NAME
shmat, shmdt - System V shared memory operations SYNOPSIS
#include
#include
void *shmat(int shmid, const void *shmaddr, int shmflg); int shmdt(const void *shmaddr); DESCRIPTION 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 SHMRND 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 SHMRDONLY 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) SHMREMAP 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 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 shmidds structure (see shmctl(2)) associated with the shared memory segment as follows: shmatime is set to the current time.
shmlpid is set to the process-ID of the calling process. shmnattch 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. On a successful shmdt() call the system updates the members of the shmidds structure associated with the shared memory segment as fol‐ lows: shmdtime is set to the current time.
shmlpid is set to the process-ID of the calling process. shmnattch is decremented by one. If it becomes 0 and the seg‐ ment 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. 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 CAPIPCOWNER capability. EIDRM shmid points to a removed identifier.
EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and SHMRND was not specified) or invalid shmaddr value, or can't attach segment at shmaddr, or SHMREMAP 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 *. (Linux libc4 and libc5 have the char * proto‐ types; glibc2 has void *.) NOTES 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 spec‐ ify 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 PAGESIZE.
The implementation places no intrinsic limit on the per-process maximum number of shared memory segments (SHMSEG). SEE ALSO brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shmover‐ view(7), svipc(7) COLOPHON
This page is part of release 3.53 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)