What is external linkage and internal linkage

Question

I want to understand the external linkage and internal linkage and their difference    I also want to know the meaning of     const variables internally link by default unless otherwise declared as extern

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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:

Internal linkage : identifiers can only be seen within a translation unit.
External linkage : identifiers can be seen (and referred to) in other translation units.
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

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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                  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   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   n is defined as an integer variable  The default linkage for variables defined outside function bodies is external    2 cpp   include  lt iostream gt  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  lt  lt   f   called    lt  lt  nCall  lt  lt    times    lt  lt  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 start of the program and not once for each 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

User · Answer

When you write an implementation file   cpp   cxx  etc  your compiler generates a translation unit  This is the source file from your implementation plus all the headers you  included in it    Internal linkage refers to everything only in scope of a translation unit   External linkage refers to things that exist beyond a particular translation unit  In other words  accessible through the whole program  which is the combination of all translation units  or object files

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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   by default  global variable is external linkage but  const global variable is internal linkage 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

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In terms of  C   Because static keyword has different meaning between  C   amp   C      Lets talk about different scope in  C   SCOPE  It is basically how long can I see something and how far    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   By default all functions are global   In case  if you need to  hide some functions in a file from outside  you can prefix the static  keyword to the function   -

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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

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As dudewat said external linkage means the symbol  function or global variable  is accessible throughout your program and internal linkage means that it is only accessible in one translation unit  You can explicitly control the linkage of a symbol by using the extern and static keywords  If the linkage is not specified then the default linkage is extern  external linkage  for non-const symbols and static  internal linkage  for const symbols     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   Note that instead of using static  internal linkage   it is better to use anonymous namespaces into which you can also put classes  Though they allow extern linkage  anonymous namespaces are unreachable from other translation units  making linkage effectively static  namespace     int i     extern by default but unreachable from other translation units   class C     extern by default but unreachable from other translation units

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I think Internal and External Linkage in C   gives a clear and concise explanation        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

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Before talking about the question  it is better to know the term translation unit  program and some basic concepts of C    actually linkage is one of them in general  precisely  You will also have to know what is a scope   I will emphasize some key points  esp  those missing in previous answers   Linkage is a property of a name  which is introduced by a declaration  Different names can denote same entity  typically  an object or a function   So talking about linkage of an entity is usually nonsense  unless you are sure that the entity will only be referred by the unique name from some specific declarations  usually one declaration  though    Note an object is an entity  but a variable is not  While talking about the linkage of a variable  actually the name of the denoted entity  which is introduced by a specific declaration  is concerned  The linkage of the name is in one of the three  no linkage  internal linkage or external linkage   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    Oops    I found what I have typed was somewhat just repeating the standard wording       There are also some other confusing points which are not covered by the language specification    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

[c++] What is external linkage and internal linkage?

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