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) page-aligned address 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 argument:

       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 contin-
	      uing 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()
       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  fol-
       lows:

	      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 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 in the user namespace that governs its IPC namespace.

       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 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

       POSIX.1-2001, POSIX.1-2008, SVr4.

       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	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 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.  When explicitly specifying an attach address in a call to shmat(), the caller should  ensure
	      that  the  address  is  a multiple 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 multiple of the system page size.  (On many Linux architectures, SHMLBA 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 4.15 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 https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15								  SHMOP(2)