Cited from course BTP300 , Chris, seneca cs web

• C and C++ compilers process source code in three distinct stages: pre-processing, compilation proper and linking
• mixed language program require linkage convention declarations to ensure that the function identifiers used in the definition and in the function call are identical

• To resolve this incompatibility, we direct the C++ compiler to use the "C" linkage convention in calling hello() rather than the default C++ convention. We do so by wrapping the include directive in a linkage convention declaration:

•  extern "C" {            /* identifies C linkage convention */
      #include "hello.h"
  }