“多个定义”，“首先在此定义”和“多个定义”。错误 [英] "Multiple definition", "first defined here" errors

问题描述

我有3个项目：服务器，客户端和 Commons 。制作标题& Commons 中的源对不会导致任何问题，我可以从服务器和客户端中自由访问这些函数。

I have 3 projects: Server, Client and Commons. Making header & source pairs in Commons doesn't cause any problems and I can access the functions freely from both Server and Client.

但是，出于某种原因，在 Server 或 Client 项目中创建其他源/头文件总是会导致多个定义（...）和首先在这里定义错误。

However, for some reason making additional source/header files within Server or Client project always causes multiple definition of (...) and first defined here errors.

示例：

commands.h （在客户端项目的根目录中）

commands.h (in root dir of the Client project)

#ifndef COMMANDS_H_
#define COMMANDS_H_

#include "commands.c"

void f123();

#endif /* COMMANDS_H_ */

commands.c （在客户端项目的根目录中）

commands.c (in root dir of the Client project)

void f123(){

}

main.c （在root中） 客户项目的目录）

main.c (in root dir of the Client project)

#include "commands.h"
int main(int argc, char** argv){

}

错误：

make: *** [Client] Error 1      Client
first defined here              Client
multiple definition of `f123'   commands.c

清理，重建索引，重建项目没有帮助。也没有重新启动计算机。

Cleaning, rebuilding index, rebuilding projects doesn't help. Neither does restarting the computer.

推荐答案

这里的问题是你包括 commands.c in commands.h 。因此，C预处理器在函数原型之前将 commands.c 的内容插入 commands.h 。 commands.c 包含函数定义。因此，函数定义在导致错误的函数声明之前结束。

The problem here is that you are including commands.c in commands.h before the function prototype. Therefore, the C pre-processor inserts the content of commands.c into commands.h before the function prototype. commands.c contains the function definition. As a result, the function definition ends up before than the function declaration causing the error.

commands.h的内容在预处理器阶段之后如下所示：

The content of commands.h after the pre-processor phase looks like this:

#ifndef COMMANDS_H_
#define COMMANDS_H_

// function definition
void f123(){

}

// function declaration
void f123();

#endif /* COMMANDS_H_ */

这是一个错误，因为你不能在C中定义后声明一个函数。如果你交换 #includecommands.c和函数声明错误不应该发生因为，现在，函数原型出现在函数声明之前。

This is an error because you can't declare a function after its definition in C. If you swapped #include "commands.c" and the function declaration the error shouldn't happen because, now, the function prototype comes before the function declaration.

但是，包含 .c 文件是不良做法，应该避免。解决此问题的更好方法是在 commands.c 中包含 commands.h 并链接编译后的命令版本到主文件。例如：

However, including a .c file is a bad practice and should be avoided. A better solution for this problem would be to include commands.h in commands.c and link the compiled version of command to the main file. For example:

commands.h

ifndef COMMANDS_H_
#define COMMANDS_H_

void f123(); // function declaration

#endif

commands.c

#include "commands.h"

void f123(){} // function definition

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