Linux C ++ undefined reference

In a C ++ project, a self-written static C library is used. But the MAKEFILE file is not correct. It is always a problem of undefined reference.

After reading this article,Undefined referenceIt is found that when the C ++ program calls the C library, its header file will be surrounded by extern "C.

I also briefly checked the cause and made it clear here:

Why
Do we need extern "C" {# include <Foo. h>} in C ++? In addition, even if the program meets the C ++ syntax but the file suffix is. C, G ++ will regard it as a C language for compilation, and may cause the same problem. It is best to change the suffix to CPP or CC for compilation. Be sure to clean the. o file first.