shmop() - Unix, Linux System Call
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NAME
shmop - shared memory operations
SYNOPSIS
#include <sys/types.h>
#include <sys/shm.h>
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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 isnt 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 processs 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:
Tag | Description |
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shm_atime is set to the current time.
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shm_lpid is set to the process-ID of the calling process.
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shm_nattch is incremented by one.
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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.
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On a successful
shmdt() call the system updates the members of the
shmid_ds structure associated with the shared memory segment as follows:
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shm_dtime is set to the current time.
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shm_lpid is set to the process-ID of the calling process.
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shm_nattch is decremented by one.
If it becomes 0 and the segment is marked for deletion,
the segment is deleted.
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SYSTEM CALLS
Tag | Description |
fork() |
After a
fork() the child inherits the attached shared memory segments.
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exec() |
After an
exec() all attached shared memory segments are detached from the process.
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exit() |
Upon
exit() all attached shared memory segments are detached from the process.
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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:
Tag | Description |
EACCES |
The calling process does not have the required permissions for
the requested attach type, and does not have the
CAP_IPC_OWNER capability.
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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.
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ENOMEM |
Could not allocate memory for the descriptor or for the page tables.
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When
shmdt() fails,
errno is set as follows:
Tag | Description |
EINVAL |
There is no shared memory segment attached at
shmaddr; or,
shmaddr is not aligned on a page boundary.
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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 behaviour and
many other implementations do not support it.
The following system parameter affects
shmat():
Tag | Description |
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). CONFORMING TO
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 *.) SEE ALSO
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