Originální popis anglicky:
shmop - shared memory operations
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/shm.h>
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 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
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
process. 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
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.
The occupied region in the user space of the calling process is unmapped.
- fork()
- After a fork() the child inherits the attached
shared memory segments.
- 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 success
shmat returns the address of the attached shared memory
segment, and
shmdt returns 0. On failure both functions return -1 with
errno indicating 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.
- 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, or SHM_REMAP
was specified and shmaddr was NULL.
- ENOMEM
- Could not allocate memory for the descriptor or for the
page tables.
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
EINVAL.
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
PAGE_SIZE.
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 *.)
brk(2),
mmap(2),
shmctl(2),
shmget(2),
ipc(5),
capabilities(7)
