How to call C functions from C++ and vice versa?

Because of the well-known name managling mechanism used in C++ compilers, a C++ function compiled by a C++ compiler cannot be called by C programs directly. Similarly, if a library file is compiled by a C compiler, then none of the functions in the library is accessible from C++ codes by default. The postings in this topic explain what name mangling is and how to stop it to make C functions are callable from C++ and vice versa.

C++ Name Mangling

Name mangling is the encoding of function and variable names into unique names so that linkers can separate common names in the language. Type names may also be mangled. The compiler generates function names with an encoding of the types of the function arguments when the module is compiled. Name mangling is commonly used to facilitate the overloading feature and visibility within different scopes. Name mangling also applies to variable names. If a variable is in a namespace, the name of the namespace is mangled into the variable name so that the same variable name can exist in more than one namespace. The C++ compiler also mangles C variable names to identify the namespace in which the C variable resides.

For example for the below C code:

int foo(double*);
double bar(int, double*);

int foo (double* d) 
{
    return 1;
}

double bar (int i, double* d) 
{
    return 0.9;
}

Its symbol table would be

[4]  0x18   44    2 1 0 0x2 bar
[5]  0x0    24    2 1 0 0x2 foo

For same file, if compile in a C++ compiler like g++, then the symbol table would be

[4]  0x0    24    2  1 0 0x2 _Z3fooPd
[5]  0x18   44    2  1 0 0x2 _Z3bariPd

``_Z3bariPd'' means a function whose name is bar and whose first arg is integer and second argument is pointer to double.

Having been said above, 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.

Following are main points discussed above

• Since C++ supports function overloading, additional information has to be added to function names (called name mangling) to avoid conflicts in binary code.
• 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.