void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);
shmat() attaches the System V 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 specified 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
In addition to SHM_RND, the following flags may be specified in the
shmflg bit-mask argument:
SHM_EXEC (Linux-specific; since Linux 2.6.9)
Allow the contents of the segment to be executed. The caller
must have execute permission on the segment.
Attach the segment for read-only access. The process must have
read permission for the segment. If this flag is not specified,
the segment is attached for read and write access, and the
process must have read and write permission for the segment.
There is no notion of a write-only shared memory segment.
This flag specifies 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 EIN-
VAL error would result if a mapping already exists in this
address range.) In this case, shmaddr must not be NULL.
The brk(2) 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 process's address space.
A successful shmat() call updates the members of the shmid_ds structure
(see shmctl(2)) associated with the shared memory segment as follows:
shm_atime is set to the current time.
shm_dtime is set to the current time.
shm_lpid is set to the process-ID of the calling process.
shm_nattch is decremented by one. If it becomes 0 and the seg-
ment is marked for deletion, the segment is deleted.
On success, shmat() returns the address of the attached shared memory
segment; on error, (void *) -1 is returned, and errno is set to indi-
cate the cause of the error.
On success, shmdt() returns 0; on error -1 is returned, and errno is
set to indicate the cause of the error.
When shmat() fails, errno is set to one of the following:
EACCES The calling process does not have the required permissions for
the requested attach type, and does not have the CAP_IPC_OWNER
EIDRM shmid points to a removed identifier.
EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and
SHM_RND was not specified) or invalid shmaddr value, or can't
attach segment at shmaddr, or SHM_REMAP was specified and
shmaddr was NULL.
ENOMEM Could not allocate memory for the descriptor or for the page
When shmdt() fails, errno is set as follows:
EINVAL There is no shared memory segment attached at shmaddr; or,
shmaddr is not aligned on a page boundary.
POSIX.1-2001, POSIX.1-2008, SVr4.
In SVID 3 (or perhaps earlier), the type of the shmaddr argument was
changed from char * into const void *, and the returned type of shmat()
from char * into void *.
After a fork(2), the child inherits the attached shared memory seg-
After an execve(2), all attached shared memory segments are detached
from the process.
Upon _exit(2), all attached shared memory segments are detached from
The following system parameter affects shmat():
SHMLBA Segment low boundary address multiple. When explicitly specify-
ing an attach address in a call to shmat(), the caller should
ensure that the address is a multiple of this value. This is
necessary on some architectures, in order either to ensure good
CPU cache performance or to ensure that different attaches of
the same segment have consistent views within the CPU cache.
SHMLBA is normally some multiple of the system page size (on
many Linux architectures, it is the same as the system page
The implementation places no intrinsic per-process limit on the number
of shared memory segments (SHMSEG).
brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_over-
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