##1. 如何直接跳出深层递归而不是一层一层跳出？
- throw exception，try catch
- 把递归函数放到一个独立线程执行，在主线程做 condition wait，递归结束时notify下，然后直接退出线程。
- C语言的setjmp + longjmp, setjump保存context, longjmp恢复context, try catch其实做的就是这个事情.
##2. Can the OS be interrupted? Sure.
##3. What creates the first process? init is the first process, hard encoded in OS. Try run pstree in your linux machine.
##4. What state do you think a process is in most of the time? Running, Ready or Waitting? Ready.
##5. How many processes can a system support? In Linux it is 65536
cat /proc/sys/kernel/pid_max
##6. What does it mean for exec to return? Does not return if OK, returns −1 on error
##7. fork() can return an error. Why might this happen? When running out of memory or exceed processes a system can support.
##8. What happens if you run “exec csh” in your shell? What happens if you run “exec ls” in your shell? Try it.
##9. What happens if a parent process exits before a child? Child process become zombie.
##10. What is double fault? On the x86 architecture, a double fault exception occurs if the processor encounters a problem while trying to service a pending interrupt or exception. An example situation when a double fault would occur is when an interrupt is triggered but the segment in which the interrupt handler resides is invalid. If the processor encounters a problem when calling the double fault handler, a triple fault is generated and the processor shuts down.