In a C function returning a string, two or three idioms are common for signalling an error.

One is to return NULL for errors.

Another is to make the string result a parameter and use a return code to signal error.

A third, less common but still out there, is an error parameter.

Failure to observe these is on the caller. Forcing the caller to consider the error is probably where rust shines.

But a good C programmer is thinking about these idioms when they encounter a function signature for the first time, or they write one.