I am doing some C exercise for self-learning, and came across the following problem:
Part a:
int main(int argc, char **argv) {
int a = 5, b = 8;
int v;
v = fork();
if(v == 0) {
// 10
a = a + 5;
// 10
b = b + 2;
exit(0);
}
// Parent code
wait(NULL);
printf("Value of v is %d.\n", v); // line a
printf("Sum is %d.\n", a + b); // line b
exit(0);
}
Part b:
int main(int argc, char **argv) {
int a = 5, b = 8;
int v;
v = vfork();
if(v == 0) {
// a = 10
a = a + 5;
// b = 6
b = b - 2;
exit(0);
}
// Parent code
wait(NULL);
printf("Value of v is %d.\n", v); // line a
printf("Sum is %d.\n", a + b); // line b
exit(0);
}
We have to compare the outputs of line a and line b.
The outputs of part a is:
Value of v is 79525.
Sum is 13.
The outputs of part b is:
Value of v is 79517.
Sum is 16.
It appears in part a, the sum is the sum of the initial declaration of a and b, whereas in part b, the sum include the summation within the child process.
My question is - why is this happening?
According to this post:
The basic difference between the two is that when a new process is created with vfork(), the parent process is temporarily suspended, and the child process might borrow the parent's address space. This strange state of affairs continues until the child process either exits, or calls execve(), at which point the parent process continues.
The definition of parent process is temporarily suspended doesn't make much sense to me. Does this mean that for 1b, the program waits until the child process to finish running (hence why the child process variables get summed) before the parent runs?
The problem statement also assumes that "the process ID of the parent process maintained by the kernel is 2500, and that the new processes are created by the operating system before the child process is created."
By this definition, what would the value of v for both programs be?
