堆栈内存地址区

Question

I am currently trying to get in touch again with C, memory usage and assembly. I am working on an Ubuntu(64-bit) VM. Given the code bellow

#include <stdio.h>

int global_var;
int global_init_var=5;

void function(){
    int stack_var;
    printf("functions stack_var is at",&stack_var);
}

int main(){
    int stack_var;
    static int static_init_var=5;
    static int static_var;
    int *heap_var_ptr;

    printf("global_init_var is at adress 0x%08x\n",&global_init_var);
    printf("static_init_var is at adress 0x%08x\n",&static_init_var);
    printf("static_var is at adress 0x%08x\n",&static_var);
    printf("global_var is at adress 0x%08x\n",&global_var);
    printf("heap_var_ptr is at adress 0x%08x\n",heap_var_ptr);
    printf("stack_var is at adress 0x%08x\n",&stack_var);
    function();
}

I get the output:

global_init_var is at 0x00601040

static_init_var is at 0x00601044

static_var is at 0x0060104c

global_var is at 0x00601050

heap_var_ptr is at 0xb9f8dd00

stack_var is at 0xb9f8dcfc

functions stack_var is at 0xb9f8dcd4

my question is: Why is the stack adress area the same as the heap address area?

Answer 1

Your heap pointer variable inside main: ' int *heap_var_ptr; '
It will be allocated in stack only (4 bytes in general).
And getting the address as in your code :
printf("heap_var_ptr is at adress 0x%08x\\n",heap_var_ptr);
This will only get you address of some garbage stored inside 'heap_var_ptr'

If you really want to print address of heap and see the difference, modify your program with below lines:

1. Allocate some address using malloc/alloc and assign the pointer of allocated memory to your variable:
   int *heap_var_ptr = (int *)malloc(sizeof(int));
   This assigns the allocated heap memory address into your pointer variable stored in stack.

2. Now the address stored in pointer variable heap_var_ptr is from heap.
   printf("heap_var_ptr is at adress %p\\n",heap_var_ptr);

You will see the difference in addresses after these changes.

Whereas in your case, you have not used any memory from heap.
And Hence stack address area is same as of your heap pointer variable.