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What is the effect of extern “C” in C++?

由 纵然是瞬间 提交于 2019-11-25 21:36:57

问题


What exactly does putting extern \"C\" into C++ code do?

For example:

extern \"C\" {
   void foo();
}

回答1:


extern "C" makes a function-name in C++ have 'C' linkage (compiler does not mangle the name) so that client C code can link to (i.e use) your function using a 'C' compatible header file that contains just the declaration of your function. Your function definition is contained in a binary format (that was compiled by your C++ compiler) that the client 'C' linker will then link to using the 'C' name.

Since C++ has overloading of function names and C does not, the C++ compiler cannot just use the function name as a unique id to link to, so it mangles the name by adding information about the arguments. A C compiler does not need to mangle the name since you can not overload function names in C. When you state that a function has extern "C" linkage in C++, the C++ compiler does not add argument/parameter type information to the name used for linkage.

Just so you know, you can specify "C" linkage to each individual declaration/definition explicitly or use a block to group a sequence of declarations/definitions to have a certain linkage:

extern "C" void foo(int);
extern "C"
{
   void g(char);
   int i;
}

If you care about the technicalities, they are listed in section 7.5 of the C++03 standard, here is a brief summary (with emphasis on extern "C"):

  • extern "C" is a linkage-specification
  • Every compiler is required to provide "C" linkage
  • a linkage specification shall occur only in namespace scope
  • all function types, function names and variable names have a language linkage See Richard's Comment: Only function names and variable names with external linkage have a language linkage
  • two function types with distinct language linkages are distinct types even if otherwise identical
  • linkage specs nest, inner one determines the final linkage
  • extern "C" is ignored for class members
  • at most one function with a particular name can have "C" linkage (regardless of namespace)
  • extern "C" forces a function to have external linkage (cannot make it static) See Richard's comment: 'static' inside 'extern "C"' is valid; an entity so declared has internal linkage, and so does not have a language linkage
  • Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved



回答2:


Just wanted to add a bit of info, since I haven't seen it posted yet.

You'll very often see code in C headers like so:

#ifdef __cplusplus
extern "C" {
#endif

// all of your legacy C code here

#ifdef __cplusplus
}
#endif

What this accomplishes is that it allows you to use that C header file with your C++ code, because the macro "__cplusplus" will be defined. But you can also still use it with your legacy C code, where the macro is NOT defined, so it won't see the uniquely C++ construct.

Although, I have also seen C++ code such as:

extern "C" {
#include "legacy_C_header.h"
}

which I imagine accomplishes much the same thing.

Not sure which way is better, but I have seen both.




回答3:


Decompile a g++ generated binary to see what is going on

main.cpp

void f() {}
void g();

extern "C" {
    void ef() {}
    void eg();
}

/* Prevent g and eg from being optimized away. */
void h() { g(); eg(); }

Compile and disassemble the generated ELF output:

g++ -c -std=c++11 -Wall -Wextra -pedantic -o main.o main.cpp
readelf -s main.o

The output contains:

     8: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 _Z1fv
     9: 0000000000000007     7 FUNC    GLOBAL DEFAULT    1 ef
    10: 000000000000000e    17 FUNC    GLOBAL DEFAULT    1 _Z1hv
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_
    12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _Z1gv
    13: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND eg

Interpretation

We see that:

  • ef and eg were stored in symbols with the same name as in the code

  • the other symbols were mangled. Let's unmangle them:

    $ c++filt _Z1fv
f()
$ c++filt _Z1hv
h()
$ c++filt _Z1gv
g()

Conclusion: both of the following symbol types were not mangled:

  • defined
  • declared but undefined (Ndx = UND), to be provided at link or run time from another object file

So you will need extern "C" both when calling:

  • C from C++: tell g++ to expect unmangled symbols produced by gcc
  • C++ from C: tell g++ to generate unmangled symbols for gcc to use

Things that do not work in extern C

It becomes obvious that any C++ feature that requires name mangling will not work inside extern C:

extern "C" {
    // Overloading.
    // error: declaration of C function ‘void f(int)’ conflicts with
    void f();
    void f(int i);

    // Templates.
    // error: template with C linkage
    template <class C> void f(C i) { }
}

Minimal runnable C from C++ example

For the sake of completeness and for the newbs out there, see also: How to use C source files in a C++ project?

