Howcome some C++ functions with unspecified linkage build with C linkage?
- by christoffer
This is something that makes me fairly perplexed.
I have a C++ file that implements a set of functions, and a header file that defines prototypes for them.
When building with Visual Studio or MingW-gcc, I get linking errors on two of the functions, and adding an 'extern "C"' qualifier resolved the error. How is this possible?
Header file, "some_header.h":
// Definition of struct DEMO_GLOBAL_DATA omitted
DWORD WINAPI ThreadFunction(LPVOID lpData);
void WriteLogString(void *pUserData, const char *pString, unsigned long nStringLen);
void CheckValid(DEMO_GLOBAL_DATA *pData);
int HandleStart(DEMO_GLOBAL_DATA * pDAta, TCHAR * pLogFileName);
void HandleEnd(DEMO_GLOBAL_DATA *pData);
C++ file, "some_implementation.cpp"
#include "some_header.h"
DWORD WINAPI ThreadFunction(LPVOID lpData) { /* omitted */ }
void WriteLogString(void *pUserData, const char *pString, unsigned long nStringLen) { /* omitted */ }
void CheckValid(DEMO_GLOBAL_DATA *pData) { /* omitted */ }
int HandleStart(DEMO_GLOBAL_DATA * pDAta, TCHAR * pLogFileName) { /* omitted */ }
void HandleEnd(DEMO_GLOBAL_DATA *pData) { /* omitted */ }
The implementations compile without warnings, but when linking with the UI code that calls these, I get a normal
error LNK2001: unresolved external symbol "int __cdecl HandleStart(struct _DEMO_GLOBAL_DATA *, wchar_t *)
error LNK2001: unresolved external symbol "void __cdecl CheckValid(struct _DEMO_MAIN_GLOBAL_DATA *
What really confuses me, now, is that only these two functions (HandleStart and CheckValid) seems to be built with C linkage. Explicitly adding "extern 'C'" declarations for only these two resolved the linking error, and the application builds and runs.
Adding "extern 'C'" on some other function, such as HandleEnd, introduces a new linking error, so that one is obviously compiled correctly.
The implementation file is never modified in any of this, only the prototypes.
