First, this isn’t really a C thing. You’re looking at a function that is written in C for some purpose. The same conventions could be used in any language.
Back in my old Unix days, there was something of a convention that 0 meant success, a positive number meant minor problems, and a negative number meant some sort of failure. Therefore, there was also a sort of convention of if (foo() >= 0) { /* success of a sort */ }.
This was doubtless related to Unix process return codes, where 0 was success.