How to use libraries compiled with MingW in MSVC?

Question

Hello,

I have compiled several libraries with MingW/MSYS... the generated static libraries are always .a files. When I try to link the library with a MSVC project, Visual Studio throws 'unresolved external symbols' ... It means that the .a static library is incompatible with MS C++ Linker. I presume it has to be converted to a MSVC compatible .lib file.

Either .a and .lib are just AR archives of .o or .obj files, so is there any way how to use MingW compiled libs in a MSVC project? Or do I have to compile/link everything just in one compiler/linker - MSVC only/MingW only? The MingW compiler is said to be compatible with MSVC.

I read a few threads about this topic, but they mostly say that renaming the file to .lib should do the work, but it unfortunately doesn't work for me.

The libraries Im trying to link are written in C.

MSVC Linker throws errors like:

error LNK2019: unresolved external symbol "int __cdecl openssl_call(struct ssl_State *,int,int,int)" (?openssl_call@@YAHPAUssl_State@@HHH@Z) referenced in function _main MyAPP.obj

... and 4 more same errors referring to other functions called from my app.

Thanks for any advice.

Answer 1

The libraries are compatible, but only if you supply a C interface. MSVC and g++ use different name-mangling schemes, so you cannot easily link C++ code created with one with code created by the other.

Answer 2

Based on this error you put in a comment:

error LNK2019: unresolved external symbol "int __cdecl openssl_call(struct ssl_State *,int,int,int)" (?openssl_call@@YAHPAUssl_State@@HHH@Z) referenced in function _main MyAPP.obj all other 4 errors are same only with other functions names

Try putting extern "C" around your include files for openssl. For example:

extern "C" {
include "openssl.h"
}

using extern "C" will instruct the compiler that the functions are using C linkage, not C++, which will stop it from performing name mangling on the functions. So it will look for the function openssl_call in the library rather than ?openssl_call@@YAHPAUssl_State@@HHH@.

Answer 3

"Either .a and .lib are just AR archives of .o or .obj files"

Are you sure that msvc's static .lib files are just like .a "ar" archives?

Answer 4

I encounted the same situations that use mingw-compiled dll in MSVC. I use following tools to make it work:
1) use gcc like that:

gcc -shared -o your_dll.dll your_dll_src.c -Wl,--output-def,your_dll.def

The bolds specify that gcc will generate a *def file that scripts your exported items.Then you need to use lib.exe, which distributed with MSVC, example like this:

lib /def:your_dll.def

Then, there will be a your_dll.lib file, comes from lib.exe.(Assume that you_dll.dll located in the same directory as your_dll.def).

currently, I can use the *.lib in my MSVC project and link the dll correctly, but I got the runtime error. Anyway, such works make your linkage workable.