PHP 4, PHP 5, PHP 7, PHP 8
shm_attach - Creates or open a shared memory segment

shm_attach( int$key, [int|null$size = null], [int$permissions = 0666] ): SysvSharedMemory|false

shm_attach returns an id that can be used to access the System V shared memory with the given key, the first call creates the shared memory segment with size and the optional perm-bits permissions.

A second call to shm_attach for the same key will return a different SysvSharedMemory instance, but both instances access the same underlying shared memory. size and permissions will be ignored.



A numeric shared memory segment ID


The memory size. If not provided, default to the sysvshm.init_mem in the php.ini, otherwise 10000 bytes.


The optional permission bits. Default to 0666.

Return Values

Returns a SysvSharedMemory instance on success, or false on failure.


Version Description
8.0.0 On success, this function returns an SysvSharedMemory instance now; previously, a resource was returned.
8.0.0 size is nullable now.

Related Functions

Function shm_attach:

  • Manual

Semaphore, Shared Memory and IPC Functions

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