IPv4 standard address to integer

ipv4-to-u32.c

#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>

enum {
    ERR_MISSING_OCTETS = 1,
    ERR_EXTRA_OCTETS,
    ERR_INVALID_OCTET
};

/* convert standard ipv4 to 32-bit uint */
int ip4toi(char *ipstr, uint32_t *res) {
    char *tok;
    char *endptr;
    long digit;
    // Clear the existing number.
    *res = 0;
    // Tokenize the input string by '.'
    for (uint8_t i = 0; i < 4; i++) {
        tok = strsep(&ipstr, ".");
        // We're expecting exactly 4 tokens.
        if (tok == NULL) {
            return ERR_MISSING_OCTETS;
        }
        // Make room for the next byte.
        *res <<= 8;
        // Convert the token to a number.
        digit = strtol(tok, &endptr, 10);
        // Check to make sure it was a valid octet.
        if (*endptr != 0 || digit < 0 || digit > 0xff) {
            return ERR_INVALID_OCTET;
        }
        // Add the digit to the result.
        *res |= digit;
    }
    // Check to make sure we had exactly 4 delimiters.
    if (strsep(&ipstr, ".") != NULL) {
        return ERR_EXTRA_OCTETS;
    }

    return 0;
}

const char *errstr(int no) {
    switch (no) {
        case ERR_MISSING_OCTETS:
            return "not enough octets; expected 4";
        case ERR_EXTRA_OCTETS:
            return "too many octets provided; expected 4";
        case ERR_INVALID_OCTET:
            return "invalid octet; expected a number between 0-255";
        default:
            return "no error";
    }
}

int main() {
    char buf[32];
    size_t input_len;
    uint32_t result;
    int err;

    while (true) {
        buf[0] = 0;
        printf(">> ");
        fgets(buf, 32, stdin);
        if (buf[0] == 0) {
            break;
        }
        input_len = strnlen(buf, 32);
        if (input_len > 0 && buf[input_len-1] == '\n') {
            buf[input_len-1] = 0;
        }
        if ((err = ip4toi(buf, &result)) != 0) {
            printf("error: %s\n", errstr(err));
            continue;
        }
        printf("%-10u (0x%08x)\n", result, result);
    }

    return 0;
}