当你编写一个实现文件(.cpp， .cxx等)时，你的编译器会生成一个翻译单元。这是你实现的源文件，加上你#包含在其中的所有头文件。

内部链接仅指翻译单元范围内的所有内容。

外部链接是指存在于特定翻译单元之外的事物。换句话说，可通过整个程序访问，这是所有翻译单元(或目标文件)的组合。

正如dudewat所说，外部链接意味着符号(函数或全局变量)可以在整个程序中访问，内部链接意味着它只能在一个翻译单元中访问。

可以通过使用extern和static关键字显式地控制符号的链接。如果没有指定链接，那么对于非const符号，默认链接是extern(外部链接)，对于const符号，默认链接是static(内部链接)。

// In namespace scope or global scope.
int i; // extern by default
const int ci; // static by default
extern const int eci; // explicitly extern
static int si; // explicitly static

// The same goes for functions (but there are no const functions).
int f(); // extern by default
static int sf(); // explicitly static 

注意，与其使用静态(内部链接)，不如使用匿名名称空间，您还可以在其中放入类。尽管它们允许外部链接，但匿名名称空间无法从其他翻译单元访问，这使得链接实际上是静态的。

namespace {
  int i; // extern by default but unreachable from other translation units
  class C; // extern by default but unreachable from other translation units
}

全局变量默认具有外部链接。通过在另一个文件中给出匹配的extern声明，可以将其作用域扩展到其他文件，而不是包含它。 通过在声明前加上关键字static，可以将全局变量的作用域限制在包含其声明的文件中。这样的变量被称为具有内部联系。

考虑下面的例子:

1. cpp

void f(int i);
extern const int max = 10;
int n = 0;
int main()
{
    int a;
    //...
    f(a);
    //...
    f(a);
    //...
}

函数f的签名将f声明为具有外部链接的函数(默认值)。它的定义必须在后面的文件或其他翻译单元中提供(如下所示)。 Max被定义为一个整数常数。常量的默认链接是内部的。使用关键字extern将其链接更改为外部链接。现在max可以在其他文件中访问。 N定义为整数变量。在函数体外定义的变量的默认链接是external。

2. cpp

#include <iostream>
using namespace std;

extern const int max;
extern int n;
static float z = 0.0;

void f(int i)
{
    static int nCall = 0;
    int a;
    //...
    nCall++;
    n++;
    //...
    a = max * z;
    //...
    cout << "f() called " << nCall << " times." << endl;
}

max is declared to have external linkage. A matching definition for max (with external linkage) must appear in some file. (As in 1.cpp) n is declared to have external linkage. z is defined as a global variable with internal linkage. The definition of nCall specifies nCall to be a variable that retains its value across calls to function f(). Unlike local variables with the default auto storage class, nCall will be initialized only once at the first invocation of f(). The storage class specifier static affects the lifetime of the local variable and not its scope.

注意:关键字static起着双重作用。在全局变量定义中使用时，它指定内部链接。当在局部变量的定义中使用时，它指定变量的生存期将是程序的持续时间，而不是函数的持续时间。

就“C”而言(因为静态关键字在“C”和“c++”之间具有不同的含义)

我们来谈谈C中的不同作用域

范围:它基本上是指我能看东西多长时间和多远。

Local variable : Scope is only inside a function. It resides in the STACK area of RAM. Which means that every time a function gets called all the variables that are the part of that function, including function arguments are freshly created and are destroyed once the control goes out of the function. (Because the stack is flushed every time function returns) Static variable: Scope of this is for a file. It is accessible every where in the file in which it is declared. It resides in the DATA segment of RAM. Since this can only be accessed inside a file and hence INTERNAL linkage. Any other files cannot see this variable. In fact STATIC keyword is the only way in which we can introduce some level of data or function hiding in 'C' Global variable: Scope of this is for an entire application. It is accessible form every where of the application. Global variables also resides in DATA segment Since it can be accessed every where in the application and hence EXTERNAL Linkage

默认情况下，所有函数都是全局的。以防万一，如果你需要 从外部隐藏一些函数在文件中，你可以前缀静态 函数的关键字。: -)

在讨论这个问题之前，最好准确地了解翻译单元、程序和c++的一些基本概念(实际上链接也是其中之一)。您还必须知道什么是作用域。

我将强调一些重点，特别是之前回答中遗漏的部分。

链接是名称的属性，由声明引入。不同的名称可以表示同一个实体(通常是一个对象或一个函数)。因此，谈论实体的链接通常是毫无意义的，除非您确定实体将仅由某些特定声明(但通常是一个声明)中的唯一名称引用。

