linkage - Why do we need extern "C"{ #include <foo.h> } in C++?

Question

Why do we need to use:

extern "C" {
#include <foo.h>
}

Specifically:

• When should we use it?

• What is happening at the compiler/linker level that requires us to use it?

• How in terms of compilation/linking does this solve the problems which require us to use it?

Answer 1

C and C++ are superficially similar, but each compiles into a very different set of code. When you include a header file with a C++ compiler, the compiler is expecting C++ code. If, however, it is a C header, then the compiler expects the data contained in the header file to be compiled to a certain format—the C++ 'ABI', or 'Application Binary Interface', so the linker chokes up. This is preferable to passing C++ data to a function expecting C data.

(To get into the really nitty-gritty, C++'s ABI generally 'mangles' the names of their functions/methods, so calling printf() without flagging the prototype as a C function, the C++ will actually generate code calling _Zprintf, plus extra crap at the end.)

So: use extern "C" {...} when including a c header—it's that simple. Otherwise, you'll have a mismatch in compiled code, and the linker will choke. For most headers, however, you won't even need the extern because most system C headers will already account for the fact that they might be included by C++ code and already extern their code.