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           #include <sys/types.h>
           #include <sys/shm.h>
           void *shmat(int shmid, const void *shmaddr, int shmflg);
           int shmdt(const void *shmaddr);


           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 mul-
           tiple 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 read-
           ing and the process must have read permission for the segment.   Other-
           wise  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 indi-
           cate that the mapping of the segment should replace any  existing  map-
           ping  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 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.
           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


           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.


           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
           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
           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.


           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 * proto-
           types; glibc2 has void *.)


           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.
           shm_overview(7), svipc(7)

    Linux 2013-02-12 SHMOP(2)


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