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

unix环境高级编程-7-process env