shmop - shared memory operations
void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);
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 isn't NULL and SHM_RND is asserted in shmflg, the attach
occurs at the address equal to shmaddr rounded down to the nearest mul-
tiple 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 read-
ing and the process must have read permission for the segment. Other-
wise 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 brk value of the calling process is not altered by the attach. The
segment will automatically be detached at process exit. The same seg-
ment may be attached as a read and as a read-write one, and more than
once, in the process's 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:
shm_atime is set to the current time.
shm_lpid is set to the process-ID of the calling process.
shm_nattch is incremented by one.
Note that the attach succeeds also if the shared memory segment is
marked to be deleted.
The function shmdt detaches the shared memory segment located at the
address specified by shmaddr from the address space of the calling pro-
cess. The to-be-detached segment must be currently attached with
shmaddr equal to the value returned by the its attaching shmat call.
On a successful shmdt call the system updates the members of the
shmid_ds structure associated with the shared memory segment as fol-
fork() After a fork() the child inherits the attached shared memory
exec() After an exec() all attached shared memory segments are detached
from the process.
exit() Upon exit() all attached shared memory segments are detached
from the process.
On failure both functions return -1 with errno indicating the error.
On success shmat returns the address of the attached shared memory seg-
ment, and shmdt returns 0.
When shmat fails, errno is set to one of the following:
EACCES The calling process has no access permissions for the
requested attach type.
EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and
SHM_RND was not specified) or invalid shmaddr value, or
failing attach at brk.
ENOMEM Could not allocate memory for the descriptor or for the page
The function shmdt can fail only if there is no shared memory segment
attached at shmaddr, in such a case at return errno will be set to EIN-
Using shmat with shmaddr equal to NULL is the preferred, portable way
of attaching a shared memory segment. Be aware that the shared memory
segment attached in this way may be attached at different addresses in
different processes. Therefore, any pointers maintained within the
shared memory must be made relative (typically to the starting address
of the segment), rather than absolute.
The following system parameter affects a shmat system call:
SHMLBA Segment low boundary address multiple. Must be page
aligned. For the current implementation the SHMBLA value is
The implementation has no intrinsic limit to the per-process maximum
number of shared memory segments (SHMSEG).
SVr4, SVID. SVr4 documents an additional error condition EMFILE. In
SVID-v4 the type of the shmaddr argument was changed from char * into
const void *, and the returned type of shmat() from char * into void *.
(Linux libc4 and libc5 have the char * prototypes; glibc2 has void *.)