#include <stdio.h>
#include <ctype.h>

typedef unsigned long ul;

ul toul(const char *cp) {
  ul result=0, value, base=10;

  if (*cp == '0') {
    base = 8;
    cp++;
    if ((*cp == 'x') && isxdigit(cp[1])) {
      cp++;
      base = 16;
    }
  }

  while (isxdigit(*cp) && (value = isdigit(*cp) ? *cp-'0' : (islower(*cp)
		? toupper(*cp) : *cp)-'A'+10) < base) {
    result = result*base + value;
    cp++;
  }

  return result;
}

int main(void) {
  printf("%ld\n", toul("123"));
  printf("%ld\n", toul("0123"));
  printf("%ld\n", toul("0x123"));
  printf("%ld\n", toul("0xff"));

  return 0;
}