C和linux中的虚拟地址分配

Question

In the program given below virtual address for both process is same. I understood the reason for global variables but could not understand for local variables.

how is virtual addresses assigned to local variables before running?

int main()
{
  int a;
  if (fork() == 0)
  { 
     a = a + 5; 
    printf(“%d,%d\n”, a, &a); 
  }
  else 
  { 
     a = a –5;
    printf(“%d, %d\n”, a, &a); 
   } 
}

Answer 1

Virtual addresses are... virtual. That means a same virtual address from two different processes (like a parent process and its child process) points to two different physical addresses.

Answer 2

While compiling, the compiler decides to use either the stack or a register for local variables. In this case, the stack.

It also decides where in the (virtual) address space to place the stack.

So for both processes the stack starts in the same (virtual) address. And since the flow of this specific program is rather deterministic, the stack frames look exactly the same for both processes, resulting in the same offset in the stack for 'a'.

Answer 3

Whatever the address of a was before the fork, it must surely be the same after the fork, so it necessarily is the same in the two processes, since their addresses for a are both equal to the same thing. In most implementations, the address of a is derived by adding an offset (determined by the compiler) to the content of the stack pointer. The content of the stack pointer is duplicated by fork.