In particular, is the following well-defined, or does it exhibit undefined behavior?
memcmp(0, 0, 0);
Does this differ between C and C++? Ideally, please provide a quote from the standard(s).
In particular, is the following well-defined, or does it exhibit undefined behavior?
It's undefined. C99 7.21.1/2 says about all the string functions:
Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values
and the description of memcmp in 7.21.4.1 doesn't explicitly state otherwise.
Does this differ between C and C++?
No, C++ defers to C for its definition of the C library functions, and doesn't have anything special to say about memcmp.