rockbox/utils/MTP/MTP_DLL/MTP_DLL.h

// The following ifdef block is the standard way of creating macros which make exporting 
// from a DLL simpler. All files within this DLL are compiled with the MTP_DLL_EXPORTS
// symbol defined on the command line. this symbol should not be defined on any project
// that uses this DLL. This way any other project whose source files include this file see 
// MTP_DLL_API functions as being imported from a DLL, whereas this DLL sees symbols
// defined with this macro as being exported.
#ifdef MTP_DLL_EXPORTS
#define MTP_DLL_API __declspec(dllexport)
#else
#define MTP_DLL_API __declspec(dllimport)
#endif

extern "C"
{
__declspec(dllexport) bool send_fw(LPWSTR file, int filesize, void (*callback)(unsigned int progress, unsigned int max));
}