Linux SIGCHLD信号处理

SIGCHLD信号处理

SIGCHLD的产生条件1、子进程终止时
2、子进程接收到SIGSTOP信号停止时
3、子进程处在停止态，接受到SIGCONT后唤醒时源代码signal_test.c#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
​
void handle_sig_child()
{
int status;
pid_t pid;
pid = waitpid(-1, &status, WUNTRACED | WCONTINUED);
printf("recv child pid %d \n", pid);
if(WIFEXITED(status))
printf("child process exited with %d\n", WEXITSTATUS(status));
else if(WIFSIGNALED(status))
printf("child process signaled with %d\n", WTERMSIG(status));
else if(WIFSTOPPED(status))
printf("child process stoped\n");
else if(WIFCONTINUED(status))
printf("child process continued\n");
}
int main(int argc, char** argv)
{
pid_t pid;
/*捕捉SIGCHLD信号*/
signal(SIGCHLD, handle_sig_child);
pid = fork();
if(pid == 0)
  {
    sleep(5);
    printf("child PID [%d]\n", getpid());
    exit(0);
  }
else if(pid != -1)
  {
    while(1)
        sleep(1);
  }
else
  {
    printf("fork error\n");
  }
}运行结果:where@ubuntu:~$ ./sigal_child_test
child PID [9881]
child PID [9881] return [0] 其他的实验测试，你可以使用kill命令给子进程发送SIGSTOP、SIGCONT以及SIGINT等信息，观察现象。​ 单个子进程的时候可以这么处理，但是如果有多个子进程，如果多个子进程在极短的时间类同时退出产生SIGCHLD信息，那么由于未决信号集不支持排队，有可能有些信号就不执行了。看下面的例子：#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
​
void handle_sig_child()
{
int status;
pid_t pid;
pid = waitpid(-1, &status, WUNTRACED | WCONTINUED);
printf("recv child pid %d \n", pid);
if(WIFEXITED(status))
printf("child process exited with %d\n", WEXITSTATUS(status));
else if(WIFSIGNALED(status))
printf("child process signaled with %d\n", WTERMSIG(status));
else if(WIFSTOPPED(status))
printf("child process stoped\n");
else if(WIFCONTINUED(status))
printf("child process continued\n");
}
int main(int argc, char** argv)
{
pid_t pid;
/*捕捉SIGCHLD信号*/
signal(SIGCHLD, handle_sig_child);
int count = 0;
​
AGAIN:
pid = fork();  //fork十次
if(pid == 0)
{
sleep(5);
printf("child PID [%d]\n", getpid());
exit(0);
}
if(++count < 10)
goto AGAIN;
while(1)
sleep(1);
}运行结果：$ ./a.out
child PID [7785]
child PID [7784]
child PID [7783]
child PID [7782]
child PID [7781]
child PID [7787]
child PID [7788]
child PID [7780]
child PID [7779]
child PID [7786]
recv child pid 7779
child process exited with 0
recv child pid 7780
child process exited with 0
recv child pid 7781
child process exited with 0
​上面的结果产生十个子进程，退出后产生十个SIGCHLD，但是只执行了3次信号处理函数。$ ps -ef
where     7778  3197  0 18:34 pts/2    00:00:00 ./a.out
where     7782  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7783  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7784  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7785  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7786  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7787  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7788  7778  0 18:34 pts/2    00:00:00 [a.out] <defunct>
where     7790  4193  0 18:34 pts/18   00:00:00 ps -ef出现未回收的情况，我们需要改进一下回收函数。#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
​
void handle_sig_child()
{
int status;
pid_t pid;
do
{
pid = waitpid(-1, &status, WUNTRACED | WCONTINUED | WNOHANG);
printf("recv child pid %d \n", pid);
if(WIFEXITED(status))
printf("child process exited with %d\n", WEXITSTATUS(status));
else if(WIFSIGNALED(status))
printf("child process signaled with %d\n", WTERMSIG(status));
else if(WIFSTOPPED(status))
printf("child process stoped\n");
else if(WIFCONTINUED(status))
printf("child process continued\n");
}while(pid != -1);
}
int main(int argc, char** argv)
{
pid_t pid;
/*捕捉SIGCHLD信号*/
signal(SIGCHLD, handle_sig_child);
int count = 0;
​
AGAIN:
pid = fork();
if(pid == 0)
{
sleep(5);
printf("child PID [%d]\n", getpid());
exit(0);
}
if(++count < 10)
goto AGAIN;
while(1)
sleep(1);
}运行结果：$ ./a.out
child PID [7851]
child PID [7852]
child PID [7847]
child PID [7850]
child PID [7849]
child PID [7853]
child PID [7854]
child PID [7855]
child PID [7856]
child PID [7848]
recv child pid 7847
child process exited with 0
recv child pid 7848
child process exited with 0
recv child pid 7849
child process exited with 0
recv child pid 7850
child process exited with 0
recv child pid 7851
child process exited with 0
recv child pid 7852
child process exited with 0
recv child pid 7853
child process exited with 0
recv child pid 7854
child process exited with 0
recv child pid 7855
child process exited with 0
recv child pid 7856
child process exited with 0
recv child pid -1
child process exited with 0
recv child pid -1
child process exited with 0 十个子进程都完美回收。