【理论知识篇】Linux混合编程+log4cpp

     由于要使用到log4cpp的库，而log4c的资料是非常的少，也懒得去研究它的用法，于是就决定试试混合编程者玩意。
      首先先引用下C++它的father: Stroustrup的一篇文章《C++ Style and Technique FAQ》（http://www2.research.att.com/~bs/bs_faq2.html）里的一小个片段：
Just declare the C function ``extern "C"'' (in your C++ code) and call it (from your C or C++ code). For example:
// C++ code
extern "C" void f(int); // one way
extern "C" { // another way
int g(double);
double h();
};
void code(int i, double d)
{
f(i);
int ii = g(d);
double dd = h();
// ...
}
The definitions of the functions may look like this:
/* C code: */
void f(int i)
{
/* ... */
}
int g(double d)
{
/* ... */
}
double h()
{
/* ... */
}
Note that C++ type rules, not C rules, are used. So you can't call function declared ``extern "C"'' with the wrong number of argument. For example:
// C++ code
void more_code(int i, double d)
{
double dd = h(i,d); // error: unexpected arguments
// ...
}
Just declare the C++ function ``extern "C"'' (in your C++ code) and call it (from your C or C++ code). For example:
// C++ code:
extern "C" void f(int);
void f(int i)
{
// ...
}
Now f() can be used like this:
/* C code: */
void f(int);

void cc(int i)
{
f(i);
/* ... */
}
Naturally, this works only for non-member functions. If you want to call member functions (incl. virtual functions) from C, you need to provide a simple wrapper. For example:
// C++ code:
class C {
// ...
virtual double f(int);
};
extern "C" double call_C_f(C* p, int i) // wrapper function
{
return p->f(i);
}
Now C::f() can be used like this:
/* C code: */
double call_C_f(struct C* p, int i);

void ccc(struct C* p, int i)
{
double d = call_C_f(p,i);
/* ... */
}
If you want to call overloaded functions from C, you must provide wrappers with distinct names for th