openbsd-compat/sys-queue.h

   1 /* OPENBSD ORIGINAL: sys/sys/queue.h */
   2
   3 /*      $OpenBSD: queue.h,v 1.25 2004/04/08 16:08:21 henning Exp $      */
   4 /*      $NetBSD: queue.h,v 1.11 1996/05/16 05:17:14 mycroft Exp $       */
   5
   6 /*
   7  * Copyright (c) 1991, 1993
   8  *      The Regents of the University of California.  All rights reserved.
   9  *
  10  * Redistribution and use in source and binary forms, with or without
  11  * modification, are permitted provided that the following conditions
  12  * are met:
  13  * 1. Redistributions of source code must retain the above copyright
  14  *    notice, this list of conditions and the following disclaimer.
  15  * 2. Redistributions in binary form must reproduce the above copyright
  16  *    notice, this list of conditions and the following disclaimer in the
  17  *    documentation and/or other materials provided with the distribution.
  18  * 3. Neither the name of the University nor the names of its contributors
  19  *    may be used to endorse or promote products derived from this software
  20  *    without specific prior written permission.
  21  *
  22  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  32  * SUCH DAMAGE.
  33  *
  34  *      @(#)queue.h     8.5 (Berkeley) 8/20/94
  35  */
  36
  37 #ifndef _SYS_QUEUE_H_
  38 #define _SYS_QUEUE_H_
  39
  40 /*
  41  * This file defines five types of data structures: singly-linked lists,
  42  * lists, simple queues, tail queues, and circular queues.
  43  *
  44  *
  45  * A singly-linked list is headed by a single forward pointer. The elements
  46  * are singly linked for minimum space and pointer manipulation overhead at
  47  * the expense of O(n) removal for arbitrary elements. New elements can be
  48  * added to the list after an existing element or at the head of the list.
  49  * Elements being removed from the head of the list should use the explicit
  50  * macro for this purpose for optimum efficiency. A singly-linked list may
  51  * only be traversed in the forward direction.  Singly-linked lists are ideal
  52  * for applications with large datasets and few or no removals or for
  53  * implementing a LIFO queue.
  54  *
  55  * A list is headed by a single forward pointer (or an array of forward
  56  * pointers for a hash table header). The elements are doubly linked
  57  * so that an arbitrary element can be removed without a need to
  58  * traverse the list. New elements can be added to the list before
  59  * or after an existing element or at the head of the list. A list
  60  * may only be traversed in the forward direction.
  61  *
  62  * A simple queue is headed by a pair of pointers, one the head of the
  63  * list and the other to the tail of the list. The elements are singly
  64  * linked to save space, so elements can only be removed from the
  65  * head of the list. New elements can be added to the list before or after
  66  * an existing element, at the head of the list, or at the end of the
  67  * list. A simple queue may only be traversed in the forward direction.
  68  *
  69  * A tail queue is headed by a pair of pointers, one to the head of the
  70  * list and the other to the tail of the list. The elements are doubly
  71  * linked so that an arbitrary element can be removed without a need to
  72  * traverse the list. New elements can be added to the list before or
  73  * after an existing element, at the head of the list, or at the end of
  74  * the list. A tail queue may be traversed in either direction.
  75  *
  76  * A circle queue is headed by a pair of pointers, one to the head of the
  77  * list and the other to the tail of the list. The elements are doubly
  78  * linked so that an arbitrary element can be removed without a need to
  79  * traverse the list. New elements can be added to the list before or after
  80  * an existing element, at the head of the list, or at the end of the list.
  81  * A circle queue may be traversed in either direction, but has a more
  82  * complex end of list detection.
  83  *
  84  * For details on the use of these macros, see the queue(3) manual page.
  85  */
  86
  87 /*
  88  * Singly-linked List definitions.
  89  */
  90 #define SLIST_HEAD(name, type)                                          \
  91 struct name {                                                           \
  92         struct type *slh_first; /* first element */                     \
  93 }
  94
  95 #define SLIST_HEAD_INITIALIZER(head)                                    \
  96         { NULL }
  97
  98 #define SLIST_ENTRY(type)                                               \
  99 struct {                                                                \
 100         struct type *sle_next;  /* next element */                      \
 101 }
 102
 103 /*
 104  * Singly-linked List access methods.
