extern C unresolved symbol with gcc 4.8?

Question

I have the following piece of code, that works with gcc 4.3 compiler, but when I compiled with gcc 4.8, got resolved symbol error ( linking time)

//test.cc
ULONG CPULimit = 200;

// test.h

namespace ABC
{
    class STAT
    {
    public:
        static ULONG getCPULimit();
    }
}

in the same test.h file itself, I have defined the getCPULimit() function inline

inline ULONG
ABC::STAT::getCPULimit()
{
    extern ULONG CPULimit; 
    return CPULimit;
}

The above code worked with 4.3 compiler, with 4.8 got unresolved symbol error.

moving extern ULONG CPULimit outside the function, will work but it exposes the global variable. now I wrapped the function with extern "C" like this

extern "C"
{
    inline ULONG
    ABC::STAT::getCPULimit()
    {
       extern ULONG CPULimit; 
       return CPULimit;
    }
}

and surprisingly it worked, 1) I'm not sure how it worked , could anyone shed some light? Is this the right way of doing it?

2) what does it mean to have two externs (one extern C and one extern)

`ULONG` is a `typedef unsigned long` in Windows headers. I suspect, you've updated your `MinGW` from `gcc 4.3` to `gcc 4.8` and got this error? — lapk, Mar 18 '14 at 16:28
Why are you extern "C"'ing a static member of a class? Isn't that exactly the opposite of what extern "C" is meant to mean (i.e. making sure that a C++ compiler understands it needs to apply a C ABI to a function call to a external C object). — Klaas van Gend, Mar 19 '14 at 08:49

score 0 · Answer 1 · answered Mar 18 '14 at 16:09

0

ULONG is not a type, but if you didn't want to use a normal one you could typedef one. EDIT: ULONG should be ulong.

answered Mar 18 '14 at 16:09

Bfitzy

1

score 0 · Accepted Answer · answered Mar 18 '14 at 18:09

The most natural way - make your global variable static and make getCPULimit function non-inlined and implemented in .cc file.

I'm almost sure having C-ABI-incompatible function within extern "C" block is either undefined or forbidden. However, your function is static, so it may be compatible with C ABI (although it looks like a bad idea). Perhaps someone could bring a quotation from C++ standard about that.

extern C unresolved symbol with gcc 4.8?

2 Answers2