Self modifying C program

selfmod.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>

int main(int argc, char* argv[])
{
  int x = 0xabcd0123;
  
  printf("x = %d\n", x);
  printf("Enter a new value: ");

  int input;
  scanf("%d", &input);
  printf("Updating x to %d\n", input);

  FILE* src = fopen("selfmod", "r");
  if (!src)
  {
    printf("Couldn't open ./selfmod for reading\n");
    perror("Error:");
    return 1;
  }

  // get file size
  fseek(src, 0, SEEK_END);
  int size = ftell(src);

  // read file contents
  char* buffer = malloc(size);
  fseek(src, 0, SEEK_SET);
  fread(buffer, 1, size, src);

  // ditch the existing file
  fclose(src);
  unlink("selfmod");

  // create a new file
  FILE* dst = fopen("selfmod", "w");
  if (!dst)
  {
    printf("Couldn't open ./selfmod for writing\n");
    perror("Error:");
    return 1;
  }

  // write the contents back out
  fwrite(buffer, size, 1, dst);
  free(buffer);
  
  // move to the variable location
  int offset = 0x000008e8;
  fseek(dst, offset, SEEK_SET);

  // and update it
  fwrite(&input, sizeof(input), 1, dst);
  fclose(dst);

  // fix permissions on the new file (chmod 755)
  chmod("selfmod", 493); 

  return 0;
}

To use:

1. Compile with gcc selfmod.c -o selfmod
2. Run xxd -u ./selfmod | grep '2301 CDAB' to find the offset in the file of x.
3. Update the variable offset as needed, and change the initial value of x if desired (but change only the variable values).
4. Recompile with the same command, and run.