fork()实现信号通信与独立闹钟：输出异常问题咨询

问题描述 #

执行「分离闹钟与父子进程信号通信」任务时，使用fork()编写的代码出现两处异常：

• 在user2函数中用getpid()获取子进程PID，预期为22732，但实际输出22731；
• 输出末尾出现未编写打印逻辑的「User defined signal 2」消息，来源不明。

原代码 #

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>

void child_handler (int signum){
        printf("Child  (PID=%d) alarm went off\n", getpid());
        // send signal to parent
        kill( getppid(), SIGUSR1 ); 
        exit(EXIT_SUCCESS);
    }
    
void parent_handler (int signum){
        printf("Parent (PID=%d) received SIGUSR1 from child after child's alarm\n", getppid());
    }
    
void user2 (int signum){
        printf("Parent (PID=%d) alarm went off\n", getppid());
        printf("Parent (PID=%d) sending SIGUSR2 to my child %d\n", getppid(), getpid());
        kill( getpid(), SIGUSR2 );
        exit(EXIT_SUCCESS);
    }
    
int main(){
        int pid;
        int status;
        
        // 1.The parent process forks a child process.
        pid = fork();
        
        if (pid == 0){
            // child process
            printf("Child  (PID=%d) setting my alarm to 3 and then wait via pause()\n", getpid());
            signal(SIGALRM, child_handler);
            alarm(3);
            pause();
            // 2. The child process sets an alarm for 3 seconds and waits for the SIGALRM signal.
        } else if (pid > 0){
            // parent process
            printf("Parent (PID=%d) preparing waiting for SIGUSR1 via pause()\n", getppid());
            
            signal(SIGUSR1, parent_handler);
            pause();     
        
            // 5.The parent process sets an alarm for 5 seconds and waits for its SIGALRM signal.
            printf("Parent (PID=%d) Setting alarm for 5 seconds and wait via pause()\n", getppid());
            signal(SIGALRM, user2);
            alarm(5);
            pause();
    
            //waitpid(pid, &status, 0);
                
            
        } else {
            // error 
            perror("Failed to fork child process.\n");
            exit(EXIT_FAILURE);
        }
        return 0;
}

原运行输出 #

Parent (PID=15925) preparing waiting for SIGUSR1 via pause()
Child  (PID=22732) setting my alarm to 3 and then wait via pause()
Child  (PID=22732) alarm went off
Parent (PID=15925) received SIGUSR1 from child after child's alarm
Parent (PID=15925) Setting alarm for 5 seconds and wait via pause()
Parent (PID=15925) alarm went off
Parent (PID=15925) sending SIGUSR2 to my child 22731
User defined signal 2

问题分析与修复 #

1. 子进程PID获取错误 #

• 原因：user2是父进程的信号处理函数，在父进程中调用getpid()返回的是父进程自身的PID，而非子进程PID。子进程PID存储在main函数的局部变量pid中，user2无法访问该变量，导致输出错误；此外原代码中父进程打印自身PID时误用getppid()（返回父进程的父进程，即Shell进程的PID），应改为getpid()。
• 修复：将子进程PID存储为全局变量，让user2能直接获取；同时修正父进程打印自身PID的调用。

2. 「User defined signal 2」消息来源 #

• 原因：user2函数中kill(getpid(), SIGUSR2)是给父进程自身发送SIGUSR2信号，但未给父进程设置该信号的自定义处理函数，系统触发默认逻辑，打印该消息并终止进程。另外子进程在3秒后已退出，即使正确发送信号也会报错，但当前核心错误是信号发送对象错误。
• 修复：将kill(getpid(), SIGUSR2)改为kill(child_pid, SIGUSR2)，给子进程发送信号；同时添加错误处理，避免子进程已退出时的报错。

修复后代码 #

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>

// 全局变量存储子进程PID，供user2函数访问
pid_t child_pid;

void child_handler (int signum){
    printf("Child  (PID=%d) alarm went off\n", getpid());
    // 向父进程发送SIGUSR1
    kill(getppid(), SIGUSR1); 
    exit(EXIT_SUCCESS);
}

void parent_handler (int signum){
    printf("Parent (PID=%d) received SIGUSR1 from child after child's alarm\n", getpid());
}

void user2 (int signum){
    printf("Parent (PID=%d) alarm went off\n", getpid());
    printf("Parent (PID=%d) sending SIGUSR2 to my child %d\n", getpid(), child_pid);
    // 向子进程发送SIGUSR2，添加错误处理
    if (kill(child_pid, SIGUSR2) == -1) {
        perror("Failed to send SIGUSR2 to child");
    }
    exit(EXIT_SUCCESS);
}

int main(){
    int status;
    
    // 父进程创建子进程
    child_pid = fork();
    
    if (child_pid == 0){
        // 子进程逻辑
        printf("Child  (PID=%d) setting my alarm to 3 and then wait via pause()\n", getpid());
        signal(SIGALRM, child_handler);
        alarm(3);
        pause();
    } else if (child_pid > 0){
        // 父进程逻辑
        printf("Parent (PID=%d) preparing waiting for SIGUSR1 via pause()\n", getpid());
        
        signal(SIGUSR1, parent_handler);
        pause();     
    
        // 设置5秒闹钟并等待
        printf("Parent (PID=%d) Setting alarm for 5 seconds and wait via pause()\n", getpid());
        signal(SIGALRM, user2);
        alarm(5);
        pause();

        // 等待子进程退出，避免僵尸进程
        waitpid(child_pid, &status, 0);
            
    } else {
        // fork失败处理
        perror("Failed to fork child process.\n");
        exit(EXIT_FAILURE);
    }
    return 0;
}

内容的提问来源于stack exchange，提问作者tof