C 包含守卫究竟是做什么的? [英] What exactly do C include guards do?

问题描述

我有一个关于 C 中包含守卫的问题.我已经阅读了一些内容，但希望能得到一些澄清.

I have a question regarding include guards in C. I've done a bit of reading but would appreciate a little bit of clarification.

假设我有一个带有函数定义的头文件header.h".

Let's say I have a header file "header.h" with a function definition.

#ifndef HEADER_FILE
#define HEADER_FILE

int two(void){
return 2;
}

#endif

这个头文件有一个包含保护.但是，我对#define HEADER_FILE 实际上在做什么感到困惑.假设我要忘记包含保护，完全忽略添加#define HEADER_FILE"对我来说是完全合法的.

This header file has an include guard. However, I'm kind of confused as to what #define HEADER_FILE is actually doing. Let's say I were to forget the include guard, it would have been perfectly legal for me to completely ignore adding '#define HEADER_FILE'.

所以我的问题是:当我们定义 HEADER_FILE 时，我们到底在做什么?我们在定义什么?为什么可以忘记包含保护，在这种情况下我们也可以忘记添加#define HEADER_FILE?

So my question: What exactly are we doing when we define HEADER_FILE? What are we defining? And why is it okay to forget the include guard in which case we can also forgot adding #define HEADER_FILE?

感谢任何帮助！

推荐答案

这是一个预处理宏.

所有这些都是预处理器语法，基本上就是说，如果尚未定义此宏，请定义它并包含 #ifndef 和 #endif 之间的所有代码

All of it is preprocessor syntax, that basically says, if this macro has not already been defined, define it and include all code between the #ifndef and #endif

它的作用是防止多次包含文件，这可能导致您的代码出现问题.

What it accomplishes is preventing the inclusion of file more than once, which can lead to problems in your code.

你的问题:

为什么可以忘记包含保护，在这种情况下我们也可以忘记添加#define HEADER_FILE?

And why is it okay to forget the include guard in which case we can also forgot adding #define HEADER_FILE?

忘记它也没关系，因为没有它它仍然是合法的 C 代码.如果没有逻辑说明为什么不应该这样做，预处理器会在编译之前处理您的文件，并在最终程序中包含指定的代码.这只是一种常见的做法，但不是必需的.

It's OK to forget it because it's still legal C code without it. The preprocessor processes your file before it's compiled and includes the specified code in your final program if there's no logic specifying why it shouldn't. It's simply a common practice, but it's not required.

一个简单的例子可能有助于说明这是如何工作的:

A simple example might help illustrate how this works:

您的头文件 header_file.h 我们会说，包含以下内容:

Your header file, header_file.h we'll say, contains this:

#ifndef HEADER_FILE
#define HEADER_FILE

int two(void){
    return 2;
}

#endif

在另一个文件 (foo.c) 中，您可能有:

In another file (foo.c), you might have:

#include "header_file.h"

void foo() {
    int value = two();
    printf("foo value=%d
", value);       
}

一旦预处理"并准备好编译，这将转化为:

What this will translate to once it's "preprocessed" and ready for compilation is this:

int two(void){
    return 2;
}

void foo() {
    int value = two();
    printf("foo value=%d
", value);       
}

所有包含保护在这里完成的是确定是否应该将 #ifndef ... 和 #endif 之间的标头内容粘贴到原来的位置#include.

All the include guard is accomplishing here is determining whether or not the header contents between the #ifndef ... and #endif should be pasted in place of the original #include.

但是，由于该函数没有声明为 extern 或 static，并且实际上是在头文件中实现的，因此如果您尝试使用它就会遇到问题在另一个源文件中，因为不会包含函数定义.

However, since that function is not declared extern or static, and is actually implemented in a header file, you'd have a problem if you tried to use it in another source file, since the function definition would not be included.

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