Question

I’m experiencing an extremely weird problem in a fresh OSX 10.4.11 + Xcode 2.5 installation. I’ve reduced it to a minimal test case. Here’s test.cpp:

#include "macros.h"

int main (void)
{
    return 1;
}

And here’s macros.h:

#ifndef __JUST_TESTING__
#define __JUST_TESTING__

template<typename T> void swap (T& pT1, T& pT2)
{
    T pTmp = pT1;
    pT1 = pT2;
    pT2 = pTmp;
}

#endif //__JUST_TESTING__

This compiles and works just fine if both files are in the same directory. HOWEVER, if I put macros.h in /usr/include/gfc2 (it’s part of a custom library I use) and change the #include in test.cpp, compilation fails with this error :

/usr/include/gfc2/macros.h:4: error: template with C linkage

I researched that error and most of the comments point to a “dangling extern C”, which doesn’t seem to be the case at all.

I’m at a complete loss here. Is g++ for some reason assuming everything in /usr/include/gfc2 is C even though it’s included from a .cpp file that doesn’t say extern “C” anywhere?

Any ideas?

EDIT : It does compile if I use the full path in the #include, ie #include "/usr/include/gfc2/macros.h"

EDIT2 : It’s not including the wrong header. I’ve verified this using cpp, g++ -E, and renaming macros.h to foobarmacros.h

Answer 1

G++ may well indeed be assuming that everything in /usr/include is C. Try compiling your code with -E and studying the line markers in the preprocessor output:

g++ -E test.cpp | grep '^#'

You’ll likely see things like

# 1 "/usr/include/gfc2/macros.h" 1 3 4

The 4 is the preprocessor hinting to G++ that it should wrap everything in extern "C", on the supposition that your platform’s ancient header files in /usr/include predate C++. See Preprocessor Output in the CPP manual.

These days G++ mostly ignores this hint, because most platforms’ C headers are no longer ancient. See the NO_IMPLICIT_EXTERN_C target macro in the GCC Internals manual. But it may be that this old version of Xcode has GCC configured without NO_IMPLICIT_EXTERN_C and thus is listening to the preprocessor’s hint. (This is set when GCC itself is built — I don’t think there’s a command-line switch to override it.)

You may be able to work around this by wrapping the contents of your header file in extern "C++".