_when the return value is an error code_, zero means success and nonzero means failure. Functions like `read`, `recv`, etc etc don't just return an error code.

_when the return value is an error code_, zero means success and nonzero means failure. Functions like `read`, `recv`, etc etc don't just return an error code. They return an actual value. Functions that only return an error code like `stat`, `connect`, and the proposed ckd_add, return 0 on success and nonzero on error.