C code compiles as C++, but not as C

Question

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

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 2

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?

Answer 3

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 4

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.