a.c

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

/*
 * Calls mprotect on the entire page that address is located in.
 */
static int mprotect_page(void *address, int prot) {
  int pagesize = getpagesize();

  address -= (long int)address % pagesize;

  return mprotect((void *)address, pagesize, prot);
}

int main() {
  /* "Hello world from 0x401000" */
  printf("Hello world from 0x%lx\n", (long int)&main);
  /* "Page size: 0x1000" */
  printf("Page size: 0x%x\n", getpagesize());

  if (mprotect_page(&main, PROT_READ | PROT_WRITE | PROT_EXEC)) {
    perror("error");
  }

  /* 0x4010db is the address of the number 1 below */
  *((int *)0x4010db) = 42;

  /* Ensures that the above instruction is run *before* 1 is moved into edx */
  asm("");

  /* "Some number: 42" */
  printf("some number: %d", 1);
}