Suposse I coded a C library which provides a bunch of "public" functions, declared in a mylib.h header file. Those functions are supposedly implemented in (say) a mylib.c file which is compiled to a (say) static lib mylib.c -> mylib.o -> mylib.a.
Is there some way to detect that I forgot to provide the implementation of some declared function in mylib.h? (Yes, I know about unit testing, good practices, etc - and, yes, I understand the meaning of a plain function declaration in C).
Suppose mylib.h declares a void func1(); and this function was not coded in the provided library. This will trigger an error only if the linker needs to use that function. Otherwise, it will compile ok and even without warnings - AFAIK. Is there a way (perhaps compiler dependent) to trigger a warning for declared but not implemented functions, or there is any other way to deal with this issue?
BTW: nm -u lists not all undefined declared functions, but only those "used" by the library, i.e., those functions that will trigger an error in the linking phase if not declared somewhere. (Which makes sense, the library object file does not know about header files, of course.)
Basically, the most reliable way is to have a program (or possibly a series of programs) which formally exercise each and every one of the functions. If one of the programs fails to link because of a missing symbol, you've goofed.
I suppose you could try to do something by editing a copy of the header into a source file (as in, file ending .c), converting the function declarations into dummy function definitions:
Original:
extern int somefunc(void);
Revised:
extern int somefunc(void){}
Then compile the modified source with minimum warnings - and ignore anything to do with "function that is supposed to return a value doesn't". Then compare the defined symbols in the object file from the revised source with the defined symbols in the library (using nm -g on Unix-like systems). Anything present in the object file that isn't present in the library is missing and should be supplied.
Note: if your header includes other headers of your own which define functions, you need to process all of those. If your header includes standard headers such as <stdio.h>, then clearly you won't be defining functions such as fopen() or printf() in the ordinary course of events. So, choose the headers you reprocess into source code carefully.