Let C plus plus compatible with C

[Chih-En Lin]

If we want to let C++ compatible with C, we need to do the following things:

/* header */

#ifdef __cplusplus
extern "C" {
#endif

void C_function();

#ifdef __cplusplus
}
#endif

The more information from Stack Overflow:

• extern "C" is a linkage-specification
• Every compiler is required to provide "C" linkage
• A linkage specification shall occur only in namespace scope
• See Richard's Comment: Only function names and variable names with external linkage have a language linkage
• Two function types with distinct language linkages are distinct types even if otherwise identical
• Linkage specs nest, inner one determines the final linkage
• extern "C" is ignored for class members
• At most one function with a particular name can have "C" linkage (regardless of namespace)
• See Richard's comment: static inside extern "C" is valid; an entity so declared has internal linkage, and so does not have a language linkage
• Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved

Here is the cppreference: Language linkage

[Chih-En Lin]

@celizon @raychen0728 @ziyuan1135 @ZitingLi

[Chih-En Lin]

We need to change all the current headers to the prior mentioned.
I will take the job.