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Extern and extern "C" for variables

I am writing a general C ++ library to use a C program. However, I have a question about externand extern "C".

Consider the following code

My header file is as follows:

#ifdef __cplusplus      
     extern "C" int global;
     extern "C" int addnumbers(int a, int b); 
 #else
      extern int global;
 #endif

This works great; I just need to declare

int global;

in my .cpp or my .c file. However, I do not understand:

What is the difference between extern "C"and externhere? I tried to comment extern "C" int globaland it works! What for?

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(MS Visual Studio) C ++ , "" extern "C" ++. :

error LNK2001: unresolved external symbol "int global" (?_global)

++, global dumpbin,

00B 00000014 SECT4  notype       External     | ?global@@3HA (int global)

, MS Visual Studio , C- - C-.

, :

namespace example {
    int global;
}

, C . ++:

namespace example {
    extern "C" int global;
}

:

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I found that extern "C" is used to make a C ++ function compiled in the C standard, and it is not related to variables, since the solutions to the function name in C and C ++ are different. For example, "void foo (int x, int y)", the C compiler will translate it to "_foo", while the C ++ compiler will translate it to "_foo_int_int".

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