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3 answers
Yes, it is generally good practice to always cast the return value of functions to (void) if not used. This is self-documenting code showing that you aren't using the return value on purpose and did not just forget it by accident.
For reference, either using the return value or casting it to void is enforced by MISRA-C (17.7) and other coding quality standards.
Hosted system compilers generally don't warn out of tradition, because that would mean that we'd have to write (void) printf("hello world"); and similar when using common functions, where the result is not commonly used. This whole sloppiness goes all the way back to the first K&R book. Which is a shame really, since in the case of scanf etc, it is definitely good practice to always check the result.
Codewarrior is more pedantic than hosted system compilers, because it is intended to be exclusively used for embedded systems and you aren't as likely to use stdio.h and similar libs there. Similarly, Codewarrior is intended to be suitable for automotive systems, where quality standards are much higher than those for PC programming.
This has nothing to do with C90 as such.
This is a bit opinionated, but personally, I would not do it. It would just clutter the code and could hide sources of bugs. But it depends a lot on what function it is.
It's also hard to give a very general answer to this. In many cases, the proper answer is "No, you should instead use the return value, because if you don't, you have no way of knowing if the function succeeded or not". So if you're casting away the result of scanf, then you're doing something wrong.
Casting is basically just a way of telling the compiler "Trust me, I know what I'm doing." I have seen lots of examples where people are trying to find bugs where a warning would have given them a very good clue, but since they are casting, the compiler yields no warning.
It's not necessarily a bad thing, but never use casting as your goto solution to make the compiler shut up. Always think it through carefully before you do it. Remember that warnings are there to help you.
If you don't want to see a particular warning, consider using compiler flags to turn it off.
I saw at least one compiler (Codewarrior for HC12) warn me if I use a function without using it's return value. [...]
Ye, for that's the right choice.
[...] Other compilers (clang/gcc) do not issue a warning though, even when using the std=90 argument.
It will, provided that the function is correctly marked warn_unused_result (https://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Warning-Options.html). The software is should be edited as making results use mandatory. Else functions are to return void.
So should I generally cast the unused return value of a function to (void)?
Never. The returned value is always meaningful and has always to be treated.
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