> If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:

• if it has pointer type, it is initialized to a null pointer;

• if it has arithmetic type, it is initialized to (positive or unsigned) zero;

[end quote]

—C99 Standard, §6.7.8

EDIT: That said, it can only relied upon to be zero at the first call of the function.