What is external linkage and internal linkage?

• A global variable has external linkage by default. Its scope can be extended to files other than containing it by giving a matching extern declaration in the other file.
• The scope of a global variable can be restricted to the file containing its declaration by prefixing the declaration with the keyword static. Such variables are said to have internal linkage.

Consider following example:

1.cpp

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

1. The signature of function f declares f as a function with external linkage (default). Its definition must be provided later in this file or in other translation unit (given below).
2. max is defined as an integer constant. The default linkage for constants is internal. Its linkage is changed to external with the keyword extern. So now max can be accessed in other files.
3. n is defined as an integer variable. The default linkage for variables defined outside function bodies is 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;
}

1. max is declared to have external linkage. A matching definition for max (with external linkage) must appear in some file. (As in 1.cpp)
2. n is declared to have external linkage.
3. z is defined as a global variable with internal linkage.
4. 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.

NB: The keyword static plays a double role. When used in the definitions of global variables, it specifies internal linkage. When used in the definitions of the local variables, it specifies that the lifetime of the variable is going to be the duration of the program instead of being the duration of the function.

Hope that helps!

Basically

• extern linkage variable is visible in all files
• internal linkage variable is visible in single file.

Explain: const variables internally link by default unless otherwise declared as extern

1. by default, global variable is external linkage
2. but, const global variable is internal linkage
3. extra, extern const global variable is external linkage

A pretty good material about linkage in C++

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

Linkage determines whether identifiers that have identical names refer to the same object, function, or other entity, even if those identifiers appear in different translation units. The linkage of an identifier depends on how it was declared. There are three types of linkages:

1. Internal linkage : identifiers can only be seen within a translation unit.
2. External linkage : identifiers can be seen (and referred to) in other translation units.
3. No linkage : identifiers can only be seen in the scope in which they are defined. Linkage does not affect scoping

C++ only : You can also have linkage between C++ and non-C++ code fragments, which is called language linkage.

Source :IBM Program Linkage

In C++

Any variable at file scope and that is not nested inside a class or function, is visible throughout all translation units in a program. This is called external linkage because at link time the name is visible to the linker everywhere, external to that translation unit.

Global variables and ordinary functions have external linkage.

Static object or function name at file scope is local to translation unit. That is called as Internal Linkage

Linkage refers only to elements that have addresses at link/load time; thus, class declarations and local variables have no linkage.