A petty trick in front of a builtin function

Author: Yanze Dai
Yanze Dai
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Lately, I was working on a project that’ll heavily concentrating on the server-side. Although those server-side stuffs aren’t really my expertise. The lucky part is that this project is about process the HTTP header and return a proper json as response.

After most of my challenges are completed, there’s one last problem for me to solve:

“How do I get only the domain name and extension name out of a host name?”

For instance, you’ve got some string domains as input:


The expected output should be:


After some research, I came to know that there’s a builtin function gethostbyname from a native C library netdb which can be directly adopted into my program. The function is well explained in the https://www.gnu.org/software/libc/manual/html_node/Host-Names.html.

Just take a look at the solution out there, it’s easily parsing the hostname into a easy-to-use data structure which completely is the final answer to look for.

Actually, before I investigated this builtin function, I’ve got myself another solution. And here is what I thought.

My own approach

Just by taking look at the input and output. It’s obviously an easy problem of processing strings. The first idea I came up with was cutting the first group of strings and return the rest of them.

My idea was obviously naive after running a couple of tests. Because some scenarios like multiple sub-domains weren’t taken into consideration.

For instance, a host name like this one:


Expected result:


If I only cut the first group of strings. The result is:


It’s still far from the correct answer. Then I realized that such “obviously easy” algorithm problem shouldn’t be solved by a brute force. There must be a certain pattern in it.

My ration told me to list all of the elements from the hostnames and then find out some aspects to brainstorm with later.

Alright. What does a hostname consist of?

Domain name + Delimiter + Extension name

For google.com

  • google as domain name
  • . as delimiter
  • com as extension name

Okay. I think I’ve found something here...

There’s actually a simple formula to balance the numbers of delimiters and sectors

delimiters = sectors - 1

Usually the extension is either 1 sector (.com) or 2 sectors (co.cc). So if I could enumerate all of the 2-sector extensions, then the domain name is apparently an easy catch.


It is quite common to say that species of the 2-sector extensions aren’t many. Most of such extensions starts with either “co” or “com”. For instance, co.jp & com.hk.

So in all, the solution would be doing the following things:

  1. Split the hostname by delimiter of “.”

    e.g. www.google.com.hk -> [“www”, “google”, “com”, “hk"]
  2. Check if the second to last element is “co” or “com”

  3. if yes, then take the third to last element as domain name.

  4. if no, then take the second to last element as domain name.


Here is my implementation in C:

function getdomainbyhost(char *http_host)
  char *res = calloc(128, sizeof(char));

  // return directly if Dev env
  if (!strcmp(http_host, "localhost") ||
      !strcmp(http_host, "") ||
      !strcmp(http_host, ""))
    sprintf(res, "%s", http_host);

    return res;
  // Get "." occurrence frequency in host
  // e.g. a.b.google.com.hk => 4
  // e.g. b.google.com.hk => 3
  // e.g. google.co.jp => 2
  // e.g. google.com.hk => 2
  // e.g. google.com => 1
  int occur = 0;
  for (int i = 0; http_host[i] != '\0'; ++i)
    if (http_host[i] == '.')

  if (occur == 1)
    sprintf(res, "%s", http_host);

    return res;

  char *arr[32];
  int i = 0;
  char domain_name[128];
  memcpy(domain_name, http_host, strlen(http_host));
  domain_name[strlen(http_host)] = '\0';
  char *token = strtok(domain_name, ".");

  while (NULL != token)
    arr[i++] = token;
    token = strtok(NULL, ".");

  // If it's 2-worded domain extension, check the second to last word
  // e.g. google.com.hk => check "com" => google.com
  // e.g. some-service.google.com => => check "google" => google.com
  regex_t regex_domain_ext;
  regcomp(&regex_domain_ext, REGEX_DOMAIN_EXT, REG_EXTENDED | REG_NOSUB);
  int match_result = regexec(&regex_domain_ext, arr[occur - 1], 0, NULL, 0);

  if (match_result != REG_NOMATCH)
    sprintf(res, "%s.%s.%s", arr[occur - 2], arr[occur - 1], arr[occur]);
    sprintf(res, "%s.%s", arr[occur - 1], arr[occur]);

  return res;

It’s perhaps never a good solution. And it may look a bit buggy and will not pass some of the QA tests. Just as you know, I'll have to use the builtin function gethostbyname.

But I believe it's a good practice of solving problems in an unique way. Maybe in the feature days when I take a look back on what I’ve done here, I’ll think myself a super idiot coming out with such dumb solution.