shmop(2)                            CLIX                            shmop(2)



  NAME

    shmop, shmat, shmdt - Shared memory operations

  LIBRARY

    Standard C Library (libc.a)

  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 );

  PARAMETERS

    shmid     Specifies a shared memory segment identifier obtained from a
              successful call to the shmget() function.

    shmaddr   Specifies the address of a shared memory segment.

    shmflg    Represents the shared memory operation to be performed.

  DESCRIPTION

    The shmat() function 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 the value of shmaddr is equal to 0, the segment is attached at the
       first available address as selected by the system.

    ⊕  If shmaddr is not equal to 0 and (shmflg & SHM_RND) is ``true'', the
       segment is attached at the address given by (shmaddr - (shmaddr modulus
       SHMLBA).

    ⊕  If shmaddr is not equal to 0 and (shmflg & SHM_RND) is ``false'', the
       segment is attached at the address given by shmaddr (see the
       <sys/shm.h> file).




  2/94 - Intergraph Corporation                                              1

  shmop(2)                            CLIX                            shmop(2)



    The shmdt() function 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 & SHM_RDONLY) is ``true'',
    otherwise it is attached for reading and writing.

  EXAMPLES

    1.  To attach a shared memory region at a specified, segment-aligned
        address:

        int shmid;
        char *addr;

        if ((shmid = shmget(75, 128 * 1024, 0666 | IPC_CREAT)) < 0) {
             perror("shmget() failed");
             exit(1);
        }

        if ((addr = shmat(shmid, 0xc040000, SHM_RND)) == (char *)-1) {
             perror("Could not attach memory");
             if (shmctl(shmid, IPC_RMID, (struct shmid_ds *)0) < 0)
                  perror("Could not remove shared memory");
             exit(1);
        }


    2.  To detach a shared memory segment:

        if (shmdt(addr) < 0)
             perror("Could not detach shared memory");


  NOTES

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

  RETURN VALUES

    Upon successful completion, the return value is as follows:

    ⊕  The shmat() function returns the data segment start address of the
       attached shared memory segment.

    ⊕  The shmdt() function returns a value of 0.

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

  ERRORS



  2                                              Intergraph Corporation - 2/94

  shmop(2)                            CLIX                            shmop(2)



    The shmat() function will fail and not attach the shared memory segment if
    one or more of the following are true:

    [EINVAL]   The value of shmid is not a valid shared memory identifier.

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

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

    [EINVAL]   The value of shmaddr is not equal to 0 (shmflg & SHM_RND) is
               ``true'', and the value of (shmaddr - (shmaddr modulus SHMLBA)
               is an illegal address.

    [EINVAL]   The value of shmaddr is not equal to 0, (shmflg &  SHM_RND) 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]   The shmdt() function will fail and not detach the shared memory
               segment if shmaddr is not the data segment start address of a
               shared memory segment.

  RELATED INFORMATION

    Functions:  exec(2), exit(2), fork(2), intro(2), shmctl(2), shmget(2)


























  2/94 - Intergraph Corporation                                              3