For GCC and clang, while -Werror turns all diagnostics into errors, even those for valid code, -pedantic-errors will only turn the standard-mandated diagnostics

For GCC and clang, while -Werror turns all diagnostics into errors, even those for valid code, -pedantic-errors will only turn the standard-mandated diagnostics into errors. This is the only reliable way I know of to determine whether the compiler considers the code to violate ISO C's rules. (I say "the compiler considers" because there are corner cases where GCC and clang disagree in their interpretation of the standard.) If your warning does not get upgraded to an error with -pedantic-errors, the warning is about something the compiler authors consider relevant to warn about but the standard allows.