导出符号

1. 好处

• 提升DSO(dynamic shared object)加载时间
• 让优化器产生更好的代码。
• 减小DSO大小
• 降低符号冲突的可能

2. 常用写法

libhv

#if defined(_MSC_VER)
    #if defined(HV_DYNAMICLIB) || defined(HV_EXPORTS) || defined(hv_EXPORTS)
        #define HV_EXPORT  __declspec(dllexport)
    #else
        #define HV_EXPORT  __declspec(dllimport)
    #endif
#elif defined(__GNUC__)
    #define HV_EXPORT  __attribute__((visibility("default")))
#else
    #define HV_EXPORT
#endif

gcc wiki

#if defined _WIN32 || defined __CYGWIN__
  #ifdef BUILDING_DLL
    #ifdef __GNUC__
      #define DLL_PUBLIC __attribute__ ((dllexport))
    #else
      #define DLL_PUBLIC __declspec(dllexport) // Note: actually gcc seems to also supports this syntax.
    #endif
  #else
    #ifdef __GNUC__
      #define DLL_PUBLIC __attribute__ ((dllimport))
    #else
      #define DLL_PUBLIC __declspec(dllimport) // Note: actually gcc seems to also supports this syntax.
    #endif
  #endif
  #define DLL_LOCAL
#else
  #if __GNUC__ >= 4
    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
    #define DLL_LOCAL  __attribute__ ((visibility ("hidden")))
  #else
    #define DLL_PUBLIC
    #define DLL_LOCAL
  #endif
#endif

extern "C" DLL_PUBLIC void function(int a);
class DLL_PUBLIC SomeClass
{
   int c;
   DLL_LOCAL void privateMethod();  // Only for use within this DSO
public:
   Person(int _c) : c(_c) { }
   static void foo(int a);
};

3. 参考

1. https://gcc.gnu.org/wiki/Visibility
2. https://akkadia.org/drepper/dsohowto.pdf