str_to_int function, with no bounds error checking

str_to_int.c

#include <stdio.h>

int char_is_int(char c) {
  return c >= '0' && c <= '9';
}

int char_to_int(char c) {
  return c - '0';
}

int str_to_int(char *s) {
  int acc = 0;
  while(*s) {
    if (char_is_int(*s)) {
      acc *= 10;
      acc += char_to_int(*s);
    }
    ++s;
  }
  return acc;
}

int main(int argc, char **argv) {
  printf("%d\n", str_to_int(argv[1]));
  return 0;
}