Thread: Iterative Fork() and Pipes in C

Hello all,

Hope you are all doing well.

I am very new to c programming and I have a task where I must make three (or more forks) communicate with the process that created them.

I used a for loop to create the forks, which did fine.
However, when trying to communicate, I used pipes (on the basis of FIFO), but I am not getting the output I need.

Let me explain:
The father must send to his sons their pids. (That is the son's pids).
He successfully sends it to the first son, but however, he sends the first son pid to the second son. And finally send the second son pid to the third one. Which is wrong.

What I want is that, the father sends to his sons their correct pids.

Here is the code and below is my output.
Can anyone help me please ?

Code:

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

#define reading 0
#define writing 1

main()
{
    int num_son,n;
    int *status;
    char c;

    //pipes declaration
    int pipe_father_son[2],pipe_son_father[2];
    
    printf("Father: I am the father and my pid is %d \n", getpid());
    
    //pipes initialisation
    pipe(pipe_son_father);
    pipe(pipe_father_son);

    int i;
    for(i=0; i<3; i++)
    {
        num_son=fork();
        if(num_son==-1)
        {
            printf("creation error \n");
            exit(1);
        }
        if(num_son!=0)    //father process
        {
            char message[15];
            int n,num;

            //the father send the son his number
            num = num_son;

            //close the pipe on read to write in it            
            close(pipe_father_son[reading]);
            n=write(pipe_father_son[writing],&num,sizeof(num));

            //the father receives a message from his son
            ////close the pipe on write to read in it
            close(pipe_son_father[writing]);
            n=read(pipe_son_father[reading],&message,sizeof(message));
            //printf("Father: i received %s\n",message);

            //the father replies to his son
            strcpy(message,"you are welcome");
            n=write(pipe_father_son[writing],&message,sizeof(message));

            //closes the pipes
            close(pipe_father_son[writing]);
            close(pipe_son_father[reading]);
            
            sleep(1);

        }
        else    //son process
        {
            int num_process,num_process_father,num;
            char message[15];

            num_process=getpid();
            num_process_father=getppid();

            //the son receives his number from his father
            close(pipe_father_son[writing]);
            n=read(pipe_father_son[reading],&num,sizeof(num));
            printf("Son: %d my pid is %d and i received %d from my father %d \n",i,num_process,num,getppid());

            //the son thanks his father
            close(pipe_son_father[reading]);
            strcpy(message,"thanks");
            n=write(pipe_son_father[writing],&message,sizeof(message));

            //the son receives the you are welcome message from his father
            n=read(pipe_father_son[reading],&message,sizeof(message));
            //printf("Son: I received %s\n",message);

            //closes the pipes
            close(pipe_father_son[reading]);
            close(pipe_son_father[writing]);
            
            sleep(1);
            exit(num_son);
            
        }
        

    }

    for(i=0; i<3; i++)
    {
        wait(&status);
        printf("Father: my son %d is over\n",i,(int) status/256);
        sleep(1);
    }
}

Father: I am the father and my pid is 1804
Son: 0 my pid is 1805 and i received 1805 from my father 1804
Son: 1 my pid is 1806 and i received 1805 from my father 1804
Son: 2 my pid is 1807 and i received 1806 from my father 1804
Father: my son 0 is over
Father: my son 1 is over
Father: my son 2 is overpost.c

First, you should #include <sys/types.h> and <sys/wait.h> for wait(), and then change status from an int* to an int. If you're using gcc, always build with the -Wall option, which will help catch such problems (there are more warnings it will yield that you should heed).

Here's a hint to help you figure out the problem: check the return value of your write() and read() calls.

thanks guys !
I'll try your suggestions soon as soon as possible and will reply.