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.
See also: ftok().
注: This function does not work on Windows systems.