 105  */
 106 #define SLIST_FIRST(head)       ((head)->slh_first)
 107 #define SLIST_END(head)         NULL
 108 #define SLIST_EMPTY(head)       (SLIST_FIRST(head) == SLIST_END(head))
 109 #define SLIST_NEXT(elm, field)  ((elm)->field.sle_next)
 110
 111 #define SLIST_FOREACH(var, head, field)                                 \
 112         for((var) = SLIST_FIRST(head);                                  \
 113             (var) != SLIST_END(head);                                   \
 114             (var) = SLIST_NEXT(var, field))
 115
 116 #define SLIST_FOREACH_PREVPTR(var, varp, head, field)                   \
 117         for ((varp) = &SLIST_FIRST((head));                             \
 118             ((var) = *(varp)) != SLIST_END(head);                       \
 119             (varp) = &SLIST_NEXT((var), field))
 120
 121 /*
 122  * Singly-linked List functions.
 123  */
 124 #define SLIST_INIT(head) {                                              \
 125         SLIST_FIRST(head) = SLIST_END(head);                            \
 126 }
 127
 128 #define SLIST_INSERT_AFTER(slistelm, elm, field) do {                   \
 129         (elm)->field.sle_next = (slistelm)->field.sle_next;             \
 130         (slistelm)->field.sle_next = (elm);                             \
 131 } while (0)
 132
 133 #define SLIST_INSERT_HEAD(head, elm, field) do {                        \
 134         (elm)->field.sle_next = (head)->slh_first;                      \
 135         (head)->slh_first = (elm);                                      \
 136 } while (0)
 137
 138 #define SLIST_REMOVE_NEXT(head, elm, field) do {                        \
 139         (elm)->field.sle_next = (elm)->field.sle_next->field.sle_next;  \
 140 } while (0)
 141
 142 #define SLIST_REMOVE_HEAD(head, field) do {                             \
 143         (head)->slh_first = (head)->slh_first->field.sle_next;          \
 144 } while (0)
 145
 146 #define SLIST_REMOVE(head, elm, type, field) do {                       \
 147         if ((head)->slh_first == (elm)) {                               \
 148                 SLIST_REMOVE_HEAD((head), field);                       \
 149         }                                                               \
 150         else {                                                          \
 151                 struct type *curelm = (head)->slh_first;                \
 152                 while( curelm->field.sle_next != (elm) )                \
 153                         curelm = curelm->field.sle_next;                \
 154                 curelm->field.sle_next =                                \
 155                     curelm->field.sle_next->field.sle_next;             \
 156         }                                                               \
 157 } while (0)
 158
 159 /*
 160  * List definitions.
 161  */
 162 #define LIST_HEAD(name, type)                                           \
 163 struct name {                                                           \
 164         struct type *lh_first;  /* first element */                     \
 165 }
 166
 167 #define LIST_HEAD_INITIALIZER(head)                                     \
 168         { NULL }
 169
 170 #define LIST_ENTRY(type)                                                \
 171 struct {                                                                \
 172         struct type *le_next;   /* next element */                      \
 173         struct type **le_prev;  /* address of previous next element */  \
 174 }
 175
 176 /*
 177  * List access methods
 178  */
 179 #define LIST_FIRST(head)                ((head)->lh_first)
 180 #define LIST_END(head)                  NULL
 181 #define LIST_EMPTY(head)                (LIST_FIRST(head) == LIST_END(head))
 182 #define LIST_NEXT(elm, field)           ((elm)->field.le_next)
 183
 184 #define LIST_FOREACH(var, head, field)                                  \
 185         for((var) = LIST_FIRST(head);                                   \
 186             (var)!= LIST_END(head);                                     \
 187             (var) = LIST_NEXT(var, field))
 188
 189 /*
 190  * List functions.
