Modern Computer: Von Neumann

ECEC 201 · Week 5 · Lecture

Every malloc has one free

• malloc returns a block or NULL; check it before you use it.
• Each block is freed exactly once: never zero times (a leak), never twice.
• After free(p), p still holds the old address. Using it is a use-after-free.
• valgrind --leak-check=full ./prog lists each block still allocated at exit, with the line that allocated it.

ECEC 201 · Week 5 · Lecture

Freeing a list without reading freed memory

#include <stdlib.h>

struct node { struct node *next; char *text; };

void list_free(struct node *head)
{
	while (head) {
		struct node *next = head->next;
		free(head->text);
		free(head);
		head = next;
	}
}

ECEC 201 · Week 5 · Lecture

A test that fails loudly

#include <assert.h>
#include "list.h"   /* your own list API */

int main(void)
{
	struct node *list = NULL;

	list = list_insert(list, "a");
	assert(list_find(list, "a") != NULL);
	list = list_remove(list, "a");
	assert(list_find(list, "a") == NULL);
	list_free(list);
	return 0;
}

ECEC 201 · Week 5 · Lecture

Padding: members start on their alignment

struct mixed { char a; int b; char c; };

The size rounds up to the largest member's alignment: 12 bytes here, with a 4-byte int.