Programming Forums > Application Development > C++

niteice · Jan 13th, 2006, 3:17 PM

I have a header file shared between some C files and some C++ files, declaring a struct (which is a bunch of entry points into the main application from a DLL) as extern, and the actual definition is in a C source file. Now, the C compiles just fine, but when the C++ compiles (this is all into one output DLL btw) it can't find the struct. I've tried about 349058349587234975^23490578349e34078 different combinations of #ifdef __cplusplus, extern "C", moving the actual definition around, etc...nothing works. Either it's undefined or multiply defined. Can anybody come up with a decent solution?

Ooble · Jan 13th, 2006, 4:16 PM

Can we see some code?

Polyphemus_ · Jan 13th, 2006, 4:17 PM

Quote:

Originally Posted by Ooble

Can we see some code?

Yeah, because at an amount of 349058349587234975^23490578349e34078 possibilities we don't know which one you missed

niteice · Jan 13th, 2006, 6:31 PM

OK, I exaggerated a bit. :p

There's one header (cg_syscalls.h) that is mostly aliasing functions from an ugly "CGAME_IMPORT.R_DrawStretchPic" to the much nicer "trap_DrawStratchPic", where CGAME_IMPORT is the struct of functions imported from the engine.

Anyway, the relevant bits of code:
cg_syscalls.h:

(Toggle Plain Text)

extern cgame_import_t	CGAME_IMPORT;

static inline void trap_Print( char *msg ) {
	CGAME_IMPORT.Print( msg );
}

static inline void trap_Error( char *msg ) {
	CGAME_IMPORT.Error( msg );
}

static inline cvar_t *trap_Cvar_Get( char *name, char *value, int flags, char *description ) {
	return CGAME_IMPORT.Cvar_Get( name, value, flags, description );
}

etc.

cg_syscalls.c:

(Toggle Plain Text)

cgame_import_t	CGAME_IMPORT;
...
cgame_export_t *GetCGameAPI( cgame_import_t *import )
{
	static cgame_export_t globals;

	CGAME_IMPORT = *import;

	globals.API = CG_API;

	globals.Init = CG_Init;
	globals.Shutdown = CG_Shutdown;

etc.

window.cpp (for example):

(Toggle Plain Text)

	trap_DrawStretchPic(dx, dy - 8, width, 16, "gfx/menu/controls/dialog_title");
	trap_DrawString(dx, dy - 8, colorBlack, title);
	// Main window
	trap_DrawStretchPic(dx, dy + 8, width, height, "gfx/menu/controls/dialog");

All of the source files for the dll include cg_syscalls.h. It works fine from the C code, but again, it's undefined inside the C++:

(Toggle Plain Text)

uicontrol.obj : error LNK2001: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A)
window.obj : error LNK2019: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) referenced in function "public: __thiscall Window::Window(void)" (??0Window@@QAE@XZ)
menubar.obj : error LNK2001: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A)

Yes, for those of you familiar with modified quake2 engines, it's QFusion-based.

The Dark · Jan 13th, 2006, 7:57 PM

When you include the cg_syscalls.h file from your C++ files, you need to tell the compiler that they contain C declarations, rather than C++.

(Toggle Plain Text)

extern "C" 
{
#include "cg_syscalls.h"
}

This tells the compiler not to "mangle" the names, so the linker can then find the functions or variables.

niteice · Jan 13th, 2006, 8:08 PM

Aha...the only thing I didn't try...that'll probably do it.

niteice · Jan 14th, 2006, 8:06 AM

Yep. Fixed it.

Thanks.

Jan 13th, 2006, 3:17 PM	#1
niteice Programmer Join Date: Aug 2005 Posts: 98 Rep Power: 4	Accessing a C struct from C++ I have a header file shared between some C files and some C++ files, declaring a struct (which is a bunch of entry points into the main application from a DLL) as extern, and the actual definition is in a C source file. Now, the C compiles just fine, but when the C++ compiles (this is all into one output DLL btw) it can't find the struct. I've tried about 349058349587234975^23490578349e34078 different combinations of #ifdef __cplusplus, extern "C", moving the actual definition around, etc...nothing works. Either it's undefined or multiply defined. Can anybody come up with a decent solution?

