Writing C for curl (via) Daniel Stenberg maintains curl - a library that deals with the most hostile of environments, parsing content from the open internet - as 180,000 lines of C89 code.
He enforces a strict 80 character line width for readability, zero compiler warnings, avoids "bad" functions like gets, sprintf, strcat, strtok and localtime (CI fails if it spots them, I found that script here) and curl has their own custom dynamic buffer and parsing functions.
They take particular care around error handling:
In curl we always check for errors and we bail out without leaking any memory if (when!) they happen.
I like their commitment to API/ABI robustness:
Every function and interface that is publicly accessible must never be changed in a way that risks breaking the API or ABI. For this reason and to make it easy to spot the functions that need this extra precautions, we have a strict rule: public functions are prefixed with “curl_” and no other functions use that prefix.