What is the difference between _imp and __imp?

Question

I came across an interesting error when I was trying to link to an MSVC-compiled library using MinGW while working in Qt Creator. The linker complained of a missing symbol that went like _imp_FunctionName. When I realized That it was due to a missing extern "C", and fixed it, I also ran the MSVC compiler with /FAcs to see what the symbols are. Turns out, it was __imp_FunctionName (which is also the way I've read on MSDN and quite a few guru bloggers' sites).

I'm thoroughly confused about how the MinGW linker complains about a symbol beginning with _imp, but is able to find it nicely although it begins with __imp. Can a deep compiler magician shed some light on this? I used Visual Studio 2010.

Answer 1

This is fairly straight-forward identifier decoration at work. The imp_ prefix is auto-generated by the compiler, it exports a function pointer that allows optimizing binding to DLL exports. By language rules, the imp_ is prefixed by a leading underscore, required since it lives in the global namespace and is generated by the implementation and doesn't otherwise appear in the source code. So you get _imp_.

Next thing that happens is that the compiler decorates identifiers to allow the linker to catch declaration mis-matches. Pretty important because the compiler cannot diagnose declaration mismatches across modules and diagnosing them yourself at runtime is very painful.

First there's C++ decoration, a very involved scheme that supports function overloads. It generates pretty bizarre looking names, usually including lots of ? and @ characters with extra characters for the argument and return types so that overloads are unambiguous. Then there's decoration for C identifiers, they are based on the calling convention. A cdecl function has a single leading underscore, an stdcall function has a leading underscore and a trailing @n that permits diagnosing argument declaration mismatches before they imbalance the stack. The C decoration is absent in 64-bit code, there is (blessfully) only one calling convention.

So you got the linker error because you forgot to specify C linkage, the linker was asked to match the heavily decorated C++ name with the mildly decorated C name. You then fixed it with extern "C", now you got the single added underscore for cdecl, turning _imp_ into __imp_.