Question 7.20

You can't use dynamically-allocated memory after you free it, can you?

No. Some early documentation for malloc stated that the contents of freed memory were ``left undisturbed,'' but this ill-advised guarantee was never universal and is not required by the C Standard.

Few programmers would use the contents of freed memory deliberately, but it is easy to do so accidentally. Consider the following (correct) code for freeing a singly-linked list:

	struct list *listp, *nextp;
	for(listp = base; listp != NULL; listp = nextp) {
		nextp = listp->next;
		free((void *)listp);
	}

and notice what would happen if the more-obvious loop iteration expression listp = listp->next were used, without the temporary nextp pointer.

References: K&R2 Sec. 7.8.5 p. 167
ANSI Sec. 4.10.3
ISO Sec. 7.10.3
Rationale Sec. 4.10.3.2
H&S Sec. 16.2 p. 387
CT&P Sec. 7.10 p. 95

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