Summary - ABI

Combine mangling, extern C and stability to write a C-compatible library.

Summary - ABI

In this group you saw the core ABI concepts: ABI definition, name mangling, Itanium ABI, extern C and ABI stability. All ensure that compiled code stays compatible.

Now you will combine them in an extern C function example. You will declare and call an unmangled function with extern "C"; this is the basic pattern for bridging C and C++.

extern C function

To define an extern "C" function, prefix with extern "C": extern "C" int compute(int x) { return x + 100; }. The symbol name is compute; no mangling is performed.

In main, std::cout << compute(23); gives 123. This is the basic pattern for calling a C library from a C++ program; it also makes a C++ function accessible from C code.

Exercise Write the extern C compute function

Write extern "C" int compute(int x) with body return x + 100;. In main write std::cout << compute(23);.

Show example solution

#include <iostream>

extern "C" int compute(int x) {
    return x + 100;
}

int main() {
    std::cout << compute(23);
    return 0;
}

Test yourself

What does extern "C" do?
```
extern "C" void greet();
```
- Shrinks class size
- Hides the code
- Speeds up the program
- Turns off name mangling
Show answer

Turns off name mangling — extern C keeps the symbol name identical to the source code.

Complete the extern C declaration

extern "____" void greet();

Show answer

extern "C" void greet();

Match each concept to its trait

ABI

Binary compatibility
name mangling

Symbol naming
extern C

Unmangled symbol
PIMPL

Hidden implementation

Next lesson: Profiling

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