 191  */
 192 #define LIST_INIT(head) do {                                            \
 193         LIST_FIRST(head) = LIST_END(head);                              \
 194 } while (0)
 195
 196 #define LIST_INSERT_AFTER(listelm, elm, field) do {                     \
 197         if (((elm)->field.le_next = (listelm)->field.le_next) != NULL)  \
 198                 (listelm)->field.le_next->field.le_prev =               \
 199                     &(elm)->field.le_next;                              \
 200         (listelm)->field.le_next = (elm);                               \
 201         (elm)->field.le_prev = &(listelm)->field.le_next;               \
 202 } while (0)
 203
 204 #define LIST_INSERT_BEFORE(listelm, elm, field) do {                    \
 205         (elm)->field.le_prev = (listelm)->field.le_prev;                \
 206         (elm)->field.le_next = (listelm);                               \
 207         *(listelm)->field.le_prev = (elm);                              \
 208         (listelm)->field.le_prev = &(elm)->field.le_next;               \
 209 } while (0)
 210
 211 #define LIST_INSERT_HEAD(head, elm, field) do {                         \
 212         if (((elm)->field.le_next = (head)->lh_first) != NULL)          \
 213                 (head)->lh_first->field.le_prev = &(elm)->field.le_next;\
 214         (head)->lh_first = (elm);                                       \
 215         (elm)->field.le_prev = &(head)->lh_first;                       \
 216 } while (0)
 217
 218 #define LIST_REMOVE(elm, field) do {                                    \
 219         if ((elm)->field.le_next != NULL)                               \
 220                 (elm)->field.le_next->field.le_prev =                   \
 221                     (elm)->field.le_prev;                               \
 222         *(elm)->field.le_prev = (elm)->field.le_next;                   \
 223 } while (0)
 224
 225 #define LIST_REPLACE(elm, elm2, field) do {                             \
 226         if (((elm2)->field.le_next = (elm)->field.le_next) != NULL)     \
 227                 (elm2)->field.le_next->field.le_prev =                  \
 228                     &(elm2)->field.le_next;                             \
 229         (elm2)->field.le_prev = (elm)->field.le_prev;                   \
 230         *(elm2)->field.le_prev = (elm2);                                \
 231 } while (0)
 232
 233 /*
 234  * Simple queue definitions.
 235  */
 236 #define SIMPLEQ_HEAD(name, type)                                        \
 237 struct name {                                                           \
 238         struct type *sqh_first; /* first element */                     \
 239         struct type **sqh_last; /* addr of last next element */         \
 240 }
 241
 242 #define SIMPLEQ_HEAD_INITIALIZER(head)                                  \
 243         { NULL, &(head).sqh_first }
 244
 245 #define SIMPLEQ_ENTRY(type)                                             \
 246 struct {                                                                \
 247         struct type *sqe_next;  /* next element */                      \
 248 }
 249
 250 /*
 251  * Simple queue access methods.
 252  */
 253 #define SIMPLEQ_FIRST(head)         ((head)->sqh_first)
 254 #define SIMPLEQ_END(head)           NULL
 255 #define SIMPLEQ_EMPTY(head)         (SIMPLEQ_FIRST(head) == SIMPLEQ_END(head))
 256 #define SIMPLEQ_NEXT(elm, field)    ((elm)->field.sqe_next)
 257
 258 #define SIMPLEQ_FOREACH(var, head, field)                               \
 259         for((var) = SIMPLEQ_FIRST(head);                                \
 260             (var) != SIMPLEQ_END(head);                                 \
 261             (var) = SIMPLEQ_NEXT(var, field))
 262
 263 /*
 264  * Simple queue functions.
