/* demo valgrind program: cc -o demo demo.c */

#include <assert.h>
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define STAT_BUFFER_SIZE 4096
#define STAT_SKIP 27

extern ptrdiff_t __libc_stack_end;

int main(void) {
  int dummy;
  
  /* lifted from os_dep.c in libgc */
  char stat_buf[STAT_BUFFER_SIZE];
  int f = open("/proc/self/stat", O_RDONLY);
  if(f < 0 || read(f, stat_buf, STAT_BUFFER_SIZE) < 2 * STAT_SKIP) {
    perror("/proc/self/stat");
    return -1;
  }
  
  size_t buf_offset = 0;
  char c = stat_buf[buf_offset++];
  size_t i;
  for(i = 0; i < STAT_SKIP; ++i) {
    while(isspace(c)) c = stat_buf[buf_offset++];
    while(!isspace(c)) c = stat_buf[buf_offset++];
  }
  while(isspace(c)) c = stat_buf[buf_offset++];
  
  ptrdiff_t result = 0;
  while(isdigit(c)) {
    result *= 10;
    result += c - '0';
    c = stat_buf[buf_offset++];
  }
  close(f);
  
  printf("kernel stack base: %p\nlibc stack base: %p\ndumb stack base: %p\ndifference between kernel and dumb: %zd bytes\n",
         (void*) result, (void*) __libc_stack_end, (void*) &dummy, (void*) (result - (ptrdiff_t) &dummy));
}