ASLR, or Address Space Layout Randomization, is a security measure that changes where a process’s memory regions are placed on each run. This makes attacks that depend on fixed addresses harder. Linux applies ASLR to the stack, which we can observe by locating a local variable in /proc/self/maps:

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#include <stdint.h>
#include <stdio.h>

int main(void) {
  char marker;
  uintptr_t stack_address = (uintptr_t)&marker;

  FILE* maps = fopen("/proc/self/maps", "r");
  if (maps == NULL) {
    perror("fopen");
    return 1;
  }

  unsigned long start, end;
  char line[256];

  while (fgets(line, sizeof(line), maps) != NULL) {
    if (sscanf(line, "%lx-%lx", &start, &end) == 2 &&
        start <= stack_address && stack_address < end) {
      unsigned long size = end - start;
      printf("stack: [0x%lx, 0x%lx], size: %lu KiB\n",
             start, end, size / 1024);
      fclose(maps);
      return 0;
    }
  }

  fclose(maps);
  fputs("stack mapping not found\n", stderr);
  return 1;
}

Compile it and run it a few times:

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clang stack_aslr.c -o stack_aslr
repeat 3 ./stack_aslr

Example output:

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stack: [0x7ffc9b704000, 0x7ffc9b726000], size: 136 KiB
stack: [0x7ffd56d9e000, 0x7ffd56dc0000], size: 136 KiB
stack: [0x7ffc30977000, 0x7ffc30999000], size: 136 KiB