Use Sbrk to Adjust the Program Break

Note: sbrk and brk are used to allocate memory.

These are deprecated (Nowadays mmap is preferred):

These were deprecated in POSIX 1997, and removed in POSIX.1-2001. That being said, most UNIXes still have them (Linux and Mac, for legacy compatibility reasons).

Here’s a possible implementation of sbrk.

static uintptr_t current_break; // Some global variable for your application.
                                // This would probably be properly tracked by the OS for the process
void *sbrk(intptr_t incr)
{
    uintptr_t old_break = current_break;
    current_break += incr;
    return old_break;
}

And let’s use it (note we’re using the OS’ sbrk, defined in <unistd.h>):

#include <stdio.h>
#include <unistd.h>
 
int main(void) {
  void *curr_address = sbrk(0);
  printf("current heap location: %p\n", curr_address); // our program break
 
  sbrk(16); // sbrk returns the old break, so this is still where curr_address was.
  void *next_address = sbrk(0);
  printf("current heap location: %p\n", next_address); // curr_address + 16
 
  sbrk(-16); // sbrk returns the old break, so this is still where curr_address was.
  void *new_address = sbrk(0);
  printf("current heap location: %p\n", new_address); // back to curr_address
}

Use Brk to Set the Program Break

Here’s a possible implementation of brk.

static uintptr_t current_break;
 
int brk(void* end_data_segment) {
  current_break = end_data_segment;
  return 0;
}

To set a pointer:

#include <unistd.h>
 
int main(void) {
  brk(0); // set current data segment to 0.
}

Caveats

Since a lot of libc functions end up using sbrk and brk under the hood, be careful with adjusting the heap with sbrk and brk. It might’ve readjusted or you might’ve readjusted the pointer for other calls.