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Last updated: Tue, 19 Sep 2006


(PHP 4 >= 4.0.4, PHP 5)

shmop_open -- Create or open shared memory block


int shmop_open ( int key, string flags, int mode, int size )

shmop_open() can create or open a shared memory block.

shmop_open() takes 4 parameters: key, which is the system's id for the shared memory block, this parameter can be passed as a decimal or hex. The second parameter are 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.

The third parameter is the mode, which are 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 ex. 0644. The last parameter is 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. On success shmop_open() will return an id that you can use to access the shared memory segment you've created.

Example 1. Create a new shared memory block

$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().

Last updated: Tue, 19 Sep 2006