shmat(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 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 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 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 4.09 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/.

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