability for knowing how it was created....
for more control, use the shmop_* series of functions, as they have finer grained control than these.
and by the way: the SHMOP functions SHOULD BE listed under "see also" for all the SHM* wrapper functions (I assume they are wrappers to the SHMOP* functions).
Since there aren't seperate functions for CREATING and ATTACHING shared memory (a mistake in my opinion), you might want to do some testing to check whether you've created it or not, as you may not want the slave of a master/slave pair to ever create the shared memory.
One way you can test this is by having your slave set the size to something small, then testing the size by putting a variable too large to fit, e.g.:
function get_shmem() {
return shm_attach(ftok('somefile.txt', 'T'), 100, 0644);
}
$shm = get_shmem();
while (!@shm_put_var($shm, 1, str_repeat('.....', 20))) {
shm_remove($shm);
sleep(1);
//we created it, so we'll remove it and sleep to wait for the master to create it, then try again.
$shm = get_shmem();
}
shm_remove_var($shm, 1);
//here we know the shared memory was already created, because we could put a variable in bigger than the size requested.
Another way you can handle it is to have all programs initialize the shared memory with the same parameters... I had a problem with this when my clients starting too fast and created the shmem without passing a memsize value, so it was defaulting to 10k which was too small.
The limit for one SHM block seems to be 32 MB, but you can split your data in several SHM blocks if you need to. The total SHM limit seems to be about 8 GB.
I'm not sure whether this is true for all configurations.
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.
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);
?>
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;
}
}
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($data2);
?>
As you can see, test2.php doesn't insert anything into shared memory.. yet it pulls out 3 totally different arrays already stored..
Hope that helps.. took me a bit to get it right.. everyone seems to have their own idea of how shm should be used. lol.
BTW, not sure how the ftok works to be honest, cause I didn't change the __FILE__ to match the file path of test.php or anything.. I would think that the file path out have to be the exact same to work correctly.. oh well, it worked as-is! haha..
- Cecil
The memsize needed for shared memory (tested on linux system) can be calculated with:
For each varialbe you want to store: 24 Bytes
+ serialized var-size (see below) alligned by 4 Bytes
+ 16 Bytes
For a update of a var with the same key, the memory for the old var will be needed extra.
4194304 means 4 MB and NOT 4 GB for shared memory on FreeBSD. You can increase the shared memory at runtime if you like.
Finding out shared memory kernel settings in FreeBSD:
sys1# sysctl -a | grep -i SHM
kern.ipc.shmmax: 4194304
kern.ipc.shmmin: 1
kern.ipc.shmmni: 96
kern.ipc.shmseg: 64
kern.ipc.shmall: 1024
kern.ipc.shm_use_phys: 0
Shows us that we can allocate a total of 4GB (wow) of shared memory, etc...
With Sun Solaris 2.x, the MAXIMUM shared memory value allowed is 1,048,576. The maximum allowed value can be determined using the command /usr/sbin/sysdef. On Linux, there does not seem to be any system enforced maximum size. To change the maximum allowed size on Solaris 2.x, use set shmsys:shminfo_shmmax=[new value].
Notice that 'int key' for shared-memory is shared with the keys used for semaphores. So you get in trouble when you use the same key value for semaphores and shared memory!
Objects are stored serialized in shm_put_var, so to find a value for memsize, you can use strlen(serialize($object_to_store_in_shm)).
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