c语言中extern关键字_了解C语言中的extern关键字

拼搏现实的明天。 2023-02-28 08:45 23阅读 0赞

c语言中extern关键字

In this article, we’ll take a look at understanding the **extern** keyword in C.

在本文中，我们将了解C中的**extern**关键字。

This is primarily used when you have multiple source files, and you want to share variables among those files.

这主要在有多个源文件并且要在这些文件之间共享变量时使用。

Let’s take a look at how we can use `extern` properly, to compile all our files without conflicts.

让我们看一下如何正确使用`extern`来编译所有文件而不会发生冲突。

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## extern关键字有什么作用？ **(****What does the extern keyword do?****)** ##

Before we look at `extern`, let’s look at two types of statements first:

在讨论`extern`之前，让我们先看两种类型的语句：

*  A variable declaration statement
    
    变量声明语句
 *  A variable definition statement
    
    变量定义语句

A declaration statement is something like this:

声明语句是这样的：

int a;
    
    char* ch;

You inform the compiler that these variables exist somewhere already, so it does not allocate memory separately for them. This is a crucial point.

您通知编译器这些变量已经存在，因此它不会为它们单独分配内存。 这是关键点。

There is another statement for **defining** a variable, where you explicitly allocate storage for that particular variable.

还有另一个用于**定义**变量的语句，您可以在其中为该特定变量显式分配存储。

To define a variable, we typically do something similar to the below snippet.

要定义变量，我们通常会执行与以下代码段相似的操作。

// Defining a
    int a = 10;
    
    // Defining ch, even if it is NULL
    char* ch = NULL;

Now, let’s move on to **extern**. We can use the extern keyword only when we are **declaring a global variable.** Let’s make this clear.

现在，让我们继续进行**extern** 。 只有在**声明全局变量**时，才能使用extern关键字**。** 让我们澄清一下。

This means that you cannot prefix the extern keyword when defining a global variable.

这意味着在定义全局变量时，不能在extern关键字前添加前缀。

// Allowed. Variable declaration
    extern int a;
    
    // Not allowed. extern with variable definition
    extern int b = 5;

To understand why, let’s look at what `extern` actually means.

要了解原因，让我们看看`extern`实际含义。

When you’re prefixing a global variable declaration with an `extern` keyword, this means that you’re telling the compiler that this variable is actually **defined** in some other file!

当给全局变量声明加上`extern`关键字前缀时，这意味着您要告诉编译器该变量实际上是在其他文件中**定义的** ！

This means that the compiler does not need to allocate the same shared variable twice, and this will not cause conflict when working with multiple source files.

这意味着编译器不需要两次分配相同的共享变量，并且在使用多个源文件时不会引起冲突。

## 在C中使用extern关键字–完整示例 **(****Using the extern keyword in C – A complete example****)** ##

Let’s look at an example to make this clear.

让我们看一个例子来阐明这一点。

Assume that we have two source files and two header files:

假设我们有两个源文件和两个头文件：

*  `file1.c`, `file1.h`
    
    `file1.c` ， `file1.h`
 *  `file2.c` `file2.h`.
    
    `file2.c` `file2.h` 。

Assume a separate main program called `main.c`, which is the driver program.

假设有一个单独的主程序`main.c` ，它是驱动程序。

So let’s consider `file1.c`, which has a global variable called `int file1_var` **defined**. So, we intend to share this to `file2.c` using `extern`.

因此，让我们考虑`file1.c` ，它具有一个称为`int file1_var`的全局变量**defined** 。 因此，我们打算使用`extern`将其共享到`file2.c` 。

I’ll provide sample code for `file1.h`, which has a simple function prototype for the functions of `file1.c`, so that other programs can find out which function to invoke, when including the header file `file1.h`.

我将为`file1.h`提供示例代码，该文件具有`file1.c`函数的简单函数原型，以便其他程序在包含头文件`file1.h`时可以找出要调用的函数。

// file1.h
    // Contains function prototypes for functions of file1.c
    
    int addition_1(int a, int b);

// file1.c
    // Contains function definition for addition_1()
    #include "file1.h"
    #include <stdio.h>
    
    // Also has a global variable called file1_var
    // We are defining it here so that it can be shared by other files
    // when they use extern on this variable
    int file1_var = 100;
    
    int addition_1(int a, int b) {
        file1_var = file1_var + 100;
        printf("Inside file1.c addition(). file1_var = %d\n", file1_var);
        return a + b;
    }

I have set the initial value of `file1_var` as 100.

