C++ supports function overloading, i.e., there can be more than one functions with same name and differences in parameters. How does C++ compiler distinguishes between different functions when it generates object code – it changes names by adding information about arguments. This technique of adding additional information to function names is called Name Mangling. C++ standard doesn’t specify any particular technique for name mangling, so different compilers may append different information to function names.

Consider following declarations of function f()

int  f (void) { return 1; }
int  f (int)  { return 0; }
void g (void) { int i = f(), j = f(0); }

A C++ compiler may mangle above names to following (Source: Wiki)

int  __f_v (void) { return 1; }
int  __f_i (int)  { return 0; }
void __g_v (void) { int i = __f_v(), j = __f_i(0); }



How to handle C symbols when linking from C++?
In C, names may not be mangled as C doesn’t support function overloading. So how to make sure that name of a symbol is not changed when we link a C code in C++. For example, see the following C++ program that uses printf() function of C.

// Save file as .cpp and use C++ compiler to compile it
int printf(const char *format,...);

int main()
{
    printf("GeeksforGeeks");
    return 0;
}

Output:

undefined reference to `printf(char const*, ...)'
        ld returned 1 exit status

The reason for compiler error is simple, name of printf is changed by C++ compiler and it doesn’t find definition of the function with new name.

The solution of problem is extern “C” in C++. When some code is put in extern “C” block, the C++ compiler ensures that the function names are unmangled – that the compiler emits a binary file with their names unchanged, as a C compiler would do.

If we change the above program to following, the program works fine and prints “GeeksforGeeks” on console.

// Save file as .cpp and use C++ compiler to compile it
extern "C"
{
    int printf(const char *format,...);
}

int main()
{
    printf("GeeksforGeeks");
    return 0;
}

Output:

GeeksforGeeks

Therefore, all C style header files (stdio.h, string.h, .. etc) have their declarations in extern “C” block.

#ifdef __cplusplus 
extern "C" {
#endif
    /* Declarations of this file */
#ifdef __cplusplus
}
#endif

Following are main points discussed above
1. Since C++ supports function overloading, additional information has to be added to function names (called name mangling) to avoid conflicts in binary code.
2. Function names may not be changed in C as C doesn’t support function overloading. To avoid linking problems, C++ supports extern “C” block. C++ compiler makes sure that names inside extern “C” block are not changed.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

         

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