shmid

    A shared memory identifier, a positive integer. It is created
    by the shmget() function and used to identify the shared
    memory segment on which to perform the control operation.

 cmd

    The control operation (IPC_STAT, IPC_SET, or IPC_RMID) to perform
    on the shared memory segment. For more information, see the
    Description.

 buf

    Pointer to a shmid_ds structre, defined in <shm.h> as follows:

    struct shmid_ds {
    struct ipc_perm shm_perm; /* permissions structure defined in <ipc.h> */
    size_t shm_segsz;         /* size of memory segment in bytes */
    pid_t shm_lpid;           /* Process ID of last memory operation */
    pid_t shm_cpid;           /* Process ID of segment creator */
    shmatt_t shm_nattch;      /* Number of current attaches */
    time_t shm_atime;         /* time of last shmat */
    time_t shm_dtime;         /* time of last shmdt */
    time_t shm_ctime;         /* time of last change */
    }