debugging the C++ preprocessor

Question

I'm trying to build Amaya. When the build failed with

error: expected unqualified-id before ‘(’ token

I ran g++ with only the preprocessor (replacing the -c option with -E) on the file that failed to compile to see what was going on. This produced an 80,000 line file, showing me that 'Blue' had been replaced by (2 << 8), which clearly explained the error. If I correct this, the file compiles fine. I guess I could live with that, but I would like to find out why this is happening.

Is there any way I can track how the preprocessor is replacing a specific string, in this case 'Blue'?

================= Update ===================

Well, I found the culprit:

> headers=`g++ [omited for brevity] -M  \
    ../../thotlib/dialogue/AmayaClassicNotebook.cpp`

> for file in $headers ; do grep -s -H Blue $file | grep "(2 << 8)";done 

/usr/local/include/gc.h:#define Blue (2 << 8)

So adding #undef Blue fixed the problem. So using this combination of -M and grep seems OK but sometimes C++ preprocessor definitions can be a real forest; I was curious whether there were some better way, some clever GNU tool maybe.

Answer 1

If nobody comes up with anything better (based on the source line info in the preprocessed file), you could use the -M option to get a list of headers included by the source file, and search those for "Blue". I expect it's possible for some kind of obfuscation to mean that this doesn't find what you're looking for, but normally you'll turn up the definition somewhere.

Answer 2

What's wrong with the perennial

find /src -exec grep Blue {} /dev/null ';'

That usually works for me, at least as a first cut.

Answer 3

I find running

g++ ... -dD -E $file > $file.ii

to be very useful in untangling preprocessing problems. From man g++:

-dD Dump all macro definitions, at the end of preprocessing,
    in addition to normal output.