shmop

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  at-
          tach 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 re-
              place 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 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
       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  seg-
              ment 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
              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 ta-
              bles.

       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 seg-
       ments.

       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 specify-
              ing 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_over-
       view(7), sysvipc(7)

COLOPHON
       This  page  is  part of release 5.05 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                             2019-08-02                          SHMOP(2)