In C, is it valid to declare a variable multiple times? -

i have below c code , expecting throw error "multiple declaration of variable", not doing so.

#include <stdio.h>  int i;         int i;      int main() {     printf("%d",i);     return 0; } 

now output 0, why?

and 1 more thing below code gives error expected

#include <stdio.h>   int main() {     int i;             int i;          printf("%d",i);     return 0; } 

o/p error saying re declaration of i

the first definition of i tentative definition (the 2nd tentative definition). "de facto" definitions though (and definitions serve declarations), no mistake it.

quote the standard:

6.9.2/2

a declaration of identifier object has file scope without initializer, , without storage-class specifier or storage-class specifier static, constitutes tentative definition. if translation unit contains 1 or more tentative definitions identifier, , translation unit contains no external definition identifier, behavior if translation unit contains file scope declaration of identifier, composite type of end of translation unit, initializer equal 0.