 265  */
 266 #define SIMPLEQ_INIT(head) do {                                         \
 267         (head)->sqh_first = NULL;                                       \
 268         (head)->sqh_last = &(head)->sqh_first;                          \
 269 } while (0)
 270
 271 #define SIMPLEQ_INSERT_HEAD(head, elm, field) do {                      \
 272         if (((elm)->field.sqe_next = (head)->sqh_first) == NULL)        \
 273                 (head)->sqh_last = &(elm)->field.sqe_next;              \
 274         (head)->sqh_first = (elm);                                      \
 275 } while (0)
 276
 277 #define SIMPLEQ_INSERT_TAIL(head, elm, field) do {                      \
 278         (elm)->field.sqe_next = NULL;                                   \
 279         *(head)->sqh_last = (elm);                                      \
 280         (head)->sqh_last = &(elm)->field.sqe_next;                      \
 281 } while (0)
 282
 283 #define SIMPLEQ_INSERT_AFTER(head, listelm, elm, field) do {            \
 284         if (((elm)->field.sqe_next = (listelm)->field.sqe_next) == NULL)\
 285                 (head)->sqh_last = &(elm)->field.sqe_next;              \
 286         (listelm)->field.sqe_next = (elm);                              \
 287 } while (0)
 288
 289 #define SIMPLEQ_REMOVE_HEAD(head, elm, field) do {                      \
 290         if (((head)->sqh_first = (elm)->field.sqe_next) == NULL)        \
 291                 (head)->sqh_last = &(head)->sqh_first;                  \
 292 } while (0)
 293
 294 /*
 295  * Tail queue definitions.
 296  */
 297 #define TAILQ_HEAD(name, type)                                          \
 298 struct name {                                                           \
 299         struct type *tqh_first; /* first element */                     \
 300         struct type **tqh_last; /* addr of last next element */         \
 301 }
 302
 303 #define TAILQ_HEAD_INITIALIZER(head)                                    \
 304         { NULL, &(head).tqh_first }
 305
 306 #define TAILQ_ENTRY(type)                                               \
 307 struct {                                                                \
 308         struct type *tqe_next;  /* next element */                      \
 309         struct type **tqe_prev; /* address of previous next element */  \
 310 }
 311
 312 /*
 313  * tail queue access methods
 314  */
 315 #define TAILQ_FIRST(head)               ((head)->tqh_first)
 316 #define TAILQ_END(head)                 NULL
 317 #define TAILQ_NEXT(elm, field)          ((elm)->field.tqe_next)
 318 #define TAILQ_LAST(head, headname)                                      \
 319         (*(((struct headname *)((head)->tqh_last))->tqh_last))
 320 /* XXX */
 321 #define TAILQ_PREV(elm, headname, field)                                \
 322         (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
 323 #define TAILQ_EMPTY(head)                                               \
 324         (TAILQ_FIRST(head) == TAILQ_END(head))
 325
 326 #define TAILQ_FOREACH(var, head, field)                                 \
 327         for((var) = TAILQ_FIRST(head);                                  \
 328             (var) != TAILQ_END(head);                                   \
 329             (var) = TAILQ_NEXT(var, field))
 330
 331 #define TAILQ_FOREACH_REVERSE(var, head, headname, field)               \
 332         for((var) = TAILQ_LAST(head, headname);                         \
 333             (var) != TAILQ_END(head);                                   \
 334             (var) = TAILQ_PREV(var, headname, field))
 335
 336 /*
 337  * Tail queue functions.
 338  */
 339 #define TAILQ_INIT(head) do {                                           \
 340         (head)->tqh_first = NULL;                                       \
 341         (head)->tqh_last = &(head)->tqh_first;                          \
 342 } while (0)
 343
 344 #define TAILQ_INSERT_HEAD(head, elm, field) do {                        \
 345         if (((elm)->field.tqe_next = (head)->tqh_first) != NULL)        \
 346                 (head)->tqh_first->field.tqe_prev =                     \
 347                     &(elm)->field.tqe_next;                             \
 348         else                                                            \
 349                 (head)->tqh_last = &(elm)->field.tqe_next;              \
 350         (head)->tqh_first = (elm);                                      \
 351         (elm)->field.tqe_prev = &(head)->tqh_first;                     \
 352 } while (0)
 353
 354 #define TAILQ_INSERT_TAIL(head, elm, field) do {                        \
