1. exit handler.
atexit(void (*func)(void)); #automatically called by exit()
exit() calls these functions in reverse order of their registeration. Ease function is called as many times as it was registered.
2. environment list
extern char ** environ
access to specific environment variable, use getenv() and putenv()
3. memory arragement
unix command size report the size(in bytes) of the text , data and bss segments
