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) 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 mul-
tiple 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 read-
ing and the process must have read permission for the segment. Other-
wise 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 indi-
cate that the mapping of the segment should replace any existing map-
ping 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
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 spec-
ified 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.
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
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.
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
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
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.
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 * proto-
types; glibc2 has void *.)
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
brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_over-
This page is part of release 3.54 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/.
Linux 2013-02-12 SHMOP(2)
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