注意，对象是实体，而变量不是。在讨论变量的链接时，实际上关注的是所表示实体的名称(由特定声明引入)。名称的联动是在三种情况之一:不联动、内部联动或外部联动。

Different translation units can share the same declaration by header/source file (yes, it is the standard's wording) inclusion. So you may refer the same name in different translation units. If the name declared has external linkage, the identity of the entity referred by the name is also shared. If the name declared has internal linkage, the same name in different translation units denotes different entities, but you can refer the entity in different scopes of the same translation unit. If the name has no linkage, you simply cannot refer the entity from other scopes.

(哦…我发现我输入的只是在重复标准措辞…)

还有一些语言规范没有涵盖的令人困惑的地方。

Visibility (of a name). It is also a property of declared name, but with a meaning different to linkage. Visibility (of a side effect). This is not related to this topic. Visibility (of a symbol). This notion can be used by actual implementations. In such implementations, a symbol with specific visibility in object (binary) code is usually the target mapped from the entity definition whose names having the same specific linkage in the source (C++) code. However, it is usually not guaranteed one-to-one. For example, a symbol in a dynamic library image can be specified only shared in that image internally from source code (involved with some extensions, typically, __attribute__ or __declspec) or compiler options, and the image is not the whole program or the object file translated from a translation unit, thus no standard concept can describe it accurately. Since symbol is not a normative term in C++, it is only an implementation detail, even though the related extensions of dialects may have been widely adopted. Accessibility. In C++, this is usually about property of class members or base classes, which is again a different concept unrelated to the topic. Global. In C++, "global" refers something of global namespace or global namespace scope. The latter is roughly equivalent to file scope in the C language. Both in C and C++, the linkage has nothing to do with scope, although scope (like linkage) is also tightly concerned with an identifier (in C) or a name (in C++) introduced by some declaration.

The linkage rule of namespace scope const variable is something special (and particularly different to the const object declared in file scope in C language which also has the concept of linkage of identifiers). Since ODR is enforced by C++, it is important to keep no more than one definition of the same variable or function occurred in the whole program except for inline functions. If there is no such special rule of const, a simplest declaration of const variable with initializers (e.g. = xxx) in a header or a source file (often a "header file") included by multiple translation units (or included by one translation unit more than once, though rarely) in a program will violate ODR, which makes to use const variable as replacement of some object-like macros impossible.

我认为c++中的内部和外部链接给出了一个清晰而简洁的解释:

A translation unit refers to an implementation (.c/.cpp) file and all header (.h/.hpp) files it includes. If an object or function inside such a translation unit has internal linkage, then that specific symbol is only visible to the linker within that translation unit. If an object or function has external linkage, the linker can also see it when processing other translation units. The static keyword, when used in the global namespace, forces a symbol to have internal linkage. The extern keyword results in a symbol having external linkage. The compiler defaults the linkage of symbols such that: Non-const global variables have external linkage by default Const global variables have internal linkage by default Functions have external linkage by default

基本上

Extern链接变量在所有文件中可见 内部链接变量在单个文件中可见。

解释:默认情况下，const变量内部链接，除非声明为extern

缺省情况下，全局变量为外部链接 但是，const全局变量是内部链接 Extra, extern const全局变量是外部链接

这是一本关于c++中的链接的很好的材料

http://www.goldsborough.me/c/c++/linker/2016/03/30/19-34-25-internal_and_external_linkage_in_c++/

链接确定具有相同名称的标识符是否引用相同的对象、函数或其他实体，即使这些标识符出现在不同的转换单元中。标识符的链接取决于它是如何声明的。 有三种类型的连杆:

内部链接:标识符只能在翻译单元中看到。 外部链接:标识符可以在其他翻译单元中看到(并引用)。 没有链接:标识符只能在定义它们的作用域中看到。 链接不影响作用域

仅限c++:还可以在c++和非c++代码片段之间进行链接，这称为语言链接。

来源:IBM Program Linkage

在c++

任何不在类或函数内嵌套的文件范围内的变量，在程序的所有翻译单元中都是可见的。这被称为外部链接，因为在链接时，该名称在翻译单元之外的任何地方都对链接器可见。

全局变量与普通函数具有外部联系。

文件作用域中的静态对象或函数名对于翻译单元是本地的。这是 称为内部联动

链接仅指在链接/加载时具有地址的元素;因此，类声明和局部变量没有关联。