 355         (elm)->field.tqe_next = NULL;                                   \
 356         (elm)->field.tqe_prev = (head)->tqh_last;                       \
 357         *(head)->tqh_last = (elm);                                      \
 358         (head)->tqh_last = &(elm)->field.tqe_next;                      \
 359 } while (0)
 360
 361 #define TAILQ_INSERT_AFTER(head, listelm, elm, field) do {              \
 362         if (((elm)->field.tqe_next = (listelm)->field.tqe_next) != NULL)\
 363                 (elm)->field.tqe_next->field.tqe_prev =                 \
 364                     &(elm)->field.tqe_next;                             \
 365         else                                                            \
 366                 (head)->tqh_last = &(elm)->field.tqe_next;              \
 367         (listelm)->field.tqe_next = (elm);                              \
 368         (elm)->field.tqe_prev = &(listelm)->field.tqe_next;             \
 369 } while (0)
 370
 371 #define TAILQ_INSERT_BEFORE(listelm, elm, field) do {                   \
 372         (elm)->field.tqe_prev = (listelm)->field.tqe_prev;              \
 373         (elm)->field.tqe_next = (listelm);                              \
 374         *(listelm)->field.tqe_prev = (elm);                             \
 375         (listelm)->field.tqe_prev = &(elm)->field.tqe_next;             \
 376 } while (0)
 377
 378 #define TAILQ_REMOVE(head, elm, field) do {                             \
 379         if (((elm)->field.tqe_next) != NULL)                            \
 380                 (elm)->field.tqe_next->field.tqe_prev =                 \
 381                     (elm)->field.tqe_prev;                              \
 382         else                                                            \
 383                 (head)->tqh_last = (elm)->field.tqe_prev;               \
 384         *(elm)->field.tqe_prev = (elm)->field.tqe_next;                 \
 385 } while (0)
 386
 387 #define TAILQ_REPLACE(head, elm, elm2, field) do {                      \
 388         if (((elm2)->field.tqe_next = (elm)->field.tqe_next) != NULL)   \
 389                 (elm2)->field.tqe_next->field.tqe_prev =                \
 390                     &(elm2)->field.tqe_next;                            \
 391         else                                                            \
 392                 (head)->tqh_last = &(elm2)->field.tqe_next;             \
 393         (elm2)->field.tqe_prev = (elm)->field.tqe_prev;                 \
 394         *(elm2)->field.tqe_prev = (elm2);                               \
 395 } while (0)
 396
 397 /*
 398  * Circular queue definitions.
 399  */
 400 #define CIRCLEQ_HEAD(name, type)                                        \
 401 struct name {                                                           \
 402         struct type *cqh_first;         /* first element */             \
 403         struct type *cqh_last;          /* last element */              \
 404 }
 405
 406 #define CIRCLEQ_HEAD_INITIALIZER(head)                                  \
 407         { CIRCLEQ_END(&head), CIRCLEQ_END(&head) }
 408
 409 #define CIRCLEQ_ENTRY(type)                                             \
 410 struct {                                                                \
 411         struct type *cqe_next;          /* next element */              \
 412         struct type *cqe_prev;          /* previous element */          \
 413 }
 414
 415 /*
 416  * Circular queue access methods
 417  */
 418 #define CIRCLEQ_FIRST(head)             ((head)->cqh_first)
 419 #define CIRCLEQ_LAST(head)              ((head)->cqh_last)
 420 #define CIRCLEQ_END(head)               ((void *)(head))
 421 #define CIRCLEQ_NEXT(elm, field)        ((elm)->field.cqe_next)
 422 #define CIRCLEQ_PREV(elm, field)        ((elm)->field.cqe_prev)
 423 #define CIRCLEQ_EMPTY(head)                                             \
 424         (CIRCLEQ_FIRST(head) == CIRCLEQ_END(head))
 425
 426 #define CIRCLEQ_FOREACH(var, head, field)                               \
 427         for((var) = CIRCLEQ_FIRST(head);                                \
 428             (var) != CIRCLEQ_END(head);                                 \
 429             (var) = CIRCLEQ_NEXT(var, field))
 430
 431 #define CIRCLEQ_FOREACH_REVERSE(var, head, field)                       \
 432         for((var) = CIRCLEQ_LAST(head);                                 \
 433             (var) != CIRCLEQ_END(head);                                 \
 434             (var) = CIRCLEQ_PREV(var, field))
 435
 436 /*
 437  * Circular queue functions.
