# Unix世界里经常看到的 pst 是 pseudo terminal 的意思啊。
# ps -t pts/6 -o pid,ppid,tty,stat,args,wchan
ps 命令的进程状态。 S表示进程在睡眠，I表示进程在等待输入，O表示进程在等待输出。当进程在S状态时，wlan指示了更详细的状态信息。
# SIGSTOP 和 SIGKILL 两个posix信号不可以被caught.
# 缺省情况下，Unix的信号是不排在队列中的。这意味着多个相同signal到达的时候如果没有来得及处理，就只会记下一个signal.如果需要稳定的信号支持，就要使用RealTime Posix接口。
# init 进程的PID是 1.
# 可以用sigprocmask函数block和unblock信号。This let us protect a critical region of code by preventing certain signals from being caught while that region of code is executing.
# 对于 Unix System V 和 Unix98, the child of a process does not become a zombie if the process sets the disposition of SIGCHLD to SIG_IGN. unfortunately, tis works only under System V & Unix98. Posix明确指票这个行为是未定义的。The portable way to handle zombies is to catch SIGCHILD & call wait or waitpid.
# waitpid 有个 WNOHANG 选项可以让waitpid立即返回。
# POSIX定义了asynchronous I/O model .但是 few systems support POSIX asynchronous I/O. The main difference between asynchronous I/O model and signal-driven I/O model is that with signal-driven I/O, the kernel tells us when an I/O operation can be initiated but with asynchronous I/O, the kernel tells us when an I/O operation is completed.
# 想得到本机到某台机的RTT。怎么做呢？kao,不要太容易啊。用 ping 啊！！！