SHMOP(2)						     Linux Programmer's Manual							  SHMOP(2)

NAME

       shmat, shmdt - 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() attaches the shared memory segment identified by shmid to the address space of the calling process.  The attaching address is spec-
       ified 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.  Oth-
       erwise 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 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 shmid_ds structure (see shmctl(2)) associated with the shared memory segment  as  fol-
       lows:

	      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.

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  indi-
       cate 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 capa-
	      bility.

       EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and SHM_RND was not specified) or invalid shmaddr value, or can't attach seg-
	      ment 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 *.  (Linux libc4 and libc5 have the char * prototypes; 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
       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).

SEE ALSO

       brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), svipc(7)

COLOPHON

       This  page is part of release 3.25 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)