Provided by: manpages-dev_4.04-2_all bug

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
       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  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()
       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.

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
              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.

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, 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 4.04 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/.