C: transitive (double) assignments

Question

I have used such construct in C:

list->head = list->tail = NULL;

and now I consider whether this really mean what I suppose.

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Answer 1

Neither of those is correct.

Since the simple assignment = operator is right-to-left associative, your expression is identical to:

list->head = (list->tail = NULL);

NULL is assigned to tail, and then tail, which has the value of a null pointer, to head.