“multiple definition of value” when compiling C program with uninitialized global in g++ but not gcc

Question

I\'m trying to understand the usage of extern and global variable declaration in header files so I came up with the following test program written in C.

The main.c f

Answer 1

C and C++ are different languages. Case in point, the above program is a valid C program but in ill-formed C++ program. You have violated C++'s one definition rule. There is no corresponding rule in C.

When compiling with gcc, you are compiling the above text as a C program. When compiling with g++, you are compiling the above text as a C++ program.

Answer 2

When compiled with gcc, uninitialized global variable (such as nValue) will be treated as a common symbol. The same common symbol occurred in different compilation unit will be merged during link time. If compiled with g++ (which means that the source program will be treated as C++ program), uninitialized global variable are implicitly initialized with a default value 0. Since global.h is included in multiple source files, the compiler will consider the symbol nValue defined multiple times.

Please also have a look at this post: Why uninitialized global variable is weak symbol?