1. the extern "C" modified variables and functions are carried out in accordance with the C language compile and link way : this is very important! ! ! !
2.extern "C" contains a double meaning
1. You can literally know, first of all, it was modified goal is to "extern" is;
2. Secondly, it was modified object code is a "C".
3. function or variable is defined extern "C" type is extern
4.extern keyword scope of functions and global variables indicate that the C / C ++ language, the keyword tells the compiler, and its stated functions and variables can be used in this module or other modules.