C code compiles as C++, but not as C [duplicate] - c++

Question

This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Convert some code from C++ to C
I've got some code that appears to be straight C. When I tell the compiler (I'm using Visual Studio 2008 Express) to compile it as c++, it compiles and links fine. When I try to compile it as C, though, it throws this error:
1>InpoutTest.obj : error LNK2019: unresolved external symbol _Out32#8 referenced in function _main
1>InpoutTest.obj : error LNK2019: unresolved external symbol _Inp32#4 referenced in function _main
The code reads from and writes to the parallel port, using Inpout.dll. I have both Inpout.lib and Inpout.dll. Here's the code:
// InpoutTest.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include "stdio.h"
#include "string.h"
#include "stdlib.h"
/* ----Prototypes of Inp and Outp--- */
short _stdcall Inp32(short PortAddress);
void _stdcall Out32(short PortAddress, short data);
/*--------------------------------*/
int main(int argc, char* argv[])
{
int data;
if(argc<3)
{
//too few command line arguments, show usage
printf("Error : too few arguments\n\n***** Usage *****\n\nInpoutTest read <ADDRESS> \nor \nInpoutTest write <ADDRESS> <DATA>\n\n\n\n\n");
}
else if(!strcmp(argv[1],"read"))
{
data = Inp32(atoi(argv[2]));
printf("Data read from address %s is %d \n\n\n\n",argv[2],data);
}
else if(!strcmp(argv[1],"write"))
{
if(argc<4)
{
printf("Error in arguments supplied");
printf("\n***** Usage *****\n\nInpoutTest read <ADDRESS> \nor \nInpoutTest write <ADDRESS> <DATA>\n\n\n\n\n");
}
else
{
Out32(atoi(argv[2]),atoi(argv[3]));
printf("data written to %s\n\n\n",argv[2]);
}
}
return 0;
}
I previously asked this question, incorrectly, here.
Any help would be appreciated.

Answer 1

You're trying to link to a C++ function, from C. That doesn't work due to name mangling- the linker doesn't know where to look for your function. If you want to call a C function from C++, you must mark it extern "C". C does not support extern "C++"- as far as I know. One of the other answers says there is. Alternatively, recompile it's source code as C.
Edit: Why ever would you compile as C if you could compile as C++, anyway?

Answer 2

It sounds like Inp32 and Out32 are defined externally in a C++ file/library, so you need to mark them as such so the compiler knows how their names will be mangled:
extern "C++" {
short _stdcall Inp32(short PortAddress);
void _stdcall Out32(short PortAddress, short data);
}

Answer 3

If you need to call a C++ routine from C code, then the C++ routine need to have "C" linkage, which is done by marking the function as extern "C". That needs to be done on the C++ side.
Put the following as the prototypes for Inp32() and Outp32() if you're able to change the existing C++ code. This should be in a header that's included by whatever calls or defined the Inp32() or Outp32() functions - whether C or C++ code:
#ifdef __cplusplus
extern "C" {
#endif
short _stdcall Inp32(short PortAddress);
void _stdcall Out32(short PortAddress, short data);
#ifdef __cplusplus
}
#endif
That will mark those functions as having a C calling convention, and those functions will be callable by either C or C++ code.
If you don't have the ability to change the C++ code, you can create your own C-compatible wrappers for the C++ functions in your own C++ module:
The wrappers.h header file:
// in wrappers.h
// C-callable wrappers
#ifndef WRAPPERS_H
#define WRAPPERS_H
#ifdef __cplusplus
extern "C" {
#endif
short Inp32_wrapper( short PortAddress);
void Out32_wrapper( short PortAddress, short data);
#ifdef __cplusplus
}
#endif
#endif /* WRAPPERS_H */
And, the wrappers.cpp implementation:
// in wrappers.cpp file:
#include "wrappers.h"
// prototypes for the C++ functions - these really should be in a
// header file...
short _stdcall Inp32(short PortAddress);
void _stdcall Out32(short PortAddress, short data);
// implementation of the wrappers
short Inp32_wrapper( short PortAddress)
{
return Inp32( PortAddress);
}
void Out32_wrapper( short PortAddress, short data)
{
Out32( PortAddress, data);
}
Now your C code can #include "wrappers.h" and call the wrapper functions which will simply call the existing C++ functions to do the work.

Answer 4

It doesn't appear to be a compiler error, but rather a linker error. The linker can't find the definitions of Inp32 and Out32. Are you linking to the library that contains the definitions? Did you spell them correctly?