How to Resolve LNK2001 Linker Errors in Mixed C and C Projects in VS2010?

Resolving Linker Errors in Mixed C and C Projects in VS2010

Issue Description

Integrating C code into a C project in different VS2010 projects has resulted in a link error when calling a C function from C code. The error, identified as LNK2001, pertains to an unresolved external symbol.

Solution

To rectify this issue, ensure proper organization of the codebase by adhering to specific guidelines:

1. Modularize Code:

   • Each C module should comprise one header file and one source file.
   • Maintain consistency in naming conventions for both files and macros.

2. Create Headers and Implementation Files:

   • Define functions in header files (e.g., functions.h) and implement them in source files (e.g., functions.c).

3. Manage Exports and Imports:

   • Utilize the FUNCTIONS_EXPORT_API macro to mark functions for export (or import) in the header file.
   • When including the header in the project that uses the functions, do not define FUNCTIONS_EXPORT_API (or define the macro with its import value).
   • Ensure that the project exporting the functions has the FUNCTIONS_EXPORT macro defined (automatically or manually) before including the header.

Example Code

functions.h

<code class="c">#pragma once

#define FUNCTIONS_EXPORT_API

#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 implementation
}

char *g_fmt(char*, double)
{
    // Function implementation
}

void freedtoa(char*)
{
    // Function implementation
}</code>

Additional Considerations

• Avoid using the extern storage specifier in function declarations within the header.
• Utilize the project-specific macro (e.g., EXPORTFUNCTIONSPROJECT_EXPORTS) to automate exports.
• Verify the macro names in the project properties under Preprocessor Definitions.

Related Issues

• [LNK2005 Error in CLR Windows Form](https://stackoverflow.com/a/17362530/1128974)
• [Excel VBA, Can't Find DLL Entry Point from a DLL file](https://stackoverflow.com/a/7214325/1128974)
• [When using fstream in a library I get linker errors in the executable](https://stackoverflow.com/a/10068252/1128974)
• [C dll export undefined](https://stackoverflow.com/a/7709927/1128974)

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