The shmget() function returns the shared memory identifier
    associated with the key.

    A shared memory identifier with its associated shmid_ds data
    structure is created for a key if one of the following is TRUE:

    o  The key argument is equal to IPC_PRIVATE.
    o  The key argument does not already have a shared memory
       identifier associated with it and (shmflg and IPC_CREAT) is
       non-zero.

    If shmflg specifies both IPC_CREAT and IPC_EXCL and a shared memory
    segment already exists for a key, shmget() function fails with
    errno set to EEXIST.

    When it is created, the shmid_ds data structure associated with the
    new shared memory identifier is initialized as follows:

    o  The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and
       shm_perm.gid are set equal to the effective user ID and effective
       group ID, respectively, of the calling process.
    o  The low order 9 bits of shm_perm.mode are set equal to the low
       order 9 bits of the shmflg argument.
    o  The variable shm_segsz is set equal to the value of the size
       argument.
    o  The variables shm_lpid, shm_nattch, shm_atime, and shm_dtime are
       set equal to zero, the variable shm_cpid is set equal to the
       Process ID of the segment creator, and the variable shm_ctime is
       set equal to the current time.