WIFEXITED/WEXITSTATUS/WIFSIGNALED

 WIFEXITED/WEXITSTATUS/WIFSIGNALED 

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，

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