Calling C from C++ is pretty easy: each C function only has one possible non-mangled symbol, so no extra work is required.

main.cpp

#include <cassert>

#include "c.h"

int main() {
    assert(f() == 1);
}

c.h

#ifndef C_H
#define C_H

/* This ifdef allows the header to be used from both C and C++. */
#ifdef __cplusplus
extern "C" {
#endif
int f();
#ifdef __cplusplus
}
#endif

#endif

c.c

#include "c.h"

int f(void) { return 1; }

Run:

g++ -c -o main.o -std=c++98 main.cpp
gcc -c -o c.o -std=c89 c.c
g++ -o main.out main.o c.o
./main.out

Without extern "C" the link fails with:

main.cpp:6: undefined reference to `f()'

because g++ expects to find a mangled f, which gcc did not produce.

Example on GitHub.

Minimal runnable C++ from C example

Calling C++ from C is a bit harder: we have to manually create non-mangled versions of each function we want to expose.

Here we illustrate how to expose C++ function overloads to C.

main.c

#include <assert.h>

#include "cpp.h"

int main(void) {
    assert(f_int(1) == 2);
    assert(f_float(1.0) == 3);
    return 0;
}

cpp.h

#ifndef CPP_H
#define CPP_H

#ifdef __cplusplus
// C cannot see these overloaded prototypes, or else it would get confused.
int f(int i);
int f(float i);
extern "C" {
#endif
int f_int(int i);
int f_float(float i);
#ifdef __cplusplus
}
#endif

#endif

cpp.cpp

#include "cpp.h"

int f(int i) {
    return i + 1;
}

int f(float i) {
    return i + 2;
}

int f_int(int i) {
    return f(i);
}

int f_float(float i) {
    return f(i);
}

Run:

gcc -c -o main.o -std=c89 -Wextra main.c
g++ -c -o cpp.o -std=c++98 cpp.cpp
g++ -o main.out main.o cpp.o
./main.out

Without extern "C" it fails with:

main.c:6: undefined reference to `f_int'
main.c:7: undefined reference to `f_float'

because g++ generated mangled symbols which gcc cannot find.

Example on GitHub.

Tested in Ubuntu 18.04.




回答4:


In every C++ program, all non-static functions are represented in the binary file as symbols. These symbols are special text strings that uniquely identify a function in the program.

In C, the symbol name is the same as the function name. This is possible because in C no two non-static functions can have the same name.

Because C++ allows overloading and has many features that C does not — like classes, member functions, exception specifications - it is not possible to simply use the function name as the symbol name. To solve that, C++ uses so-called name mangling, which transforms the function name and all the necessary information (like the number and size of the arguments) into some weird-looking string processed only by the compiler and linker.

So if you specify a function to be extern C, the compiler doesn't performs name mangling with it and it can be directly accessed using its symbol name as the function name.

This comes handy while using dlsym() and dlopen() for calling such functions.




回答5:


C++ mangles function names to create an object-oriented language from a procedural language

Most programming languages aren't built on-top of existing programming languages. C++ is built on-top of C, and furthermore it's an object-oriented programming language built from a procedural programming language, and for that reason there are C++ expressions like extern "C" which provide backwards compatibility with C.

Let's look at the following example:

#include <stdio.h>

// Two functions are defined with the same name
// but have different parameters

void printMe(int a) {
  printf("int: %i\n", a);
}

void printMe(char a) {
  printf("char: %c\n", a);
}

int main() {
  printMe("a");
  printMe(1);
  return 0;
}

A C compiler will not compile the above example, because the same function printMe is defined twice (even though they have different parameters int a vs char a).

gcc -o printMe printMe.c && ./printMe;
1 error. PrintMe is defined more than once.

A C++ compiler will compile the above example. It does not care that printMe is defined twice.

g++ -o printMe printMe.c && ./printMe;

This is because a C++ compiler implicitly renames (mangles) functions based on their parameters. In C, this feature was not supported. However, when C++ was built over C, the language was designed to be object-oriented, and needed to support the ability to create different classes with methods (functions) of the same name, and to override methods (method overriding) based on different parameters.

extern "C" says "don't mangle C function names"

However, imagine we have a legacy C file named "parent.c" that includes function names from other legacy C files, "parent.h", "child.h", etc. If the legacy "parent.c" file is run through a C++ compiler, then the function names will be mangled, and they will no longer match the function names specified in "parent.h", "child.h", etc - so the function names in those external files would also need to be mangled. Mangling function names across a complex C program, those with lots of dependencies, can lead to broken code; so it might be convenient to provide a keyword which can tell the C++ compiler not to mangle a function name.

