Extern Variable

Question

int main()
{
  extern int i;
  i=20;
  printf("%d",i);
  return 0;
}

The compiler is giving an Error that 'i' is undefined

Whats the Reason?

Answer 1

Difference :

1.int i; // i is defined to be an integer type in the current function/file
2.extern int i; // i is defined in some other file and only a proto-type is present here.

Hence while compiling, the compiler(LDD) will look for the original definition of the variable and if it does'nt find, it'll throw an error 'undefined reference to `i'.

Answer 2

By saying extern you tell the compiler that i is defined in a different translation unit, which I'm guessing you don't have. There's a difference between declaration and definition in C. In short, the former is telling the compiler the type of the variable, the latter is telling to allocate storage for it.

Just drop that extern for now.