allocate two pages, in C

sbrk.c

#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>

// same as calling sbrk(0)
void *get_brk (void)
{
  return (void *) syscall (SYS_brk, 0);
}

int main ()
{
  void *start_brk, *end_brk, *new_brk;
  long page_size = sysconf (_SC_PAGESIZE);
  printf ("Page Size: %ld\n", page_size);
  // Get the initial starting address of the heap
  start_brk = get_brk ();
  printf ("Initial heap start: %p\n", start_brk);

  // Allocate an entire page using sbrk
  new_brk = sbrk (page_size);
  end_brk = get_brk ();
  printf ("New heap start after allocating one page: %p\n", new_brk);
  printf ("New heap end after allocating one page: %p\n", end_brk);
  printf ("Address diff: %ld\n", end_brk - start_brk);

  // Extend the heap to two pages using brk
  new_brk = (void *) ((char *) start_brk + 2 * page_size);
  if (brk (new_brk) != 0)
  {
	perror ("brk");
  }

  end_brk = get_brk ();
  printf ("New heap start after extending to two pages: %p\n", start_brk);
  printf ("New heap end after extending to two pages: %p\n", end_brk);
  printf ("Address diff: %ld\n", end_brk - start_brk);

  return 0;
}

An example output of this program:

Page Size: 4096
Initial heap start: 0x5596cc2a6000
New heap start after allocating one page: 0x5596cc2a6000
New heap end after allocating one page: 0x5596cc2a7000
Address diff: 4096
New heap start after extending to two pages: 0x5596cc2a6000
New heap end after extending to two pages: 0x5596cc2a8000
Address diff: 8192