The extern "C" keyword tells a C++ compiler not to mangle (rename) C function names.

For example:

extern "C" void printMe(int a);




回答6:


It changes the linkage of a function in such a way that the function is callable from C. In practice that means that the function name is not mangled.




回答7:


Not any C-header can be made compatible with C++ by merely wrapping in extern "C". When identifiers in a C-header conflict with C++ keywords the C++ compiler will complain about this.

For example, I have seen the following code fail in a g++ :

extern "C" {
struct method {
    int virtual;
};
}

Kinda makes sense, but is something to keep in mind when porting C-code to C++.




回答8:


It informs the C++ compiler to look up the names of those functions in a C-style when linking, because the names of functions compiled in C and C++ are different during the linking stage.




回答9:


extern "C" is meant to be recognized by a C++ compiler and to notify the compiler that the noted function is (or to be) compiled in C style. So that while linking, it link to the correct version of function from C.




回答10:


I used 'extern "C"' before for dll(dynamic link library) files to make etc. main() function "exportable" so it can be used later in another executable from dll. Maybe an example of where I used to use it can be useful.

DLL

#include <string.h>
#include <windows.h>

using namespace std;

#define DLL extern "C" __declspec(dllexport)
//I defined DLL for dllexport function
DLL main ()
{
    MessageBox(NULL,"Hi from DLL","DLL",MB_OK);
}

EXE

#include <string.h>
#include <windows.h>

using namespace std;

typedef LPVOID (WINAPI*Function)();//make a placeholder for function from dll
Function mainDLLFunc;//make a variable for function placeholder

int main()
{
    char winDir[MAX_PATH];//will hold path of above dll
    GetCurrentDirectory(sizeof(winDir),winDir);//dll is in same dir as exe
    strcat(winDir,"\\exmple.dll");//concentrate dll name with path
    HINSTANCE DLL = LoadLibrary(winDir);//load example dll
    if(DLL==NULL)
    {
        FreeLibrary((HMODULE)DLL);//if load fails exit
        return 0;
    }
    mainDLLFunc=(Function)GetProcAddress((HMODULE)DLL, "main");
    //defined variable is used to assign a function from dll
    //GetProcAddress is used to locate function with pre defined extern name "DLL"
    //and matcing function name
    if(mainDLLFunc==NULL)
    {
        FreeLibrary((HMODULE)DLL);//if it fails exit
        return 0;
    }
    mainDLLFunc();//run exported function 
    FreeLibrary((HMODULE)DLL);
}



回答11:


extern "C" is a linkage specification which is used to call C functions in the Cpp source files. We can call C functions, write Variables, & include headers. Function is declared in extern entity & it is defined outside. Syntax is

Type 1:

extern "language" function-prototype

Type 2:

extern "language"
{
     function-prototype
};

eg:

#include<iostream>
using namespace std;

extern "C"
{
     #include<stdio.h>    // Include C Header
     int n;               // Declare a Variable
     void func(int,int);  // Declare a function (function prototype)
}

int main()
{
    func(int a, int b);   // Calling function . . .
    return 0;
}

// Function definition . . .
void func(int m, int n)
{
    //
    //
}



回答12:


This answer is for the impatient/ have deadlines to meet to, only a part/simple explanation is below:

  • in C++, you can have same name in class via overloading (for example, since they are all same name can't be exported as-is from dll, etc.) solution to these problems is they are converted to different strings (called symbols), symbols accounts the name of function, also the arguments, so each of these functions even with same name, can be uniquely identified (also called, name mangling)
  • in C, you don't have overloading, the function name is unique (so, a separate string for identifying the a function name uniquely is not required, so symbol is function name itself)

So
in C++, with name mangling uniquely identities each function
in C, even without name mangling uniquely identities each function

To change the behaviour of C++, that is, to specify that name mangling should not happen for a particular function, you can use extern "C" before the function name, for whatever reason, like exporting a function with a specific name from a dll, for use by its clients.

Read other answers, for more detailed/more correct answers.




回答13:


When mixing C and C++ (i.e., a. calling C function from C++; and b. calling C++ function from C), the C++ name mangling causes linking problems. Technically speaking, this issue happens only when the callee functions have been already compiled into binary (most likely, a *.a library file) using the corresponding compiler.

So we need to use extern "C" to disable the name mangling in C++.



来源：https://stackoverflow.com/questions/1041866/what-is-the-effect-of-extern-c-in-c

标签
c++
c
linkage
name-mangling
extern-c
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