shmop_delete
=== Checking if a shared memory exists ===
The solution provided by Mitchell_Shnier at ieee dot orgZ doesn't work on my computer - I get a warning "Invalid flag ac".
In order to check if a shared-memory exists, you just have to open it with the "a" or "w" flag, while hiding the warnings using the "@" operator:
<?php
@$shid = shmop_open($systemId, "a", 0666, 0);
if (!empty($shid)) {
... shared memory exists
} else {
... shared memory doesn't exist
}
?>
shmop_open
(PHP 4 >= 4.0.4, PHP 5)
shmop_open — Create or open shared memory block
Description
int shmop_open
( int
$key
, string $flags
, int $mode
, int $size
)shmop_open() can create or open a shared memory block.
Parameters
-
key -
System's id for the shared memory block. Can be passed as a decimal or hex.
-
flags -
The flags that you can use:
- "a" for access (sets SHM_RDONLY for shmat) use this flag when you need to open an existing shared memory segment for read only
- "c" for create (sets IPC_CREATE) use this flag when you need to create a new shared memory segment or if a segment with the same key exists, try to open it for read and write
- "w" for read & write access use this flag when you need to read and write to a shared memory segment, use this flag in most cases.
- "n" create a new memory segment (sets IPC_CREATE|IPC_EXCL) use this flag when you want to create a new shared memory segment but if one already exists with the same flag, fail. This is useful for security purposes, using this you can prevent race condition exploits.
-
mode -
The permissions that you wish to assign to your memory segment, those are the same as permission for a file. Permissions need to be passed in octal form, like for example 0644
-
size -
The size of the shared memory block you wish to create in bytes
Note:
Note: the 3rd and 4th should be entered as 0 if you are opening an existing memory segment.
Return Values
On success shmop_open() will return an id that you can
use to access the shared memory segment you've created. FALSE is
returned on failure.
Examples
Example #1 Create a new shared memory block
<?php
$shm_key = ftok(__FILE__, 't');
$shm_id = shmop_open($shm_key, "c", 0644, 100);
?>
This example opened a shared memory block with a system id returned by ftok().
add a note
User Contributed Notes
shmop_open - [9 notes]
erelsgl at gmail dot com ¶
5 years ago
Mitchell_Shnier at ieee dot orgZ ¶
11 years ago
To check whether a particular shared memory segment is already created, you need to concatenate the "a" and "c" flags. For example (where $SystemKey is the Unix key used by the other process(es) with which you want to share this memory segment)...<BR>
$shm_id = shmop_open($SystemKey, "ac", 0, 0);
if ($shm_id) {
#it is already created
} else {
#you need to create it with shmop_open using "c" only
}<BR>
Using only "a" does not work (just as using only IPC_EXCL in the Unix shmget() call is meaningless). Also, use the ipcs shell command to see your shared memory segments.
a ¶
2 years ago
On win xp 32bit php 5.2.6 recreation of shared memory block with the same id but bigger size fails.
<?php
$shm_key = 0xff3;
$shm_id = shmop_open($shm_key, "c", 0666, 128);
shmop_delete($shm_id);
shmop_close($shm_id);
$shm_id = shmop_open($shm_key, "c", 0666, 32);
shmop_delete($shm_id);
shmop_close($shm_id);
$shm_id = shmop_open($shm_key, "c", 0666, 1024); // on win32 fails there
shmop_delete($shm_id);
shmop_close($shm_id);
?>
DinosauRUS ¶
4 years ago
If you faced with any problem you're going to solve with shared memmory, but your server doesn't support it, you can use files instead. I've wrote simple wrapper for this and its suites for me. Hope it will be usefull for you too.