Jan 13th, 2006, 4:16 PM	#2
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	Can we see some code? __________________ Me :: You :: Them

Jan 13th, 2006, 6:31 PM	#4
niteice Programmer Join Date: Aug 2005 Posts: 98 Rep Power: 4	OK, I exaggerated a bit. :p There's one header (cg_syscalls.h) that is mostly aliasing functions from an ugly "CGAME_IMPORT.R_DrawStretchPic" to the much nicer "trap_DrawStratchPic", where CGAME_IMPORT is the struct of functions imported from the engine. Anyway, the relevant bits of code: cg_syscalls.h: (Toggle Plain Text) extern cgame_import_t CGAME_IMPORT; static inline void trap_Print( char msg ) { CGAME_IMPORT.Print( msg ); } static inline void trap_Error( char msg ) { CGAME_IMPORT.Error( msg ); } static inline cvar_t trap_Cvar_Get( char name, char value, int flags, char description ) { return CGAME_IMPORT.Cvar_Get( name, value, flags, description ); } extern cgame_import_t CGAME_IMPORT; static inline void trap_Print( char msg ) { CGAME_IMPORT.Print( msg ); } static inline void trap_Error( char msg ) { CGAME_IMPORT.Error( msg ); } static inline cvar_t trap_Cvar_Get( char name, char value, int flags, char description ) { return CGAME_IMPORT.Cvar_Get( name, value, flags, description ); } etc. cg_syscalls.c: (Toggle Plain Text) cgame_import_t CGAME_IMPORT; ... cgame_export_t GetCGameAPI( cgame_import_t import ) { static cgame_export_t globals; CGAME_IMPORT = import; globals.API = CG_API; globals.Init = CG_Init; globals.Shutdown = CG_Shutdown; cgame_import_t CGAME_IMPORT; ... cgame_export_t GetCGameAPI( cgame_import_t import ) { static cgame_export_t globals; CGAME_IMPORT = import; globals.API = CG_API; globals.Init = CG_Init; globals.Shutdown = CG_Shutdown; etc. window.cpp (for example): (Toggle Plain Text) trap_DrawStretchPic(dx, dy - 8, width, 16, "gfx/menu/controls/dialog_title"); trap_DrawString(dx, dy - 8, colorBlack, title); // Main window trap_DrawStretchPic(dx, dy + 8, width, height, "gfx/menu/controls/dialog"); trap_DrawStretchPic(dx, dy - 8, width, 16, "gfx/menu/controls/dialog_title"); trap_DrawString(dx, dy - 8, colorBlack, title); // Main window trap_DrawStretchPic(dx, dy + 8, width, height, "gfx/menu/controls/dialog"); All of the source files for the dll include cg_syscalls.h. It works fine from the C code, but again, it's undefined inside the C++: (Toggle Plain Text) uicontrol.obj : error LNK2001: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) window.obj : error LNK2019: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) referenced in function "public: __thiscall Window::Window(void)" (??0Window@@QAE@XZ) menubar.obj : error LNK2001: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) uicontrol.obj : error LNK2001: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) window.obj : error LNK2019: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) referenced in function "public: __thiscall Window::Window(void)" (??0Window@@QAE@XZ) menubar.obj : error LNK2001: unresolved external symbol "struct cgame_import_t CGAME_IMPORT" (?CGAME_IMPORT@@3Ucgame_import_t@@A) Yes, for those of you familiar with modified quake2 engines, it's QFusion-based.

Jan 13th, 2006, 7:57 PM	#5
The Dark Expert Programmer Join Date: Jun 2005 Posts: 825 Rep Power: 4	When you include the cg_syscalls.h file from your C++ files, you need to tell the compiler that they contain C declarations, rather than C++. (Toggle Plain Text) extern "C" { #include "cg_syscalls.h" } extern "C" { #include "cg_syscalls.h" } This tells the compiler not to "mangle" the names, so the linker can then find the functions or variables.

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Jan 13th, 2006, 8:08 PM	#6
niteice Programmer Join Date: Aug 2005 Posts: 98 Rep Power: 4	Aha...the only thing I didn't try...that'll probably do it.

Jan 14th, 2006, 8:06 AM	#7
niteice Programmer Join Date: Aug 2005 Posts: 98 Rep Power: 4	Yep. Fixed it. Thanks.

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Programming Forums > Application Development > C++

Accessing a C struct from C++