Name mangling and extern "C" in C + +

Original link: http://www.geeksforgeeks.org/extern-c-in-c/

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 no particular technique for name mangling, so different compilers may append different infor Mation to function names.

Consider following declarations of function f ()

1 int  F (voidreturn1;} 2 int  F (int)  return0;} 3 void g (voidint i = f (), j = f (0);}

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

1 int  __f_v (voidreturn1;} 2 int  __f_i (int)  return0;} 3 void __g_v (voidint i = __f_v (), j = __f_i (0);}

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

1 //Save file as. cpp and use C + + compiler to compile it2 intprintfConst Char*format,...);3 4 intMain ()5 {6printf"Geeksforgeeks");7     return 0;8}

Output:

 1 undefined reference to ' printf (charconst*, ...) '  1 exit status

The reason for compiler error was 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 was put in extern "C" block, the C + + compiler ensures that the function names was unmangled–that the Compi Ler 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.

1 //Save file as. cpp and use C + + compiler to compile it2 extern "C"3 {4     intprintfConst Char*format,...);5 }6 7 intMain ()8 {9printf"Geeksforgeeks");Ten     return 0; One}

Output:

 1 geeksforgeeks

Therefore, all C style header files (stdio.h, String.h,.. etc) has their declarations in extern "C" block.

12extern"C"  {3 #endif  4/     **/5#ifdef __cplusplus6}  7#endif

Following is main points discussed above
1. Since C + + supports function overloading, additional information have to is added to function names (called Name mangling) To avoid conflicts in binary code.
2. function names May is 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 is not changed.

Name mangling and extern "C" in C + +