Language linkage

Provides for linkage between program units written in different programming languages.

Explanation

Every function type, every function name with external linkage, and every variable name with external linkage, has a property called language linkage. Language linkage encapsulates the set of requirements necessary to link with a program unit written in another programming language: calling convention, name mangling (name decoration) algorithm, etc.

Only two language linkages are guaranteed to be supported:

1. "C++", the default language linkage.
2. "C", which makes it possible to link with functions written in the C programming language, and to define, in a C++ program, functions that can be called from the units written in C.

extern "C"
{
    int open(const char *path_name, int flags); // C function declaration
}
 
int main()
{
    int fd = open("test.txt", 0); // calls a C function from a C++ program
}
 
// This C++ function can be called from C code
extern "C" void handler(int)
{
    std::cout << "Callback invoked\n"; // It can use C++
}

Since language linkage is part of every function type, pointers to functions maintain language linkage as well. Language linkage of function types (which represents calling convention) and language linkage of function names (which represents name mangling) are independent of each other:

extern "C" void f1(void(*pf)()); // declares a function f1 with C linkage,
                             // which returns void and takes a pointer to a C function
                             // which returns void and takes no parameters
 
extern "C" typedef void FUNC(); // declares FUNC as a C function type that returns void
                                // and takes no parameters
 
FUNC f2;            // the name f2 has C++ linkage, but its type is C function
extern "C" FUNC f3; // the name f3 has C linkage and its type is C function void()
void (*pf2)(FUNC*); // the name pf2 has C++ linkage, and its type is
                    // "pointer to a C++ function which returns void and takes one
                    // argument of type 'pointer to the C function which returns void
                    // and takes no parameters'"
 
extern "C"
{
    static void f4(); // the name of the function f4 has internal linkage (no language)
                      // but the function's type has C language linkage
}

Two functions with the same name and the same parameter list in the same namespace cannot have two different language linkages (note, however, that linkage of a parameter may permit such overloading, as in the case of std::qsort and std::bsearch). Likewise, two variables in the same namespace cannot have two different language linkages.

Special rules for "C" linkage

When class members, friend functions with a trailing requires-clause, (since C++20) or non-static member functions appear in a "C" language block, the linkage of their types remains "C++" (but parameter types, if any, remain "C"):

extern "C"
{
    class X
    {
        void mf();           // the function mf and its type have C++ language linkage
        void mf2(void(*)()); // the function mf2 has C++ language linkage;
                             // the parameter has type “pointer to C function”
    };
}

When a function or a variable is declared (in any namespace) with "C" language linkage, all declarations of functions (in any namespace) and all declarations of variables in global scope with the same unqualified name must refer to the same function or variable.

int x;
 
namespace A
{
    extern "C" int x(); // error: same name as global-namespace variable x
}

namespace A
{
    extern "C" int f();
}
 
namespace B
{
    extern "C" int f();   // A::f and B::f refer to the same function f with C linkage
}
 
int A::f() { return 98; } // definition for that function

namespace A
{
    extern "C" int g() { return 1; }
}
 
namespace B
{
    extern "C" int g() { return 1; } // error: redefinition of the same function
}

namespace A
{
    extern "C" int h();
}
 
extern "C" int h() { return 97; } // definition for the C linkage function h
                                  // A::h and ::h refer to the same function

template<typename T>
struct A { struct B; };
 
extern "C"
{
    template<typename T>
    struct A<T>::B
    {
        friend void f(B*) requires true {} // C language linkage ignored
    };
}
 
namespace Q
{
    extern "C" void f(); // not ill-formed 
}

Notes

Language specifications can only appear in namespace scope.

The braces of the language specification do not establish a scope.

When language specifications nest, the innermost specification is the one that is in effect.

A function can be re-declared without a linkage specification after it was declared with a language specification, the second declaration will reuse the first language linkage. The opposite is not true: if the first declaration has no language linkage, it is assumed "C++", and redeclaring with another language is an error.

A declaration directly contained in a language linkage specification is treated as if it contains the extern specifier for the purpose of determining the linkage of the declared name and whether it is a definition.

extern "C" int x; // a declaration and not a definition
// The above line is equivalent to extern "C" { extern int x; }
 
extern "C" { int x; } // a declaration and definition
 
extern "C" double f();
static double f(); // error: linkage conflict
 
extern "C" static void g(); // error: linkage conflict

extern "C" makes it possible to include header files containing declarations of C library functions in a C++ program, but if the same header file is shared with a C program, extern "C" (which is not allowed in C) must be hidden with an appropriate #ifdef, typically __cplusplus:

#ifdef __cplusplus
extern "C" int foo(int, int); // C++ compiler sees this
#else
int foo(int, int);            // C compiler sees this
#endif

The only modern compiler that differentiates function types with "C" and "C++" language linkages is Oracle Studio, others do not permit overloads that are only different in language linkage, including the overload sets required by the C++ standard (std::qsort, std::bsearch, std::signal, std::atexit, and std::at_quick_exit): GCC bug 2316, Clang bug 6277, CWG issue 1555.

extern "C"   using c_predfun   = int(const void*, const void*);
extern "C++" using cpp_predfun = int(const void*, const void*);
 
// ill-formed, but accepted by most compilers
static_assert(std::is_same<c_predfun, cpp_predfun>::value,
              "C and C++ language linkages shall not differentiate function types.");
 
// following declarations do not declare overloads in most compilers
// because c_predfun and cpp_predfun are considered to be the same type
void qsort(void* base, std::size_t nmemb, std::size_t size, c_predfun*   compar);
void qsort(void* base, std::size_t nmemb, std::size_t size, cpp_predfun* compar);

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

References

• C++23 standard (ISO/IEC 14882:2023):
• 9.11 Linkage specifications [dcl.link]
• C++20 standard (ISO/IEC 14882:2020):
• 9.11 Linkage specifications [dcl.link]
• C++17 standard (ISO/IEC 14882:2017):
• 10.5 Linkage specifications [dcl.link]
• C++14 standard (ISO/IEC 14882:2014):
• 7.5 Linkage specifications [dcl.link]
• C++11 standard (ISO/IEC 14882:2011):
• 7.5 Linkage specifications [dcl.link]
• C++03 standard (ISO/IEC 14882:2003):
• 7.5 Linkage specifications [dcl.link]