我已将`file1_var`的初始值设置为100。

Similarly, I will do the same for `file2.h` and `file2.h`. But now, since we want a reference to `file1_var`, we need to use `extern`, and only declare it.

同样，我将对`file2.h`和`file2.h`进行相同的`file2.h` 。 但是现在，由于我们要引用`file1_var` ，因此需要使用`extern` ，仅对其进行声明。

// file2.h
    
    int addition_2(int a, int b);

// file2.c
    // Contains function definition for addition_2()
    #include "file2.h"
    #include <stdio.h>
    
    // Makes an extern reference to file1_var
    extern int file1_var;
    
    int addition_2(int a, int b) {
        file1_var = file1_var + 200;
        printf("Inside file2.c addition(). file1_var = %d\n", file1_var);
        return a + b;
    }

Now, the two files share a variable `file1_var`, which also gets updated when any of the `addition()` functions gets called.

现在，这两个文件共享一个变量`file1_var` ，当调用任何`file1_var` `addition()`函数时，该文件也会更新。

Let’s write the code for the driver program `main.c`, which calls both functions. So, we must include both header files `file1.h` and `file2.h`.

让我们为驱动程序`main.c`编写代码，该程序调用这两个函数。 因此，我们必须同时包含头文件`file1.h`和`file2.h` 。

#include "file1.h"
    #include "file2.h"
    #include <stdio.h>
    
    // We can also use the file1_var reference here!
    extern int file1_var;
    
    // We must have only one main() reference
    // Since this is our driver program, the main function must be only here
    int main() {
        int res1 = addition_1(10, 20);
        int res2 = addition_2(30, 40);
        printf("file1_var = %d\n", file1_var);
        return 0;
    }

I’ll be using `gcc` to compile all our files:

我将使用`gcc`编译所有文件：

gcc -o main.out main.c file1.c file2.c

This command compiles and links the files `main.c`, `file1.c` and `file2.c`, and creates an executable called `main.out`.

该命令编译并链接文件`main.c` ， `file1.c`和`file2.c` ，并创建一个名为`main.out`的可执行文件。

To execute, type `./main.out`.

要执行，请输入`./main.out` 。

./main.out

**Output**

**输出量**

Inside file1.c addition(). file1_var = 200
    Inside file2.c addition(). file1_var = 400
    file1_var = 400

The output indeed seems to match what we expect. `file1_var` will get incremented by 100, when `addition_1()` gets called.

输出确实确实符合我们的预期。 当调用`addition_1()`时， `file1_var`将增加100。

Now, again, it will increment by 200, when `addition_2()` gets called. So finally, the updated value of `file1_var` is 400!

现在，再次调用`addition_2()`时，它将增加200。 因此，最后， `file1_var`的更新值为400！

We have successfully used `extern` to share variables between multiple source files!

我们已经成功地使用`extern`在多个源文件之间共享变量！

**NOTE**: To understand more about compiling and using multiple source and header files, refer [this ][this]link. This mentions the best practices to avoid re-declaration errors when linking multiple files, so please read this too.

**注意** ：要了解有关编译和使用多个源文件和头文件的更多信息，请参考[此][this]链接。 这里提到了避免链接多个文件时重新声明错误的最佳实践，因此也请阅读此内容。

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## 结论 **(****Conclusion****)** ##

In this article, we learned how we could use the extern keyword in C++ to share variables across source files. This is essential if you’re working with multiple files and global variables.

在本文中，我们学习了如何在C ++中使用extern关键字在源文件之间共享变量。 如果要使用多个文件和全局变量，这是必不可少的。

## 参考资料 **(****References****)** ##

*  [StackOverflow answer][this] on dealing with extern references (Recommended read)
    
    有关处理外部引用的[StackOverflow答案][this] （推荐阅读）

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> 翻译自: [https://www.journaldev.com/38985/extern-keyword-in-c][https_www.journaldev.com_38985_extern-keyword-in-c]

c语言中extern关键字

[this]: https://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files
[https_www.journaldev.com_38985_extern-keyword-in-c]: https://www.journaldev.com/38985/extern-keyword-in-c