The function shmat attaches the shared memory segment identified by shmid to the address space of the calling process. The attaching address is specified by shmaddr with one of the following criteria:
If shmaddr is NULL, the system chooses a suitable (unused) address at which to attach the segment.
If shmaddr isnt NULL and SHM_RND is asserted in shmflg, the attach occurs at the address equal to shmaddr rounded down to the nearest multiple of SHMLBA. Otherwise shmaddr must be a page-aligned address at which the attach occurs.
If SHM_RDONLY is asserted in shmflg, the segment is attached for reading and the process must have read permission for the segment. Otherwise the segment is attached for read and write and the process must have read and write permission for the segment. There is no notion of a write-only shared memory segment.
The (Linux-specific) SHM_REMAP flag may be asserted in shmflg to indicate that the mapping of the segment should replace any existing mapping in the range starting at shmaddr and continuing for the size of the segment. (Normally an EINVAL error would result if a mapping already exists in this address range.) In this case, shmaddr must not be NULL.
The brk value of the calling process is not altered by the attach. The segment will automatically be detached at process exit. The same segment may be attached as a read and as a read-write one, and more than once, in the processs address space.
On a successful shmat call the system updates the members of the shmid_ds structure associated to the shared memory segment as follows: