cppannotations/yo/intro/cfunc.yo

Normal bf(C) functions, e.g., which are compiled and collected in a run-time
library, can also be used in bf(C++) programs. Such functions, however, must be
declared as bf(C) functions.

As an example, the following code fragment declares a function tt(xmalloc)
as a bf(C) function:
        verb(
    extern "C" void *xmalloc(int size);
        )
    This declaration is analogous to a declaration in bf(C), except that the
prototype is prefixed with ti(extern "C").

    A slightly different way to declare bf(C) functions is the following:
        verb(
    extern "C"
    {
        // C-declarations go in here
    }
        )
    It is also possible to place preprocessor directives at the location of
the declarations. E.g., a bf(C) header file tt(myheader.h) which declares
bf(C) functions can be included in a bf(C++) source file as follows:
        verb(
    extern "C"
    {
        #include <myheader.h>
    }
        )
    Although these two approaches may be used, they are actually seldom
encountered in bf(C++) sources. We will encounter a more frequently used
method to declare external bf(C) functions in the next section.