Pass the value from child to parent process

Question

I have this code that is supposed to create three child process' and each will perform a small mathematical operation. Then, the parent is supposed to use the results from all the child process' and get a final answer but I can't find a way to actually read the result from the child in the parent. Is there a way to do this?

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

int main(void)
{
   int pid1, pid2, pid3, status;
   int a=1, b=2, c=5, d=4, e=6, f=3, g;
   int t1, t2, t3;

   printf("Hello World!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!\n");
   printf("Here I am  before use of forking\n");
   printf("I am the PARENT process and pid is : %d\n",getpid());

   pid1 = fork( );
   if (pid1 == 0)
   {      
    printf("\n\nHere I am just after child forking1\n");
    printf("I am the Child process and pid1 is :%d\n",getpid());      
    printf("My parent's pid is :%d\n",getppid());   
    t1 = a+b;
    printf("The answer for t1 is: %d\n", t1);       
    exit(0);
   }
   else
   {
    wait(&status);
        printf("\nHere I am just after parent forking1\n");
        printf("I am the Parent process and pid is: %d\n",getpid());
   }

   pid2 = fork( );
   if (pid2 == 0)
   {      
    printf("\n\nHere I am just after child forking2\n");
    printf("I am the Child process and pid2 is :%d\n",getpid());      
    printf("My parent's pid is :%d\n",getppid());   
    t2 = c+d;
    printf("The answer for t2 is: %d\n", t2);   
    exit(0);    
   }
   else
   {
    wait(&status);
        printf("\nHere I am just after parent forking2\n");
        printf("I am the Parent process and pid is: %d\n",getpid());
   }

   pid3 = fork( );
   if (pid3 == 0)
   {      
    printf("\n\nHere I am just after child forking3\n");
    printf("I am the Child process and pid3 is :%d\n",getpid());      
    printf("My parent's pid is :%d\n",getppid());   
    t3 = e/f;   
    printf("The answer for t3 is: %d\n", t3);   
    exit(0);
   }
   else
   {
    wait(&status);
        printf("\nHere I am just after parent forkingALL\n");
        printf("I am the Parent process and pid is: %d\n",getpid());
   }


   printf("\n\nThe final answer for t1 is: %d\n", t1);
   printf("The final answer for t2 is: %d\n", t2);
   printf("The final answer for t3 is: %d\n", t3);


   g = t1*t2-t3;
   printf("The final answer for g is: %d\n", g);
}

Answer 1

You can do this with a very easy technique, which is shared memory. I will give a complete example of how it works.

First, let's assume I want to write a program to print the first nterms in Fibonacci series (and I know it is not that logical to do so, but it is an easy example so everyone can understand it).

1. I have a parent which reads an integer value representing the first n terms
2. Then the parent process will create a child and pass n to it
3. Then the child should calculate the first n terms and return them back to the parent.

---

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>

void printFibo(int n, int *fibo)
{
    int i;
    for(i=0; i<=n; i++)
        printf("%d -> %d\n" ,i, fibo[i]);
}

void computeFibo(int n, int *fibo) 
{
    int i;
    fibo[0] = 0;
    fibo[1] = 1;

    for (i=2; i<=n; i++) 
        fibo[i] = fibo[i-1] + fibo[i-2];
}

int main(int argc, char *argv[])
{
    pid_t childPID;
    int status;
    int shm_fd;
    int* shared_memory;
    int msize; // the size (in bytes) of the shared memory segment 
    const char *name = "FIBONACCI_SERIES";
    int n;

    if (argc!=2) 
    {
        fprintf(stderr, "usage: %s <Fibonacci number to be generated>\n", argv[0]);
        return -1;
    }

    n = atoi(argv[1]);
    if (n < 0) 
    {
        fprintf(stderr, "Illegal fibonacci number: %s\n", argv[1]);
        return -2;
    }

    // calculating the array size based on the number of terms being passed from child to parent
    msize = (n+2)*sizeof(int); 

    // open the memory
    shm_fd = shm_open (name, O_CREAT | O_EXCL | O_RDWR, S_IRWXU | S_IRWXG);
    if (shm_fd < 0) 
    {
        fprintf(stderr,"Error in shm_open()");
        return -3;
    }

    printf("Created shared memory object %s\n", name);

    // attach the shared memory segment
    ftruncate(shm_fd, msize);
    printf("shmat returned\n");

    // allocating the shared memory
    shared_memory = (int *) mmap(NULL, msize, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
    if (shared_memory == NULL) 
    {
        fprintf(stderr,"Error in mmap()");
        return -3;
    }

    printf("Shared memory segment allocated correctly (%d bytes).\n", msize);

    shared_memory[0] = n;

    childPID=fork();
    if ( childPID == -1 ) 
    {
        fprintf(stderr, "Cannot proceed. fork() error");
        return -4;
    }
    if (childPID  == 0) 
    {
        // then we're the child process
        computeFibo(shared_memory[0],shared_memory+1);
        exit(0);
    }
    else
    {
        // parent will wait until the child finished
        wait(&status);

        // print the final results in the 
        printFibo(shared_memory[0], shared_memory+1);

        // now detach the shared memory segment
        shm_unlink(name);
    }
    return 0;
}

Answer 2

If you want to do it without using any way of communication i.e pipes, shared memory then you will have to use exit() system call. The exit system call return a signal that is then caught by wait() system call in parent process. Here I am giving you a code in which I am sending a value from child to parent. One last thing you have to divide the signal caught by wait by 255 to get the exact value. `

    #include<unistd.h>
    #include<stdio.h>
    #include<stdlib.h>
    #include<sys/types.h>
    #include<sys/wait.h>
    int main(int argc,char *argv[])
    {
       pid_t pid=fork();
       if(pid==0)
       {//child
            int sum=5+7;
            exit(sum);//sending exiting status or any value to parent
       }
       else
       {//parent
            int childval=-1;
            wait(&childval);//catching signal sent by exit of(child) 
            printf("%d",childval/255);//changing signal to exact value  
       }    
        return 0;
    }

`