Is extern "C" required also for linking global variables used in Cpp file to the one defined in a c file?
It is used for linking function from C++ file which is referenced in C file because of the name mangling of function names in C++ files. Does the C compiler also changes the name of variables??
Is
extern "C"required also for linking global variables used in Cpp file to the one defined in a c file?
Portably, yes.
You might find that leaving out extern "C" works for your compiler (for example, GCC, which doesn't mangle C++ variable names in the global namespace), but that's not something you can rely on for all compilers.
Does the C compiler also changes the name of variables??
It depends on the compiler (specifically, on the ABI it uses). The language standards don't specify how language-level names map to linker symbols, so different compilers can use different name-mangling schemes.
It is not required for variables. Extern "C" is required for functions because in C++ the functions can be defined multiple times for different number and type of parameters. Each function name includes parameters in encoded form. But the variables cannot be redefined and the names are identical (compatible) in C and C++.
来源：https://stackoverflow.com/questions/27938862/is-extern-c-required-also-for-linking-global-variables-used-in-cpp-file-to-the