Ask HN: What is a snippet/small lib of C code that you include on every project?

I was reading through an old post: https://news.ycombinator.com/item?id=25176531

I noticed someone posted a URL to a website containing lots of interesting small C libraries: https://ccodearchive.net/list.html

I was wondering if anyone has a small lesser known library (created or found) that they use and that they'd like to share.