I'd say that the commonly accepted definition for a blocking call is one that may depend on I/O to complete, releasing control of the CPU core while waiting. B

I'd say that the commonly accepted definition for a blocking call is one that may depend on I/O to complete, releasing control of the CPU core while waiting. By that definition, getpid() is definitely nonblocking, though it doesn't have an upper bound in execution time. POSIX does not offer hard realtime guarantees. close() in general would probably be blocking (as a filesystem may need to do I/O), but I'd expect it to behave nonblocking in most cases, especially when operating on virtual files opened read-only. Unfortunately, I don't think those kinds of behavioral details are documented.