Links error when C-code from different VS2010 projects

#ifndef G_FMT_H
#define G_FMT_H 
#ifdef __cplusplus
extern "C" {
#endif
 extern char *dtoa(double, int, int, int *, int *, char **);
 extern char *g_fmt(char *, double);
 extern void freedtoa(char*);

#ifdef __cplusplus
}
#endif
#endif //G_FMT_H

#include " header.h"
...
g_fmt(my_array, 2.0);

error LNK2001: unresolved external symbol g_fmt

#pragma once

#if defined (WIN32)
#if defined(FUNCTIONS_STATIC) 
#define FUNCTIONS_API
#else
#if defined(FUNCTIONS_EXPORTS)
#define FUNCTIONS_API __declspec(dllexport)
#else
#define FUNCTIONS_API __declspec(dllimport )
#endif
#endif
#else
#define FUNCTIONS_API
#endif

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

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

#if defined(__cplusplus)
}
#endif

#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
}

I am trying to include some C code I found in the C project. The function is defined in the C file as follows.

#ifdef __cplusplus
extern "C" {
#endif
 extern char *dtoa(double, int, int, int *, int *, char **);
 extern char *g_fmt(char *, double);
 extern void freedtoa(char*);
#ifdef __cplusplus
 }
#endif

 char *
g_fmt(register char *b, double x)
{

The VS project I included in it is creating a dll. The file is being compiled to C, the project Other files are being compiled to C.

I added a header to be included in my C file

#ifndef G_FMT_H
#define G_FMT_H
#ifdef __cplusplus
extern "C" {
#endif
 extern char *dtoa(double, int, int, int *, int *, char **);< br /> extern char *g_fmt(char *, double);
 extern void freedtoa(char*);

#ifdef __cplusplus
}
#endif
#endif //G_FMT_H

In another project of the solution, I included my title and tried to call the g_fmt function.

#include "header.h"
...
g_fmt(my_arr ay, 2.0);

This project is linked to another project, and I can call C functions in the first library without any problems. However, adding the above line will give me a lnk2001 error .

error LNK2001: unresolved external symbol g_fmt

I found some other questions about mixing C and C, and I seem to have used it in the right place The extern keyword does all the necessary things, but I still can’t link. What do I need to do in VS2010?

In C or C, considering that the module may be composed of multiple objects, each object should be clearly separated. In addition, each object (. c or .cpp files) should have their own header files. Therefore, you should have a header file and a c(xx) file for C functions (3 in your example). Also, as a general guideline, in Try to be consistent when naming files and macros: the header.h file uses the G_FMT_H macro as the include protection. Therefore, try to reorganize the .h and .c files into:

functions.h:

#pragma once

#if defined (WIN32)
#if defined(FUNCTIONS_STATIC)
#define FUNCTIONS_API
# else
#if defined(FUNCTIONS_EXPORTS)
#define FUNCTIONS_API __declspec(dllexport)
#else
#define FUNCTIONS_API __declspec(dllimport)
#endif
# endif
#else
#define FUNCTIONS_API
#endif

#if defined(__cplusplus)
extern "C" {
#endif< br />
 FUNCTIONS_API char *dtoa(double, int, int, int *, int *, char **);
 FUNCTIONS_API char *g_fmt(char *, double);
 FUNCTIONS_API void freedtoa(char*);

#if defined(__cplusplus)
}
#endif

and the corresponding implementation (functions.c):

#define FUNCTIONS_EXPORTS
#include "functions.h"

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

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

void freedtoa(char*) {
 //function statements
}

Notes in the header file Two things (except for reorganization and renaming):

>The extern storage specifier of each function has disappeared.
>Export logic: Your project will now define FUNCTIONS_EXPORT (from functions.c) , Or hopefully a VStudio project setup-anyway, somewhere before #include “functions.h”)

>When this project (functions.c) is compiled, functions will be exported (due to Macro definition)
>When the header file will be included in another project (FUNCTIONS_EXPORTS is not defined), the functions will be marked as imported and the linker will search for them in the imported .lib-one of them should be generated by this project To be more strict, you can replace FUNCTIONS_EXPORTS with a macro automatically defined by VStudio IDE (and delete its definition from functions.c): ${YOUR_PROJECT_NAME} _EXPORTS (for example, if your Dll project is named ExportFunctionsProject. vc(x) proj, then the macro name will be EXPORTFUNCTIONSPROJECT_EXPORTS)
>You can check the macro name in the VStudio(2010) IDE: Project Properties -> C/C++ -> Preprocessor -> Preprocessor Definition. Related For more detailed information, please see [MS.Docs]: /D (Preprocessor Definitions)

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EDIT0: Added “support” for static build. Edit 1: Cross-platform some corrections.