shmop_open

Create or open shared memory block

&reftitle.description; ``Shmop``false```shmop_open` `int` `key` `string` `mode` `int` `permissions` `int` `size`

`shmop_open` can create or open a shared memory block.

&reftitle.parameters;

`key`

- System's id for the shared memory block. Can be passed as a decimal or hex.

`mode`

- 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.

`permissions`

- 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: the 3rd and 4th should be entered as 0 if you are opening an existing memory segment.

&reftitle.returnvalues;

On success `shmop_open` will return a `Shmop` instance that you can use to access the shared memory segment you've created. &false; is returned on failure.

&reftitle.errors;

If `mode` is invalid, or `size` is less than or equal to zero, a `ValueError` is thrown. On other failures, `E_WARNING` is emitted.

&reftitle.changelog;

&Version; | &Description;
8.0.0 | On success, this function returns an `Shmop` instance now; previously, a `resource` was returned.
8.0.0 | If `mode` is invalid, or `size` is less than or equal to zero, a `ValueError` is thrown; previously `E_WARNING` was emitted instead, and the function returned &false;.

&reftitle.examples;

### 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`.

&reftitle.seealso;

- `shmop_close`
- `shmop_delete`