Howcome some C ++ functions with C-linked unspecified linking?
This is what makes me rather 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 the "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 to the UI code that calls them, I get 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 is that only these two functions (HandleStart and CheckValid) seem to be built with a C reference. By explicitly adding "extern" C '' declarations for only these two, the bind error is resolved and the application builds and runs. Adding "extern" C "to some other function, such as HandleEnd, introduces a new linking error, so it obviously compiled correctly.
The implementation file never changes in any of them, only in prototypes.
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The error indicates that nothing is happening to your implementation file or file (as used by the implementation file). The link error indicates that the generated functions were generated using C ++ linkage. Its UI file incorrectly searched for C-Linkage Feature Options. Correcting the definitions in the title fixes your implementation to match possibly the wrong user interface requirements, and not vice versa.
Your user interface file is a .m or .c file, OR if your user interface file is a .cpp file you have a dome like this:
// ui.cpp
extern "C" {
#include "some_header.h"
}
Of course, if your user interface file is a .c file - you need to either change it to cpp or explicitly define the functions with C-linkage so that they can be called from C.
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