shmop

NAME

       shmop - shared memory operations

SYNOPSIS

       # include <sys/types.h>
       # include <sys/ipc.h>
       # include <sys/shm.h>

       char *shmat ( int shmid, char *shmaddr, int shmflg )

       int shmdt ( char *shmaddr)

DESCRIPTION

       The  function  shmat  attaches  the  shared memory segment
       identified by shmid to the data  segment  of  the  calling
       process.   The  attaching  address is specified by shmaddr
       with one of the following criteria:

              If shmaddr is  0,  the  system  tries  to  find  an
              unmapped region in the range 1 - 1.5G starting from
              the upper value and coming down from there.

              If shmaddr isn't 0 and SHM_RND is asserted in  shm-
              flg,  the  attach  occurs  at  address equal to the
              rounding down of shmaddr to a multiple  of  SHMLBA.
              Otherwise shmaddr must be a page aligned address at
              which the attach occurs.

       If SHM_RDONLY  is  asserted  in  shmflg,  the  segment  is
       attached for reading and the process must have read access
       permissions to the  segment.   Otherwise  the  segment  is
       attached for read and write and the process must have read
       and write access permissions to the segment.  There is  no
       notion of write-only shared memory segment.

       The brk value of the calling process is not altered by the
       attach.  The segment will automatically detached  at  pro-
       cess 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.

       On  a successful shmat call the system updates the members
       of the structure shmid_ds associated to 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.

       Note that the attach succeeds also if  the  shared  memory
       segment is marked as to be deleted.
       The  function  shmdt  detaches  from the calling process's
       data segment the shared  memory  segment  located  at  the
       address specified by shmaddr.  The detaching shared memory
       segment must be one among the currently attached ones  (to
       the  process's  address  space)  with shmaddr equal to the
       value returned by the its attaching shat call.

       On a successful shmdt call the system updates the  members
       of  the structure shmid_ds associated to 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.

       The  occupied region in the user space of the calling pro-
       cess is unmapped.

SYSTEM CALLS

       fork() After a fork()  the  child  inherits  the  attached
              shared memory segments.

       exec() After an exec() all attached shared memory segments
              are detached (not destroyed).

       exit() Upon exit() all attached shared memory segments are
              detached (not destroyed).

RETURN VALUE

       On  a failure both functions return -1 with errno indicat-
       ing the error, otherwise shmat returns the address of  the
       attached shared memory segment, and shmdt returns 0.

ERRORS

       When shmat fails, at return errno will be set to one among
       the following values:

       EACCES     The calling process has no  access  permissions
                  for the requested attach type.

       EINVAL     Invalid shmid value, unaligned (i.e., not page-
                  aligned  and  SHM_RND  was  not  specified)  or
                  invalid  shmaddr  value,  or  failing attach at
                  brk.

       ENOMEM     Could not allocate memory for the descriptor or
                  for the page tables.

       The  function  shmdt  can fails only if there is no shared
       memory segment attached at shmaddr,  in  such  a  case  at
       return errno will be set to EINVAL.

NOTES

       On executing a fork(2) system call, the child inherits all
       the attached shared memory segments.

       The shared memory segments attached to a process executing
       an exec(2) system call will not be attached to the result-
       ing process.

       The following is a system parameter affecting a shmat sys-
       tem call:

       SHMLBA     Segment low boundary address multiple.  Must be
                  page aligned.  For the  current  implementation
                  the SHMBLA value is PAGE_SIZE.

       The  implementation has no intrinsic limit to the per pro-
       cess maximum number of shared memory segments (SHMSEG)

SEE ALSO

       ipc(5), shmctl(2), shmget(2).