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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 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.

       If SHM_RDONLY is specified in shmflg, the segment is attached for reading and the  process
       must have read permission for the segment.  Otherwise 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  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() 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 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.

       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

       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

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       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.