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shmat(2) - phpMan
SHMOP(2) Linux Programmer's Manual SHMOP(2)
NAME
shmat, shmdt - System V shared memory operations
SYNOPSIS
#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 argu‐
ment:
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 seg‐
ment. (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 cur‐
rently 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 speci‐
fied) 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
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 *.
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 main‐
tained 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 implemen‐
tations 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 multi‐
ple 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 multi‐
ple 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).
SEE ALSO
brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), svipc(7)
COLOPHON
This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.
Linux 2014-07-08 SHMOP(2)
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