NAME¶
shmat, shmdt - shared memory operations
SYNOPSIS¶
#include <sys/types.h>
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);
DESCRIPTION¶
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 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.
If
SHM_RDONLY is specified 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 specified 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(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.
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.
RETURN VALUE¶
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.
ERRORS¶
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 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.
SVr4, POSIX.1-2001.
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 *. (Linux libc4 and libc5
have the
char * prototypes; glibc2 has
void *.)
NOTES¶
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.1-2001 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. Must be page
aligned. For the current implementation the SHMLBA value is
PAGE_SIZE.
The implementation places no intrinsic limit on the per-process maximum number
of shared memory segments (
SHMSEG).
SEE ALSO¶
brk(2),
mmap(2),
shmctl(2),
shmget(2),
capabilities(7),
shm_overview(7),
svipc(7)
COLOPHON¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.