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Convert the tail queue to a semi-circle queue and use the XOR swap

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trick to swap the prev pointers during append.
This commit is contained in:
Todd C. Miller
2007-08-24 00:14:21 +00:00
parent 444e77946a
commit 6f732db02b
2 changed files with 23 additions and 20 deletions
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37
parse.h
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@@ -52,10 +52,12 @@ struct cmndtag {
* modelled after the yacc grammar.
*
* Other than the alias struct, which is stored in a red-black tree,
* the data structure used is basically a tail queue without a separate
* head struct--the first entry acts as the head. This makes it possible
* to trivally append sub-lists. Note, however, that the "last" field is
* only valid in the first entry (the list head).
* the data structure used is basically a doubly-linked tail queue without
* a separate head struct--the first entry acts as the head where the prev
* pointer does double duty as the tail pointer. This makes it possible
* to trivally append sub-lists. In addition, the prev pointer is always
* valid (even if it points to itself). Unlike a circle queue, the next
* pointer of the last entry is NULL and does not point back to the head.
*/
/*
@@ -64,7 +66,7 @@ struct cmndtag {
struct userspec {
struct member *user; /* list of users */
struct privilege *privileges; /* list of privileges */
struct userspec *last, *next;
struct userspec *prev, *next;
};
/*
@@ -73,7 +75,7 @@ struct userspec {
struct privilege {
struct member *hostlist; /* list of hosts */
struct cmndspec *cmndlist; /* list of Cmnd_Specs */
struct privilege *last, *next;
struct privilege *prev, *next;
};
/*
@@ -83,7 +85,7 @@ struct cmndspec {
struct member *runaslist; /* list of runas users */
struct member *cmnd; /* command to allow/deny */
struct cmndtag tags; /* tag specificaion */
struct cmndspec *last, *next;
struct cmndspec *prev, *next;
};
/*
@@ -93,7 +95,7 @@ struct member {
char *name; /* member name */
short type; /* type (see gram.h) */
short negated; /* negated via '!'? */
struct member *last, *next;
struct member *prev, *next;
};
/*
@@ -115,7 +117,7 @@ struct defaults {
struct member *binding; /* user/host/runas binding */
int type; /* DEFAULTS{,_USER,_RUNAS,_HOST} */
int op; /* TRUE, FALSE, '+', '-' */
struct defaults *last, *next;
struct defaults *prev, *next;
};
/*
@@ -126,7 +128,7 @@ struct defaults {
(r)->var = (v1); \
(r)->val = (v2); \
(r)->op = (o); \
(r)->last = NULL; \
(r)->prev = (r); \
(r)->next = NULL; \
} while (0)
@@ -137,19 +139,20 @@ struct defaults {
(r) = emalloc(sizeof(struct member)); \
(r)->name = (n); \
(r)->type = (t); \
(r)->last = NULL; \
(r)->prev = (r); \
(r)->next = NULL; \
} while (0)
/*
* Append a list (or single entry) to a tail queue.
* Append one queue (or single entry) to another using the
* circular properties of the prev pointer to simplify the logic.
* We use XOR to swap the two prev pointers to avoid a temp variable.
*/
#define LIST_APPEND(h, e) do { \
if ((h)->last != NULL) \
(h)->last->next = (e); \
else /* if ((h)->next == NULL) */ \
(h)->next = (e); \
(h)->last = (e)->last ? (e)->last : (e); \
(h)->prev->next = (e); \
(long)(e)->prev ^= (long)(h)->prev; \
(long)(h)->prev ^= (long)(e)->prev; \
(long)(e)->prev ^= (long)(h)->prev; \
} while (0)
/*