Question

Consider the following char array:

char str[128] = "abcd";

Are all the remaining uninitialized chars in the rest of the array (from str[4] to str[127]) zero/null filled?

Answer 1

Yes, if there are fewer elements explicitly given in an initialiser than the aggregate contains, then the remaining elements are initialised as if the aggregate had static storage duration. For integer types (and char is one) that means with 0s.
The relevant section of the standard is 6.7.9 (21):

If there are fewer initializers in a brace-enclosed list than there are elements or members
of an aggregate, or fewer characters in a string literal used to initialize an array of known
size than there are elements in the array, the remainder of the aggregate shall be
initialized implicitly the same as objects that have static storage duration.

String literals as initialisers for char arrays are equivalent to brace-encloded initialisers in that respect.