| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
SHMOP(2) Linux Programmer's Manual SHMOP(2)
shmat, shmdt - 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 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.
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 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 *.)
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).
brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7),
svipc(7)
This page is part of release 3.32 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 2008-06-03 SHMOP(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface