当父母将数据写入与分叉子项共享的管道并且两者都正在读取数据时，如何防止父母在孩子之前阅读它？

Question

I have a C language code implementing pipes for parent and child communication. I constantly need to read and write from both parent and child. A smaller version of code is as follows:

int main(){
    int cpid, var;
    int fds[2];

    pipe(fds);

    if((cpid=fork())==0){
        read(fds[0], &var, sizeof(int));
        printf("Recieved in child - %d\n",var);

        var = 3;
        write(fds[1], &var, sizeof(int));
        printf("Writing from child - %d\n",var);

        exit(0);
    }

    var = 5;
    write(fds[1], &var, sizeof(int));
    printf("Writing from parent - %d\n",var);

    // I need something here to execute following read only after child writes

    read(fds[0], &var, sizeof(int));
    printf("Recieved in parent - %d\n",var);

    waitpid(cpid,NULL,0);
    return 0;
}

But I am facing problem that the read in parent process is getting executed just after write from parent. I need to prevent this from happening. Is there any way we can stop read in parent until child has written in above code? Any algorithms or suggestions are welcome.

PS: I have tried using signals like SUGUSR1 and pausing the parent until child sends the signal but as signals are not queued the concept of signals in this case fails for multiple child parent pipe communication.

Answer 1

According to comments from @JonathanLeffler and @EugeneSh. single pipe is not suited for bidirectional communication. So the above code can be modified to use one pipe for parent-child and one pipe for child-parent communication as follows:

int main(){
    int cpid, var;
    int p2c[2], c2p[2];

    pipe(p2c);
    pipe(c2p);

    if((cpid=fork())==0){
        read(p2c[0], &var, sizeof(int));
        printf("Recieved in child - %d\n",var);

        var = 3;
        write(c2p[1], &var, sizeof(int));
        printf("Writing from child - %d\n",var);

        exit(0);
    }

    var = 5;
    write(p2c[1], &var, sizeof(int));
    printf("Writing from parent - %d\n",var);

    read(c2p[0], &var, sizeof(int));
    printf("Recieved in parent - %d\n",var);

    waitpid(cpid,NULL,0);
    return 0;
}

This would apply to multiple child-parent logic also.