Alternate linkage specifications
What happens if you’re writing a
program in C++ and you want to use a C library? If you make the C function
declaration,
float f(int a, char b);
the C++ compiler will decorate this name
to something like _f_int_char to support function overloading (and
type-safe linkage). However, the C compiler that compiled your C library has
most definitely not decorated the name, so its internal name will be
_f. Thus, the linker will not be able to resolve your C++ calls to
f( ).
The escape mechanism provided in C++ is
the alternate linkage specification, which was produced in the language
by overloading the extern
keyword. The extern is followed by a string that
specifies the linkage you want for the declaration, followed by the
declaration:
extern "C" float f(int a, char b);
This tells the compiler to give C linkage
to f( ) so that the compiler doesn’t decorate the
name. The only two types of
linkage specifications supported by the standard are “C” and
“C++,” but compiler vendors have the option of supporting
other languages in the same way.
If you have a group of declarations with
alternate linkage, put them inside braces, like this:
extern "C" {
float f(int a, char b);
double d(int a, char b);
}
extern "C" {
#include "Myheader.h"
}
Most C++ compiler vendors handle the
alternate linkage specifications inside their header files that work with both C
and C++, so you don’t have to worry about
it.
