Name Mangling and Extern C in C++ Concept

Name mangling is a technique used by C++ compilers to encode the names of functions and variables in a way that includes type information. This is primarily done to support function overloading and to allow C++ to link with C code. When C++ code is compiled, the names of functions and variables are transformed into a mangled form to include type information, which helps the compiler distinguish between overloaded functions.

For example, consider these two C++ functions:

int add(int a, int b);
float add(float a, float b);

In the object code generated by the C++ compiler, these functions may have mangled names like _Z3addii and _Z3addff. These mangled names include information about the function name, the number, and types of parameters, and sometimes other details.

Now, consider the use case where you have C++ code that you want to link with C code. C does not have name mangling because it does not support function overloading or type-based differentiation. Therefore, you need a way to inform the C++ compiler that the functions you are trying to link with have C linkage and should not be name-mangled. This is where the extern "C" linkage specification comes into play.

When you declare a function or variable with extern "C", you are telling the C++ compiler to use C linkage for that particular item. This means that the compiler will not apply name mangling to it, making it compatible with C code.

Here’s an example:

// C++ code
extern "C" {
    int add(int a, int b);
    // ...
}

In this example, the add function is declared with C linkage using extern "C". When this code is compiled, the add function will not undergo name mangling, and its symbol will be compatible with C code.

This is commonly used when you have a C++ program that needs to interface with C libraries or when you are writing a C++ wrapper around a C library. The extern "C" linkage specification ensures that the C++ code can call C functions without any name-mangling issues.