You can request warnings about features that did not exist, or worked
differently, in traditional C with the -Wtraditional option.
GCC does not warn about features of ISO C which you must use when you
are using a conforming compiler, such as the # and ##
operators.
Presently -Wtraditional warns about:
Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but does not in ISO C.
In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a directive
if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide directives like
#pragma not understood by traditional C by indenting them. Some
traditional implementations would not recognize #elif, so it
suggests avoiding it altogether.
A function-like macro that appears without an argument list. In some
traditional preprocessors this was an error. In ISO C it merely means
that the macro is not expanded.
The unary plus operator. This did not exist in traditional C.
The U and LL integer constant suffixes, which were not
available in traditional C. (Traditional C does support the L
suffix for simple long integer constants.) You are not warned about
uses of these suffixes in macros defined in system headers. For
instance, UINT_MAX may well be defined as 4294967295U, but
you will not be warned if you use UINT_MAX.
You can usually avoid the warning, and the related warning about
constants which are so large that they are unsigned, by writing the
integer constant in question in hexadecimal, with no U suffix. Take
care, though, because this gives the wrong result in exotic cases.