shm_attach() returns an id that that can be used to access the System V shared memory with the given key, the first call creates the shared memory segment with mem_size (default: sysvshm.init_mem in the configuration file, otherwise 10000 bytes) and the optional perm-bits (default: 0666).
A second call to shm_attach() for the same key will return a different shared memory identifier, but both identifiers access the same underlying shared memory. Memsize and perm will be ignored.
Note: This function does not work on Windows systems.