shmat (2)
Leading comments
Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. S...
NAME
shmat, shmdt - System V shared memory operationsSYNOPSIS
#include <sys/types.h> #include <sys/shm.h> void *shmat(int shmid, const void *shmaddr, int shmflg); int shmdt(const void *shmaddr);
DESCRIPTION
shmat()
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()
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.
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.
- 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.
CONFORMING TO
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 *.
NOTES
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).