extern-cpp.md

title	extern (C++) | Microsoft Docs
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extern (C++)

Syntax

extern string-literal { declaration-list }
extern string-literal declaration

Remarks

The extern keyword declares a variable or function and specifies that it is defined (or assigned a value) in another translation unit. The compiler treats the extern variable and the variable defined elsewhere as the same name. Such a variable has external linkage (its name is visible from files other than the one in which it's defined). Declarations of non-const variables and functions at global scope are external by default. See Program and linkage for more information.

The extern keyword is ignored if the declaration is also a definition. In the following, when the compiler sees the int foo = 9; if fileA.cpp, it treats it as a separate name from foo in FileB.cpp, and a linker error will be raised because a name can only appear once in a given scope. To fix the error, change extern int foo = 9; to extern int foo; so that the linker treats this foo variable as the same name as the one in fileB.cpp:

//fileA.cpp
extern int foo = 9; // Error. extern is ignored.

//fileB.cpp
int foo = 0;

Example

// specifying_linkage1.cpp
int i = 1;
void other();

int main() {
   // Reference to i, defined above:
   extern int i;
}

void other() {
   // Address of global i assigned to pointer variable:
   static int *external_i = &i;

   // i will be redefined; global i no longer visible:
   // int i = 16;
}

In C++, when used with a string, extern specifies that the linkage conventions of another language are being used for the declarator(s). C functions and data can be accessed only if they are previously declared as having C linkage. However, they must be defined in a separately compiled translation unit.

Microsoft C++ supports the strings "C" and "C++" in the string-literal field. All of the standard include files use the extern "C" syntax to allow the run-time library functions to be used in C++ programs.

Example

The following example shows alternative ways to declare names that have C linkage:

// specifying_linkage2.cpp
// compile with: /c
// Declare printf with C linkage.
extern "C" int printf( const char *fmt, ... );

//  Cause everything in the specified header files
//   to have C linkage.
extern "C" {
   // add your #include statements here
   #include <stdio.h>
}

//  Declare the two functions ShowChar and GetChar
//   with C linkage.
extern "C" {
   char ShowChar( char ch );
   char GetChar( void );
}

//  Define the two functions ShowChar and GetChar
//   with C linkage.
extern "C" char ShowChar( char ch ) {
   putchar( ch );
   return ch;
}

extern "C" char GetChar( void ) {
   char ch;

   ch = getchar();
   return ch;
}

// Declare a global variable, errno, with C linkage.
extern "C" int errno;

If a function has more than one linkage specification, they must agree; it is an error to declare functions as having both C and C++ linkage. Furthermore, if two declarations for a function occur in a program — one with a linkage specification and one without — the declaration with the linkage specification must be first. Any redundant declarations of functions that already have linkage specification are given the linkage specified in the first declaration. For example:

extern "C" int CFunc1();
...
int CFunc1();            // Redeclaration is benign; C linkage is
                         //  retained.

int CFunc2();
...
extern "C" int CFunc2(); // Error: not the first declaration of
                         //  CFunc2;  cannot contain linkage
                         //  specifier.

See Also

Keywords extern Storage-Class Specifier in C Behavior of Identifiers in C Linkage in C