Provides for linkage between program units written in different programming languages.
This can also be used to detach a declaration from its module. See Module ownership.
(since C++20)extern string-literal { declaration-seq (optional) } (1) extern string-literal declaration (2)
1) Applies the language specification string-literal to all function types, function names with external linkage and variables with external linkage declared in declaration-seq.
2) Applies the language specification string-literal to a single declaration or definition.
string-literal - an unevaluated string literal that names the required language linkage declaration-seq - a sequence of declarations, which may include nested linkage specifications declaration - a declaration [edit] ExplanationEvery 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:
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
}
If two declarations of an entity give it different language linkages, the program is ill-formed; no diagnostic is required if neither declaration is reachable from the other. A redeclaration of an entity without a linkage specification inherits the language linkage of the entity and its type (if exists).
extern "C" int f(); extern "C++" int f(); // Error: different language linkages extern "C" int g(); int g(); // OK, has C language linkage int h(); // has C++ language linkage by default extern "C" int h(); // Error: different language linkages[edit] 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 âpointer to C functionâ
};
}
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
}
Let C be a declaration that declares a function or variable with "C" language linkage. If another declaration D declares an entity with the same name, and it satisfies any of the following conditions, C and D declare the same entity:
D declares a variable that belongs to the global scope.C declares a variable, D also declares a variable.C declares a function, D also declares a function.Unlike regular redeclarations, C and D can have different target scopes:
extern "C"
{
int x;
int f();
int g() { return 1; }
}
namespace A
{
int x; // Error: redefines âxâ
int f(); // OK, redeclares âfâ
int g() { return 1; } // Error: redefines âgâ
}
However, the restrictions of such declarations still apply, which means they should either both declare functions or both declare variables, and the declared entities must have the same type:
namespace A
{
extern "C" int x();
extern "C" int y();
}
int x; // Error: redeclares âxâ as a different kind of entity
namespace B
{
void y(); // Error: redeclares âyâ with a different type
}[edit] 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 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);[edit] Keywords
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior CWG 4 C++98 names with internal linkage can have language linkages limited to names with external linkage CWG 341 C++98 a function with "C" language linkage canRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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