How to Fix Linker Errors When Calling C Functions from a C Project in Visual Studio?

Linker Error When Attempting to Call C Function from C Code in Different Project

This issue occurs when you attempt to include C code in a C project within different Visual Studio 2010 (or any version) projects. The following steps can assist in resolving this error:

1. Organize Header and Source Files

For clarity and modularity, ensure each C function has a dedicated header file and source file.

2. Header File (functions.h)

• Remove the extern specifier for each function.
• Define export logic using macros to determine whether to mark functions as exported or imported.

Example:

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

char *dtoa(double, int, int, int*, int*, char**);
char *g_fmt(char*, double);
void freedtoa(char*);</code>

3. Source File (functions.c)

• Define a macro to indicate that the functions are exported.
• Include the header file and implement the functions.

Example:

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

char *dtoa(double, int, int, int*, int*, char**)
{
    // Function implementation
}</code>

4. Project Export Settings

• In Visual Studio settings, configure the project that includes the header file to define the FUNCTIONS_EXPORTS macro. This ensures the functions are marked as imported.

5. Linker Settings

• For the project that defines the FUNCTIONS_EXPORTS macro, ensure that the linker is configured to include the library generated from the other project containing the functions.

By following these steps, you can successfully mix C and C code in different projects and resolve the linker error related to the g_fmt function call.

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