 438  */
 439 #define CIRCLEQ_INIT(head) do {                                         \
 440         (head)->cqh_first = CIRCLEQ_END(head);                          \
 441         (head)->cqh_last = CIRCLEQ_END(head);                           \
 442 } while (0)
 443
 444 #define CIRCLEQ_INSERT_AFTER(head, listelm, elm, field) do {            \
 445         (elm)->field.cqe_next = (listelm)->field.cqe_next;              \
 446         (elm)->field.cqe_prev = (listelm);                              \
 447         if ((listelm)->field.cqe_next == CIRCLEQ_END(head))             \
 448                 (head)->cqh_last = (elm);                               \
 449         else                                                            \
 450                 (listelm)->field.cqe_next->field.cqe_prev = (elm);      \
 451         (listelm)->field.cqe_next = (elm);                              \
 452 } while (0)
 453
 454 #define CIRCLEQ_INSERT_BEFORE(head, listelm, elm, field) do {           \
 455         (elm)->field.cqe_next = (listelm);                              \
 456         (elm)->field.cqe_prev = (listelm)->field.cqe_prev;              \
 457         if ((listelm)->field.cqe_prev == CIRCLEQ_END(head))             \
 458                 (head)->cqh_first = (elm);                              \
 459         else                                                            \
 460                 (listelm)->field.cqe_prev->field.cqe_next = (elm);      \
 461         (listelm)->field.cqe_prev = (elm);                              \
 462 } while (0)
 463
 464 #define CIRCLEQ_INSERT_HEAD(head, elm, field) do {                      \
 465         (elm)->field.cqe_next = (head)->cqh_first;                      \
 466         (elm)->field.cqe_prev = CIRCLEQ_END(head);                      \
 467         if ((head)->cqh_last == CIRCLEQ_END(head))                      \
 468                 (head)->cqh_last = (elm);                               \
 469         else                                                            \
 470                 (head)->cqh_first->field.cqe_prev = (elm);              \
 471         (head)->cqh_first = (elm);                                      \
 472 } while (0)
 473
 474 #define CIRCLEQ_INSERT_TAIL(head, elm, field) do {                      \
 475         (elm)->field.cqe_next = CIRCLEQ_END(head);                      \
 476         (elm)->field.cqe_prev = (head)->cqh_last;                       \
 477         if ((head)->cqh_first == CIRCLEQ_END(head))                     \
 478                 (head)->cqh_first = (elm);                              \
 479         else                                                            \
 480                 (head)->cqh_last->field.cqe_next = (elm);               \
 481         (head)->cqh_last = (elm);                                       \
 482 } while (0)
 483
 484 #define CIRCLEQ_REMOVE(head, elm, field) do {                           \
 485         if ((elm)->field.cqe_next == CIRCLEQ_END(head))                 \
 486                 (head)->cqh_last = (elm)->field.cqe_prev;               \
 487         else                                                            \
 488                 (elm)->field.cqe_next->field.cqe_prev =                 \
 489                     (elm)->field.cqe_prev;                              \
 490         if ((elm)->field.cqe_prev == CIRCLEQ_END(head))                 \
 491                 (head)->cqh_first = (elm)->field.cqe_next;              \
 492         else                                                            \
 493                 (elm)->field.cqe_prev->field.cqe_next =                 \
 494                     (elm)->field.cqe_next;                              \
 495 } while (0)
 496
 497 #define CIRCLEQ_REPLACE(head, elm, elm2, field) do {                    \
 498         if (((elm2)->field.cqe_next = (elm)->field.cqe_next) ==         \
 499             CIRCLEQ_END(head))                                          \
 500                 (head).cqh_last = (elm2);                               \
 501         else                                                            \
 502                 (elm2)->field.cqe_next->field.cqe_prev = (elm2);        \
 503         if (((elm2)->field.cqe_prev = (elm)->field.cqe_prev) ==         \
 504             CIRCLEQ_END(head))                                          \
 505                 (head).cqh_first = (elm2);                              \
 506         else                                                            \
 507                 (elm2)->field.cqe_prev->field.cqe_next = (elm2);        \
 508 } while (0)
 509
 510 #endif  /* !_SYS_QUEUE_H_ */