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) page-aligned address 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:
Allow the contents of the segment to be executed. The caller must have execute permission on the segment.
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.
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 byshmaddr
from the address space of the calling process. The to-be-detached segment must be currently attached withshmaddr
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.
When shmat() fails, errno
is set to one of the following:
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.
shmid
points to a removed identifier.
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.
Could not allocate memory for the descriptor or for the page tables.
When shmdt() fails, errno
is set as follows:
There is no shared memory segment attached at shmaddr
; or, shmaddr
is not aligned on a page boundary.
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():
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, SHMLBA 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)
This page is part of release 4.15 of the Linux man-pages
project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.