如何在C中告诉子进程另一个子进程的pid？

Question

I am writing a program in which I need to create two child processes, a producer and a consumer. The producer writes on a file what is read from stdin, the consumer reads the same file after the producer has written the line. I need to synchronize the two processes and I wanted to do so by using signals, but I have now a problem in that I cannot send (using the kill() function) the signals from the consumer to the producer.

This is my program:

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

void catcherp(){};
void catcherc(){};

pid_t producer, consumer;

int main () {
int status_consumer, status_producer;
char string[128], reading[128];
FILE * fout, *finn;


producer = fork();
if (producer == 0){
    signal(SIGUSR2, catcherp);
    // producer process, child
    while(1){
        fout = fopen ("test.txt", "w");
        printf ("?: ");
        scanf ("%s", string);
        fputs(string, fout);        
        fclose(fout);

        kill(consumer, SIGUSR1);
        pause();
    }

    exit(0);
}   else {
    // parent process
    consumer = fork ();
    if (consumer == 0) { 
        signal(SIGUSR1, catcherc);
        // consumer process, child
        while(1) {              
            pause();
            finn = fopen ("test.txt", "r");
            fgets(reading, 128, finn);
            printf("%s\n", reading);
            fclose(finn);               

            kill (producer, SIGUSR2);

        }
        exit(0);
    } else {
        printf("This is the parent process\n");
        waitpid(producer, &status_producer, 0);
        waitpid(consumer, &status_consumer, 0);
        printf("The children exited\n");
    }
}
return 0;
}

The exit(0) commands in both child processes are there because I still have to implement the exit condition for the loop. I am pretty sure that my problem lies in how I create the consumer process after creating the producer process. That means that the producer sees the "consumer" pid to be 0, which terminates the program.

Now, I would like to understand how I'm supposed to create two concurrent processes using the fork() function (if it's possible), can someone enlighten me?

Answer 1

In the end I managed to solve the problem, but I had to use the temporary file in order to get the pid of the producer process when inside the consumer process. I was hoping to find a smarter way but the solution given by the course was basically the same.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

void catcher(){};

int main () {
    int status_consumer, status_producer; // needed for the waitpid functions
    pid_t producer, consumer; // pids of the child processes
    char string[128]; // input string
    FILE * f; // fp

    signal (SIGUSR1, catcher);

    consumer = fork();
    if (consumer == 0) { 
        // child process
        while (1) {     
            pause(); // wait for the ready signal from the sender
            f = fopen ("tmp.txt", "r");
            fscanf (f, "%d %s", &producer, string); // read string and the pid of the sender
            printf("%s\n", string);   
            fclose(f); 

            if (strcmp("end", string) == 0) {
                break; // exit the loop when the word "end" is read
            }
            kill (producer, SIGUSR1);       
        }

        exit(0);
    } else {
        producer = fork (); 
        if (producer == 0) {
            // child process
            while(1) {      
                f = fopen ("tmp.txt", "w");
                printf("?: ");
                scanf("%s", string); // read from stdin the string
                fprintf(f, "%d\t%s\n", getpid(), string); // write on tmp.txt the string
                fclose(f);

                kill(consumer, SIGUSR1);
                if (strcmp("end", string) == 0){
                    break; // exit the loop when the word "end" is read
                }           
                pause();
            }
        } else {
            // parent process
            waitpid(producer, &status_producer, 0);
            waitpid(consumer, &status_consumer, 0);
        }

    }
     return 0;  // end of program
 }

Thanks for the comments