WIFEXITED(wstatus)
returns true if the child terminated normally, that is, by call‐
ing exit(3) or _exit(2), or by returning from main().
WEXITSTATUS(wstatus)
returns the exit status of the child. This consists of the
least significant 8 bits of the status argument that the child
specified in a call to exit(3) or _exit(2) or as the argument
for a return statement in main(). This macro should be employed
only if WIFEXITED returned true.
WIFSIGNALED(wstatus)
returns true if the child process was terminated by a signal.
SYNOPSIS
#include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
DESCRIPTION
The behavior of signal() varies across UNIX versions, and has also varied historically across different versions of Linux. Avoid its use: use sigaction(2) instead. See
Portability below.
signal() sets the disposition of the signal signum to handler, which is either SIG_IGN, SIG_DFL, or the address of a programmer-defined function (a "signal handler").
If the signal signum is delivered to the process, then one of the following happens:
* If the disposition is set to SIG_IGN, then the signal is ignored.
* If the disposition is set to SIG_DFL, then the default action associated with the signal (see signal(7)) occurs.
* If the disposition is set to a function, then first either the disposition is reset to SIG_DFL, or the signal is blocked (see Portability below), and then handler is
called with argument signum. If invocation of the handler caused the signal to be blocked, then the signal is unblocked upon return from the handler.
The signals SIGKILL and SIGSTOP cannot be caught or ignored.
RETURN VALUE
signal() returns the previous value of the signal handler, or SIG_ERR on error. In the event of an error, errno is set to indicate the cause.
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <signal.h>
6 #include <sys/types.h>
#include <sys/wait.h> 7 static void sig_child(int signo);
9 int main()
11 pid_t pid;
12 //创建捕捉子进程退出信号
13 signal(SIGCHLD,sig_child);
14pid = fork();
15if (pid < 0)
17 perror("fork error:");
18 exit(1);
20else if (pid == 0) {
21 printf("I am child process,pid id %d.I am exiting.\n",getpid());
22 exit(0);
24 printf("I am father process.I will sleep two seconds\n");
25//等待子进程先退出
26 sleep(2);
27 //输出进程信息
28 system("ps -o pid,ppid,state,tty,command");
29printf("father process is exiting.\n");
30 return 0;
33 static void sig_child(int signo)
35 pid_t pid;
36 int stat;
37 //处理僵尸进程
38 while ((pid = wait(&stat) >0)
39 printf("child %d terminated.\n", pid);
演示结果：
三、第三招：使用waitpid函数回收子进程：
1、还是先来看waitpid()函数的原型：
#include <sys/types.h>
#include <sys/wait.h>
pid_t waitpid(pid_t pid, int *status, int options);
The waitpid() system call suspends execution of the calling process until a child specified by pid argument has changed state. By default, waitpid() waits only for termi‐
nated children, but this behavior is modifiable via the options argument, as described below.
The value of pid can be:
< -1 meaning wait for any child process whose process group ID is equal to the absolute value of pid.
-1 meaning wait for any child process.
0 meaning wait for any child process whose process group ID is equal to that of the calling process.
> 0 meaning wait for the child whose process ID is equal to the value of pid.
The value of options is an OR of zero or more of the following constants:
WNOHANG return immediately if no child has exited.
WUNTRACED also return if a child has stopped (but not traced via ptrace(2)). Status for traced children which have stopped is provided even if this option is not speci‐
fied.
WCONTINUED (since Linux 2.6.10)
also return if a stopped child has been resumed by delivery of SIGCONT.
