How to Resolve Linker Errors When Calling C Functions from a Separate C Project in Visual Studio 2010?

Linker Error when Calling a C Function from C Code in Separate VS2010 Project

Understanding the Issue:

A developer encountered a linker error when attempting to call a C function from a C project in a different Visual Studio 2010 solution. Despite using extern keywords and including a header, the project failed to link to the C library, resulting in an "unresolved external symbol" error.

Correcting the Structure:

To resolve the issue, the project structure for the C functions needs to be reorganized and renamed. Instead of including the C function definitions in the header, the header should simply declare extern functions with the correct calling conventions. The implementation of these functions should be moved to a separate C source file.

Exporting Functions:

Exporting the C functions is done by defining a macro in the C source file. When the project is compiled, this macro defines the functions as exported. In the C project, the header should be included first to define the extern functions, followed by appropriate macros to mark the functions as imported.

Suggested File Structure:

• functions.h - Header file with extern declarations of C functions
• functions.c - Implementation file with function definitions and necessary export macros

Example Files:

functions.h

<code class="c">#pragma once

#if defined(_WIN32)
    #if defined(FUNCTIONS_STATIC)
        #define FUNCTIONS_EXPORT_API
    #else
        #if defined(FUNCTIONS_EXPORTS)
            #define FUNCTIONS_EXPORT_API __declspec(dllexport)
        #else
            #define FUNCTIONS_EXPORT_API __declspec(dllimport)
        #endif
    #endif
#else
    #define FUNCTIONS_EXPORT_API
#endif

#if defined(__cplusplus)
extern "C" {
#endif

FUNCTIONS_EXPORT_API char *dtoa(double, int, int, int*, int*, char**);
FUNCTIONS_EXPORT_API char *g_fmt(char*, double);
FUNCTIONS_EXPORT_API void freedtoa(char*);

#if defined(__cplusplus)
}
#endif</code>

functions.c

<code class="c">#define FUNCTIONS_EXPORTS
#include "functions.h"

char *dtoa(double, int, int, int*, int*, char**)
{
    //function statements
}

char *g_fmt(char*, double)
{
    //function statements
}

void freedtoa(char*)
{
    //function statements
}</code>

By implementing these changes, the linker error should be resolved, and the C project will be able to successfully link to and call the C functions defined in the separate project.

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