SYSTEM CALL: shmdt();
PROTOTYPE: int shmdt ( char *shmaddr );
RETURNS: -1 on error: errno = EINVAL (Invalid attach address passed)
After a shared memory segment is no longer needed by a process, it should be detached by calling
this system call. As mentioned earlier, this is not the same as removing the segment from the kernel!
After a detach is successful, the shm_nattch member of the associates shmid_ds structure is decremented
by one. When this value reaches zero (0), the kernel will physically remove the segment.