If the exit status value (*note Program Termination::) of the child
process is zero, then the status value reported by `waitpid' or `wait'
is also zero. You can test for other kinds of information encoded in
the returned status value using the following macros. These macros are
defined in the header file `sys/wait.h'.
-- Macro: int WIFEXITED (int STATUS)
This macro returns a nonzero value if the child process terminated
normally with `exit' or `_exit'.
-- Macro: int WEXITSTATUS (int STATUS)
If `WIFEXITED' is true of STATUS, this macro returns the low-order
8 bits of the exit status value from the child process. *Note
Exit Status::.
-- Macro: int WIFSIGNALED (int STATUS)
This macro returns a nonzero value if the child process terminated
because it received a signal that was not handled. *Note Signal
Handling::.
子进程的结束状态返回后存于status，底下有几个宏可判别结束情况
WIFEXITED(status)如果子进程正常结束则为非0值。
WEXITSTATUS(status)取得子进程exit()返回的结束代码，一般会先用WIFEXITED 来判断是否正常结束才能使用此宏。
WIFSIGNALED(status)如果子进程是因为信号而结束则此宏值为真
WTERMSIG(status)取得子进程因信号而中止的信号代码，一般会先用WIFSIGNALED 来判断后才使用此宏。
WIFSTOPPED(status)如果子进程处于暂停执行情况则此宏值为真。一般只有使用WUNTRACED 时才会有此情况。
WSTOPSIG(status)取得引发子进程暂停的信号代码
