How to Resolve \'Unresolved External Symbol\' Errors When Calling C Functions from C in Visual Studio 2010?

Linker Error in Cross-Project C Function Calls in C Code in Visual Studio 2010

To address the linker error "unresolved external symbol g_fmt" encountered while calling a C function from C code in Visual Studio 2010, the following steps may assist:

1. Organization and Naming:

Ensure that each C module has its own header and implementation (extension .c). Use consistent naming conventions for files and macros, such as G_FMT_H as an include guard.

2. Header File Modifications:

Replace the header.h header file with functions.h, which includes macros for exporting functions.

<code class="c">#define FUNCTIONS_EXPORT_API __declspec(dllexport) // For DLL export
#ifdef __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*);

#ifdef __cplusplus
}
#endif</code>

3. Implementation File Modifications:

Create a corresponding implementation file, functions.c, and include the header file. Define the functions and macros for exporting.

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

char *dtoa(double, int, int, int*, int*, char**) {} // Define functions
char *g_fmt(char*, double) {}
void freedtoa(char*) {}</code>

4. Exporting Functions:

Define the FUNCTIONS_EXPORT macro in the project building the DLL (or as a project setting in Visual Studio) to mark the functions for export. Alternatively, use the macro automatically defined by Visual Studio IDE: ${YOUR_PROJECT_NAME}_EXPORTS.

Additional Considerations:

• Use the same include guards in the header file and corresponding implementation file.
• Ensure that the project linking to the DLL has the correct import libraries or references the correct import DLLs.
• Check the preprocessor definitions in Project Properties to verify that the export macro is correctly defined.

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