extern C

Learn C compatibility with extern C.

extern C

The extern "C" linkage specification requests C linkage for a function or variable; that is, name mangling is turned off. The symbol stays exactly as in the source code.

Example: extern "C" void greet() produces the symbol greet (no mangling). This lets a C-compiled library be called from a C++ program. Conversely, C code can call a C++ function declared with extern "C". It is the bridge between C and C++.

Mixing C and C++

If a header file is used from both C and C++, the extern "C" block is guarded with #ifdef __cplusplus. The C compiler does not see this block; the C++ compiler does.

Example: #ifdef __cplusplus\nextern "C" {\n#endif\nvoid greet();\n#ifdef __cplusplus\n}\n#endif. This idiom is common in standard C libraries and embedded systems. The C++ side calls the unmangled greet symbol; the C side does not mangle anyway.

Exercise Write hello with extern C

Write extern "C" void hello() with body std::cout << "Hello";. In main write hello();.

Show example solution

#include <iostream>

extern "C" void hello() {
    std::cout << "Hello";
}

int main() {
    hello();
    return 0;
}

Test yourself

What does extern "C" do?
```
extern "C" void greet();
```
- Translates code to C
- Turns off name mangling
- Speeds up the program
- Removes classes
Show answer

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

Complete the extern C block

extern "____" { void legacy(); }

Show answer

extern "C" { void legacy(); }

Match each piece to its role

extern "C"

C linkage
void greet()

Unmangled symbol
_Z3foov

Mangled symbol
namespace

Namespace

Next lesson: ABI stability

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