ctype contains two sets of implementations for all functions. The standard is a set of macros which translate into accesses to a lookup table.
This table uses 257 bytes of memory, so an implementation using real functions is provided. These are accessible if the macros are undefined first. After #undef isupper, isupper is translated into a call to function isupper(). Without the undef, isupper is replaced by the corresponding macro.
Using the functions instead of the macros of course saves RAM and code size - at the expense of some additional function call overhead.