The shmctl() function provides a variety of shared memory control
    operations as specified by the cmd argument. It can have the
    following values:

    IPC_STAT

        Place the current value of each member of the data structure
        associated with shmid into the structure pointed to by buf. The
        contents of this structure are defined in <shm.h>.

    IPC_SET

        Set the value of the following members of the data structureure
        associated with shmid to the corresponding value found in the
        structure pointed to by buf:

        shm_perm.uid
        shm_perm.gid
        shm_perm.mode /* only low 9 bits */

        This cmd can be executed by a process that has an effective
        user ID equal to either that of a user having appropriate
        privileges or to the value of either shm_perm.uid or
        shm_perm.cuid in the data structure associated with shmid.

        IPC_RMID

        Remove the shared memory identifier specified by shmid from the
        system and delete the shared memory segment and data structure
        associated with it. If the segment is attached to one or more
        processes, the segment key is changed to IPC_PRIVATE and the
        segment is marked as removed. The segment disappears when the
        last attached process detaches it. This cmd can be executed by
        a process that has an effective user ID equal to either that of
        a user with appropriate privileges or to the value of either
        shm_perm.uid or shm_perm.cuid in the data structure associated
        shmid.