从不同 VS2010 项目中的 C++ 代码调用 C 函数时出现链接器错误

2021-12-14 00:00:00 c c++ visual-studio-2010 extern lnk2001

我正在尝试包含在我们的 C++ 项目中找到的一些 C 代码.该函数在 C 文件中是这样定义的.

I'm trying to include some C code I found in our C++ project. The function is defined like this in the C file. 

#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)
{

我包含它的 VS 项目正在创建一个 dll.该文件正在编译为 C，项目中的其他文件正在编译为 C++.

The VS project I'm including this in is creating a dll. The file is being compiled as C, other files in the project are being compiled as C++.

我添加了一个头文件以包含在我的 C++ 文件中

I added a header to include in my C++ files

#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

在解决方案的另一个项目中，我包含我的标题并尝试调用 g_fmt 函数.

In another project in the solution I include my header and try calling the g_fmt function.

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

此项目正在链接到另一个项目，我可以毫无问题地调用第一个库中的 C++ 函数.但是，添加上面的行会给我一个 lnk2001 错误.

This project is linking to the other one, I'm able to call C++ functions in the first library without any issues. However, adding the above line gives me a lnk2001 error .

error LNK2001: unresolved external symbol g_fmt

我发现了一些关于混合 C 和 C++ 的其他问题，我似乎已经在正确的位置使用 extern 关键字完成了所有必要的操作，但是我仍然无法链接.在 VS2010 中我需要做什么具体的事情吗?

I've found a number of other questions about mixing C and C++ and I seem to have done everything necessary with the extern keywords in the right places, however I'm still not able to link. Is there anything specific I need to do in VS2010?

推荐答案

问题针对 VStudio 2010，但适用于任何版本.

The question is for VStudio 2010, but applies to any version.

在 C 或 C++ 中，考虑到一个模块可能由多个对象组成，每个对象都应该明确界定.此外，每个对象(.c 或 .cpp 文件)都应该有自己的头文件.因此，您应该有一个头文件和一个 .c(xx) 文件用于 C 函数(在您的示例中为 3).
此外，作为一般准则，在命名文件和宏时尽量保持一致:您的 header.h 文件使用 G_FMT_H 宏作为包含保护.
尝试将您的 .h 和 .c 文件重新组织为以下内容:

In C or C++, considering that a module may consist of multiple objects, every object should be clearly delimited. Also, every object (.c or .cpp file) should have its own header file. So, you should have one header file and one .c(xx) file for the C functions (3 in your example).
Also, as a general guideline, try to be consistent when naming files and macros: your header.h file uses G_FMT_H macro as include guard.
Try reorganizing your .h and .c files to something like:

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

    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

和相应的实现(functions.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
}

头文件中需要注意的 2 件事(除了重组和重命名):

2 things to notice here (besides reorganizing and renaming) in the header file:

  • 每个函数的 extern 存储说明符不见了
  • 导出逻辑:您的项目现在将定义FUNCTIONS_EXPORT(来自functions.c，或希望成为VStudioem> 项目设置 - 无论如何，在之前 #include "functions.h")
    • 编译此项目 (functions.c) 时，将导出函数(由于宏定义)
    • 当头文件将包含在另一个项目中(没有定义FUNCTIONS_EXPORTS)时，函数将被标记为已导入，链接器将在导入的中搜索它们.lib(s) - 其中之一应该是这个项目生成的
    • 更严格地说，您可以用自动定义的宏替换FUNCTIONS_EXPORTS(并从functions.c中删除其定义)通过 VStudio IDE:${YOUR_PROJECT_NAME}_EXPORTS(例如，如果您的 Dll 项目被称为 ExportFunctionsProject.vc(x)projem>，那么宏名称将是 EXPORTFUNCTIONSPROJECT_EXPORTS)
    • 您可以在 VStudio (2010) IDE 中查看宏名称:项目属性 -> C/C++ -> 预处理器 -> 预处理器定义.有关更多详细信息，请查看 [MS.Docs]:/D(预处理器定义)
    • The extern storage specifier for each function is gone
    • Exporting logic: your project will define now FUNCTIONS_EXPORT (from functions.c, or desirable to be a VStudio project setting - anyway, somewhere before #include "functions.h")
      • When this project (functions.c) will be compiled, the functions will be exported (due to the macro definition)
      • When the header file will be included in another project (that doesn't define FUNCTIONS_EXPORTS), the functions will be marked as imported, and the linker will search for them in the imported .lib(s) - one of them should be the one generated by this project
      • To be more rigorous, you could replace the FUNCTIONS_EXPORTS (and remove its definition from functions.c) by a macro that's automatically defined by VStudio IDE: ${YOUR_PROJECT_NAME}_EXPORTS (e.g. if your Dll project is called ExportFunctionsProject.vc(x)proj, then the macro name will be EXPORTFUNCTIONSPROJECT_EXPORTS)
      • You can check the macro name in VStudio (2010) IDE: Project Properties -> C/C++ -> Preprocessor -> Preprocessor definitions. For more details, check [MS.Docs]: /D (Preprocessor Definitions)

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