Why doesn't array work by using fork() function?

Question

I try to solve the question.

Question is that

I should creat a process creating child process.

Child process should write 1 to 10 to the int array size of 10 { initial value of 0 }.

Mother process should wait until the child done writing. AFter the child process finishes, the mother process print out the array ( ex. 1,2,...,10) and replace all to -1 and print out the array one more time.

Child is a producer and Mother is a consumer. ( Only one child and Only one consumer. and they do this only once.)

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

int main(){
int arr[10]={0,};
pid_t pid;
int state;
pid = fork();

if(pid == -1) // error
{
    printf("can't fork, error\n");
    exit(-1);
}

else if (pid == 0) // Child ( producer )
{
    printf("\nProducer is created.\n");

    printf("array: ");
    for(c=0; c<10; c++)
    {
        printf("%d ", arr[c]);
        arr[c]=c+1;
    }

}

else // Mother ( consumer )
{
    pid=wait(&state);
    sleep(1);
    printf("\nConsumer takes control of array");


    printf("\narray:");

    for(j=0;j<10;j++) 
    {
        printf(" %d", arr[j]);
    }


    printf("\nConsumer is done.");

    printf("\narray: ");
    for ( i =0; i<10; i++)
    {

        arr[i]=-1;
        printf("%d ", arr[i]);


    }
    printf("\ndone\n");
    exit(0);
}
return 0;}

I want output

0 0 0 0 0 0 0 0 0 0

1 2 3 4 5 6 7 8 9 10

-1 -1 -1 -1 -1 -1 -1 -1 -1 -1

but

my out put is

0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0

-1 -1 -1 -1 -1 -1 -1 -1 -1 -1

what's the problem with my code?

Answer 1

The memory address spaces of the processes will "copy on write". So the array is not shared between the two processes.

Answer 2

If you want to share substantial data between different processes (like parent and child), there are a number of ways. Three of the most common are

1. Producer writes to a file; Consumer reads the file
2. Shared Memory, as provided by the shm* API of the Unix kernel
3. Memory mapped files, as provided by the mmap API of the Unix kernel

Suggestion: read everything you can find by the late great W. Richard Stevens, such as Programming in the Unix Environment .