Linux SIGCHLD訊號處理

嚇人的猿發表於2018-02-27
LinuxGC

SIGCHLD訊號處理

SIGCHLD的產生條件

1、子程式終止時
2、子程式接收到SIGSTOP訊號停止時
3、子程式處在停止態，接受到SIGCONT後喚醒時

原始碼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訊號*/
    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");
    }
}

執行結果:

where@ubuntu:~$ ./sigal_child_test 
child PID [9881]
child PID [9881] return [0]

其他的實驗測試，你可以使用kill命令給子程式傳送SIGSTOP、SIGCONT以及SIGINT等資訊，觀察現象。

​

單個子程式的時候可以這麼處理，但是如果有多個子程式，如果多個子程式在極短的時間類同時退出產生SIGCHLD資訊，那麼由於未決訊號集不支援排隊，有可能有些訊號就不執行了。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訊號*/
    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);
}

執行結果：

$ ./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
​

上面的結果產生十個子程式，退出後產生十個SIGCHLD，但是隻執行了3次訊號處理函式。

$ 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

出現未回收的情況，我們需要改進一下回收函式。

#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訊號*/
    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);
}

執行結果：

$ ./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

十個子程式都完美回收。

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