where memory for extern variable will be stored and by which file

Question

I read about extern variable, but no where found answer related to its memory allocation, My question is Who will allocate memory for Extern variable, and in which memory segment.

int a; // file 1

extern int a; // file 2

here file 1 will allocate memory for a or file 2. In data segment or in stack ?

Thanks.

Answer 1

The extern keyword means "declare without defining". In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition.

So in file2, you just declared the variable without definition (no memory allocated). In file1, you declared and defined a variable of type integer. Here you allocated memory on the BSS segment because you have an uninitialized global (for C).

In C++, the globals are stored in a per-process data area.

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Difference between declaration and definition :

To understand how external variables relate to the extern keyword, it is necessary to know the difference between defining and declaring a variable.

When a variable is defined, the compiler allocates memory for that variable and possibly also initializes its contents to some value. When a variable is declared, the compiler requires that the variable be defined elsewhere.

The declaration informs the compiler that a variable by that name and type exists, but the compiler need not allocate memory for it since it is allocated elsewhere.

Answer 2

In file 2 an integer type variable called a has been declared (remember no definition i.e. no memory allocation for a so far). And we can do this declaration as many times as needed. Where is in file 1 an integer type variable called a has been declared as well as defined. (remember that definition is the super set of declaration). Here the memory for a is also allocated.