Question

Write a C program that has a local and a global variable. The program uses a fork to create a child process. The parent process modifies both variables to be 10 and 20 and prints out the values. Then the child process modifies both variables to be 100 and 200 and prints out the values?

Explain the program output?

Answer 1

#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>
int global;
int main()
{
int local;
pid_t id1 = fork();
if (id1==0)
{
wait(NULL);
global=100;
local=200;
printf("Global:%d\tLocal:%d\n",global,local);
}
else
{
global=10;
local=20;
printf("Global:%d\tLocal:%d\n",global,local);   
wait(NULL);
}
return 0;
}

Explanation:

if (id1==0) // if id1==0 then it is child process, so we make it wait until parent is executed
{
wait(NULL);
global=100;
local=200;
printf("Global:%d\tLocal:%d\n",global,local);
}

// id pid>0 then it is parent process, so we modify the variables, print them and then make the parent wait until child also modifies variables and print them

else

{
global=10;
local=20;
printf("Global:%d\tLocal:%d\n",global,local);   
wait(NULL);
}