my_aton.c

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

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

int aton(const char* ip, int* inet_addr) {
  uint32_t val;
  char c;
  unsigned int parts[4] = {0};
  unsigned int * pp = parts;
  int saw_digit = 0;
  int num_read = 0;

  while((c = *ip++) != '\0') {
    if (isdigit(c)) {
      val = (val * 10) + (c - '0');
      if (val > 255) {
        return 0;
      }
      if (! saw_digit) {
        if (++num_read > 4) {
          return 0;
        }
        saw_digit = 1;
      }

    }
    else if (c == '.' && saw_digit) {
      if (num_read == 4) {
        return 0;
      }
      *pp++ = val;
      saw_digit = 0;
      val = 0;
    } else {
      return 0;
    }
  }

  if (num_read < 4) {
    return 0;
  }

  val |= (parts[0] << 24) | (parts[1] << 16) | (parts[2] << 8);
  *inet_addr = val;

}

int main() {
  const char* ip = "101.227.12.253";
  int addr = 0;
  if (aton(ip, &addr)) {
    printf("addr_int = %d", addr);
  }
  assert(addr == 1709378813);
  return 0;
}