<?php
define(kSHARED_FOLDER, "shared/");
define(kSHARED_MAX_ATTEMPS, 10);
define(kSESSION_SHARED, "shared_");
class Shared {
var $id = 0;
var $filename = '';
var $filepointer;
var $data = array();
var $date = 0;
function Shared($id) {
$this->id = $id;
$this->filename = kSHARED_FOLDER.$this->id;
if(empty($this->filename))
{
print "no filename";
return false;
}
$this->date = $_SESSION[kSESSION_SHARED.$id];
}
function clear() {
if ($this->id == null)
{
return false;
}
$counter = 0;
ignore_user_abort(true);
if(($this->filepointer = @fopen($this->filename, "w")) == false) {
ignore_user_abort(false);
return false;
}
while(true) {
if ($counter >= kSHARED_MAX_ATTEMPS) {
fclose($this->filepointer);
ignore_user_abort(false);
return false;
}
if(flock($this->filepointer, LOCK_EX) == false) {
$counter++;
usleep(rand(1, 25000));
}
else
break;
}
if(flock($this->filepointer, LOCK_UN) == false) {
ignore_user_abort(false);
return false;
}
unset($this->data);
$this->data = array();
fclose($this->filepointer);
$this->date = $_SESSION[kSESSION_SHARED.$id] = filemtime($this->filename);
ignore_user_abort(false);
return true;
}
function setObjectForKey($value, $key) {
if ($this->id == null)
return false;
$counter = 0;
ignore_user_abort(true);
if(($this->filepointer = @fopen($this->filename, "a+")) == false) {
ignore_user_abort(false);
print "can not open file<br>";
return false;
}
while(true) {
if ($counter >= kSHARED_MAX_ATTEMPS) {
fclose($this->filepointer);
print("1 aborted...");
ignore_user_abort(false);
return false;
}
$block;
if(flock($this->filepointer, LOCK_EX, $block) == false) {
$counter++;
print("1 waiting...");
usleep(rand(1, 25000));
}
else
break;
}
$data = file_get_contents($this->filename);
$array = array();
if (!empty($data))
$array = unserialize($data);
$array[$key] = $value;
$data = serialize($array);
ftruncate($this->filepointer, 0);
fseek($this->filepointer, 0, SEEK_SET);
fwrite($this->filepointer, $data);
$this->data = $array;
if(flock($this->filepointer, LOCK_UN) == false) {
ignore_user_abort(false);
return false;
}
fclose($this->filepointer);
$this->date = $_SESSION[kSESSION_SHARED.$id] = filemtime($this->filename);
ignore_user_abort(false);
return true;
}
function getObjectForKey($key) {
if ($this->id == null)
return null;
$counter = 0;
ignore_user_abort(true);
if(($this->filepointer = @fopen($this->filename, "a+")) == false) {
ignore_user_abort(false);
print("can not open<br>");
return null;
}
if ($this->date == filemtime($this->filename)) {
fclose($this->filepointer);
return $this->data[$key];
}
while(true) {
if ($counter >= kSHARED_MAX_ATTEMPS) {
fclose($this->filepointer);
ignore_user_abort(false);
print("2 aborted<br>");
return null;
}
if(flock($this->filepointer, LOCK_SH ) == false) {
$counter++;
print("2 waiting...<br>");
usleep(rand(1, 25000));
}
else
break;
}
fseek($this->filepointer, 0);
$data = file_get_contents($this->filename);
$array = array();
if (!empty($data))
$array = unserialize($data);
$data = $array[$key];
$this->data = $array;
if(flock($this->filepointer, LOCK_UN) == false) {
ignore_user_abort(false);
return $data;
}
fclose($this->filepointer);
$this->date = $_SESSION[kSESSION_SHARED.$id] = filemtime($this->filename);
ignore_user_abort(false);
return $data;
}
}
?>
Craig Manley ¶
8 years ago
To: macmaster at pobox dot com:
To clear up some new confusion: you said the shm key is 8 bytes long. As far as I know it's 4 bytes (32bits).
Check out the output of ipcs on Linux below to see what I mean.
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x6e6a694d 65538 mijnbel 644 65536 0
0x326e794d 98307 mijnbel 644 65536 0
0x62417347 131076 smsklap 644 65536 0
daniele_dll at yahoo dot it ¶
9 years ago
There is a little ftok function. This function isn't included into php for windows so i've grabbed it directly from linux glibc 2.3.2 source code. I hope that this can be useful.
There is the code:
<?php
function ftok($pathname, $proj_id) {
$st = @stat($pathname);
if (!$st) {
return -1;
}
$key = sprintf("%u", (($st['ino'] & 0xffff) | (($st['dev'] & 0xff) << 16) | (($proj_id & 0xff) << 24)));
return $key;
}
echo ftok($_SERVER["SCRIPT_FILENAME"], 250);
?>
sorry for my english :)
Chris Petersen ¶
9 years ago
Be warned that if you try to shmop_open with a key set to zero, shmop_open will seemingly work, and you can write to it, but you will not be able to read from it or delete it. If you're not careful, you can continue doing this - creating more and more shared memory blocks at "zero" until eventually you WILL start getting errors saying that php can't access or create the shared memory block, and you will have to restart your machine to free up all of those "zero" blocks.
me at here dot com ¶
8 years ago
Just an alternative idea if 'shared memory' is what you need for your websites, you can use tmpfs (on Linux):
Get root to do this:
mkdir /home/myname/tmpfs
chown myname:mygroup /home/myname/tmpfs
..and this in a script executed at boot time:
mount -t tmpfs /mnt/tmpfs /home/myuser/tmpfs
Now you can use regular file functions (including locking) to access shared memory between all your processes.
More info: http://docsun.cites.uiuc.edu/sun_docs/C/
...and this because the note editor doesn't accept long lines...
solaris_9/SUNWaadm/SYSADV1/p150.html
macmaster at pobox dot com ¶
12 years ago
the key is a LONG variable type, meaning that the key can only be eight (8) bytes long, which can be too short if you're using any form of automagic key generation (like a parsed filename)