shmop(2)                                                           shmop(2)


   NAME
         shmop: shmat, shmdt - shared memory operations

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

         void *shmat(int shmid, void *shmaddr, int shmflg);

         int shmdt (void *shmaddr);

   DESCRIPTION
         shmat attaches the shared memory segment associated with the shared
         memory identifier specified by shmid to the data segment of the
         calling process.  The segment is attached at the address specified by
         one of the following criteria:

               If shmaddr is equal to (void *) 0, the segment is attached at
               the first available address as selected by the system.

               If shmaddr is not equal to (void *) 0 and (shmflg&SHMRND) is
               true, the segment is attached at the address given by (shmaddr
               - (shmaddr modulus SHMLBA)).

               If shmaddr is not equal to (void *) 0 and (shmflg&SHMRND) is
               false, the segment is attached at the address given by shmaddr.

         shmdt detaches from the calling process's data segment the shared
         memory segment located at the address specified by shmaddr.

         The segment is attached for reading if (shmflg&SHMRDONLY) is true
         {READ}, otherwise it is attached for reading and writing
         {READ/WRITE}.

         shmat fails and does not attach the shared memory segment if one or
         more of the following are true:

         EINVAL         shmid is not a valid shared memory identifier.

         EACCES         Operation permission is denied to the calling process
                        [see intro(2)].

         ENOMEM         The available data space is not large enough to
                        accommodate the shared memory segment.

         EINVAL         shmaddr is not equal to zero, and the value of
                        (shmaddr - (shmaddr modulus SHMLBA)).  is an illegal
                        address.




   7/91                                                                 Page 1

   shmop(2)                                                           shmop(2)


         EINVAL         shmaddr is not equal to zero, (shmflg&SHMRND) is
                        false, and the value of shmaddr is an illegal address.

         EMFILE         The number of shared memory segments attached to the
                        calling process would exceed the system-imposed limit.

         EINVAL         shmdt fails and does not detach the shared memory
                        segment if shmaddr is not the data segment start
                        address of a shared memory segment.

   SEE ALSO
         intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2).

   DIAGNOSTICS
         Upon successful completion, the return value is as follows:

               shmat returns the data segment start address of the attached
               shared memory segment.

               shmdt returns a value of 0.

         Otherwise, a value of -1 is returned and errno is set to indicate the
         error.

   NOTES
         The user must explicitly remove shared memory segments after the last
         reference to them has been removed.


























   Page 2                                                                 7/91