SEO

Hi,

Yesterday I committed a change to the queue.h macros to be able to
find some specific bugs in the usage of these macros.

pedro@ found out the hard way that removing the same element from the
list twice can produce very funny results, as demonstrated by the
program below, based on his example (the example output is created
with the old queue.h). Here elements that were removed earlier
suddenly reappear.

The change I made causes a kernel panic or a core dump when the actual
wrong operation takes place, rather than producing a bogus list that
results in problems that are very hard to debug.

This may uncover bugs in existing software. So please be aware of
this change when trying to debug programs on -currennt.

-Otto


#include <sys/queue.h>
#include <stdlib.h>
#include <stdio.h>

LIST_HEAD(head, element) list;

struct element {
char val;
LIST_ENTRY(element) entries;
};

int
main(void)
{
struct element *a, *b, *c, *d, *z;

LIST_INIT(&list);

a = malloc(sizeof(struct element));
a->val = 'A';
LIST_INSERT_HEAD(&list, a, entries);

b = malloc(sizeof(struct element));
b->val = 'B';
LIST_INSERT_HEAD(&list, b, entries);

c = malloc(sizeof(struct element));
c->val = 'C';
LIST_INSERT_HEAD(&list, c, entries);

d = malloc(sizeof(struct element));
d->val = 'D';
LIST_INSERT_HEAD(&list, d, entries);

LIST_FOREACH(z, &list, entries)
printf("%c\n", z->val);

LIST_REMOVE(d, entries);
LIST_REMOVE(c, entries);
LIST_REMOVE(b, entries);
LIST_REMOVE(a, entries);

printf("== should be empty\n");
LIST_FOREACH(z, &list, entries)
printf("%c\n", z->val);

LIST_REMOVE(d, entries);

printf("== after second remove of d\n");
LIST_FOREACH(z, &list, entries)
printf("%c\n", z->val);
}

[otto@fonzo:57]$ ./a.out
D
C
B
A
== should be empty
== after second remove of d
C
B
A
[otto@fonzo:58]$