shmat, shmdt - System V shared memory operations
#include <sys/types.h>
#include <sys/shm.h>
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 attach occurs.
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.
- SHM_RDONLY
- 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.
- SHM_REMAP (Linux-specific)
- 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 EINVAL 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_lpid is set to the process-ID of the calling
process.
- shm_nattch is incremented by one.
shmdt() detaches the shared memory segment located at the address
specified by
shmaddr from the address space of the calling process. The
to-be-detached segment must be currently attached with
shmaddr equal to
the value returned by the 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
follows:
- 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 segment 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 indicate 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
capability in the user namespace that governs its IPC namespace.
- 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 tables.
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 segments.
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
process.
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.
On Linux, it is possible to attach a shared memory segment even if it is already
marked to be deleted. However, POSIX.1 does not specify this behavior and many
other implementations do not support it.
The following system parameter affects
shmat():
- SHMLBA
- Segment low boundary address multiple. When explicitly
specifying 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 size).
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_overview(7),
svipc(7)
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