How to get Xcode 8 C preprocessor to ignore // comments in #defines

Question

The C preprocessor (cpp) seems like it should handle this code correctly:

#define A 1 // hello there

int foo[A];

I would expect to replace A with 1.

What happens is that A is replaced with 1 // hello there, which results in the following output from cpp -std=c99 test.c:

# 1 "test.c"

int foo[1 // hello there];

Which is not valid C and fails to compile.

How can I get cpp to perform the proper replacement?

Note on compiler: Using cpp from the latest (8.2.1, Dec 2016) Xcode on mac, so I doubt it's due to an outdated compiler.

Answer 1

Somewhat to my surprise, I can reproduce the problem on my Mac (macOS Sierra 10.12.2; Apple LLVM version 8.0.0 (clang-800.0.42.1)) using /usr/bin/cpp which is the XCode cpp — but not using GNU cpp (which I invoke using just cpp).

Workarounds include:

/usr/bin/gcc -E -std=c99 test.c

This uses the clang wrapper gcc to run the C preprocessor and correctly handles the version. You could add a -v option and see what it runs; I didn't see it running cpp per se (it runs clang -cc1 -E with lots of other information).

You can also use:

clang -E -std=c99 test.c

It's effectively the same thing.

You could also install GCC and use that instead of XCode. There are questions with answers about how to get that done (but it isn't for the faint of heart).