c++ - Global variable dll export/import -

i got usual macros below:

#ifdef platform_windows     #ifdef build_shared         #define api __declspec(dllexport)     #else         #define api __declspec(dllimport)     #endif #else     #define api #endif   

(the lib compiles /dbuild_shared)

then have global union object want export/import:
header file:

#ifdef __cplusplus   extern "c" {   #endif   //definition of union type    api extern union u global;   #ifdef __cplusplus   }  #endif  

source file:

// include header api union u global; 

the lib compiles fine, while linking executable linker error unresolved external symbol _global. i'm using msvc (vs 2015). not sure if c linkage has it. i'm not sure if uses c linkage because intellisense shows __cplusplus not defined in header file quoted above.