include/apr-0/apr_ring.h

   1 /* Licensed to the Apache Software Foundation (ASF) under one or more
   2  * contributor license agreements.  See the NOTICE file distributed with
   3  * this work for additional information regarding copyright ownership.
   4  * The ASF licenses this file to You under the Apache License, Version 2.0
   5  * (the "License"); you may not use this file except in compliance with
   6  * the License.  You may obtain a copy of the License at
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 /*
  18  * This code draws heavily from the 4.4BSD <sys/queue.h> macros
  19  * and Dean Gaudet's "splim/ring.h".
  20  * <http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/sys/queue.h>
  21  * <http://www.arctic.org/~dean/splim/>
  22  *
  23  * We'd use Dean's code directly if we could guarantee the
  24  * availability of inline functions.
  25  */
  26
  27 #ifndef APR_RING_H
  28 #define APR_RING_H
  29
  30 /**
  31  * @file apr_ring.h
  32  * @brief APR Rings
  33  */
  34
  35 /*
  36  * for offsetof()
  37  */
  38 #include "apr_general.h"
  39
  40 /**
  41  * @defgroup apr_ring Ring Macro Implementations
  42  * @ingroup APR
  43  * A ring is a kind of doubly-linked list that can be manipulated
  44  * without knowing where its head is.
  45  * @{
  46  */
  47
  48 /**
  49  * The Ring Element
  50  *
  51  * A ring element struct is linked to the other elements in the ring
  52  * through its ring entry field, e.g.
  53  * <pre>
  54  *      struct my_element_t {
  55  *          APR_RING_ENTRY(my_element_t) link;
  56  *          int foo;
  57  *          char *bar;
  58  *      };
  59  * </pre>
  60  *
  61  * An element struct may be put on more than one ring if it has more
  62  * than one APR_RING_ENTRY field. Each APR_RING_ENTRY has a corresponding
  63  * APR_RING_HEAD declaration.
  64  *
  65  * @warning For strict C standards compliance you should put the APR_RING_ENTRY
  66  * first in the element struct unless the head is always part of a larger
  67  * object with enough earlier fields to accommodate the offsetof() used
  68  * to compute the ring sentinel below. You can usually ignore this caveat.
  69  */
  70 #define APR_RING_ENTRY(elem)                                            \
  71     struct {                                                            \
  72         struct elem *next;                                              \
  73         struct elem *prev;                                              \
  74     }
  75
  76 /**
  77  * The Ring Head
  78  *
  79  * Each ring is managed via its head, which is a struct declared like this:
  80  * <pre>
  81  *      APR_RING_HEAD(my_ring_t, my_element_t);
  82  *      struct my_ring_t ring, *ringp;
  83  * </pre>
  84  *
  85  * This struct looks just like the element link struct so that we can
  86  * be sure that the typecasting games will work as expected.
  87  *
  88  * The first element in the ring is next after the head, and the last
  89  * element is just before the head.
  90  */
  91 #define APR_RING_HEAD(head, elem)                                       \
  92     struct head {                                                       \
  93         struct elem * volatile next;                                    \
  94         struct elem * volatile prev;                                    \
  95     }
  96
  97 /**
  98  * The Ring Sentinel
  99  *
 100  * This is the magic pointer value that occurs before the first and
 101  * after the last elements in the ring, computed from the address of
 102  * the ring's head.  The head itself isn't an element, but in order to
 103  * get rid of all the special cases when dealing with the ends of the
 104  * ring, we play typecasting games to make it look like one.
 105  *
 106  * Here is a diagram to illustrate the arrangements of the next and
 107  * prev pointers of each element in a single ring. Note that they point
 108  * to the start of each element, not to the APR_RING_ENTRY structure.
 109  *
 110  * <pre>
 111  *     +->+------+<-+  +->+------+<-+  +->+------+<-+
 112  *     |  |struct|  |  |  |struct|  |  |  |struct|  |
 113  *    /   | elem |   \/   | elem |   \/   | elem |  \
 114  * ...    |      |   /\   |      |   /\   |      |   ...
 115  *        +------+  |  |  +------+  |  |  +------+
 116  *   ...--|prev  |  |  +--|ring  |  |  +--|prev  |
 117  *        |  next|--+     | entry|--+     |  next|--...
 118  *        +------+        +------+        +------+
 119  *        | etc. |        | etc. |        | etc. |
 120  *        :      :        :      :        :      :
 121  * </pre>
 122  *
 123  * The APR_RING_HEAD is nothing but a bare APR_RING_ENTRY. The prev
 124  * and next pointers in the first and last elements don't actually
 125  * point to the head, they point to a phantom place called the
 126  * sentinel. Its value is such that last->next->next == first because
 127  * the offset from the sentinel to the head's next pointer is the same
 128  * as the offset from the start of an element to its next pointer.
 129  * This also works in the opposite direction.
 130  *
 131  * <pre>
 132  *        last                            first
 133  *     +->+------+<-+  +->sentinel<-+  +->+------+<-+
 134  *     |  |struct|  |  |            |  |  |struct|  |
 135  *    /   | elem |   \/              \/   | elem |  \
 136  * ...    |      |   /\              /\   |      |   ...
 137  *        +------+  |  |  +------+  |  |  +------+
 138  *   ...--|prev  |  |  +--|ring  |  |  +--|prev  |
 139  *        |  next|--+     |  head|--+     |  next|--...
 140  *        +------+        +------+        +------+
 141  *        | etc. |                        | etc. |
 142  *        :      :                        :      :
 143  * </pre>
 144  *
 145  * Note that the offset mentioned above is different for each kind of
 146  * ring that the element may be on, and each kind of ring has a unique
 147  * name for its APR_RING_ENTRY in each element, and has its own type
 148  * for its APR_RING_HEAD.
 149  *
 150  * Note also that if the offset is non-zero (which is required if an
 151  * element has more than one APR_RING_ENTRY), the unreality of the
 152  * sentinel may have bad implications on very perverse implementations
 153  * of C -- see the warning in APR_RING_ENTRY.
 154  *
 155  * @param hp   The head of the ring
 156  * @param elem The name of the element struct
 157  * @param link The name of the APR_RING_ENTRY in the element struct
 158  */
 159 #define APR_RING_SENTINEL(hp, elem, link)                               \
 160     (struct elem *)((char *)(hp) - APR_OFFSETOF(struct elem, link))
 161
 162 /**
 163  * The first element of the ring
 164  * @param hp   The head of the ring
 165  */
 166 #define APR_RING_FIRST(hp)      (hp)->next
 167 /**
 168  * The last element of the ring
 169  * @param hp   The head of the ring
 170  */
 171 #define APR_RING_LAST(hp)       (hp)->prev
 172 /**
 173  * The next element in the ring
 174  * @param ep   The current element
 175  * @param link The name of the APR_RING_ENTRY in the element struct
 176  */
 177 #define APR_RING_NEXT(ep, link) (ep)->link.next
 178 /**
 179  * The previous element in the ring
 180  * @param ep   The current element
 181  * @param link The name of the APR_RING_ENTRY in the element struct
 182  */
 183 #define APR_RING_PREV(ep, link) (ep)->link.prev
 184
 185
 186 /**
 187  * Initialize a ring
 188  * @param hp   The head of the ring
 189  * @param elem The name of the element struct
 190  * @param link The name of the APR_RING_ENTRY in the element struct
 191  */
 192 #define APR_RING_INIT(hp, elem, link) do {                              \
 193         APR_RING_FIRST((hp)) = APR_RING_SENTINEL((hp), elem, link);     \
 194         APR_RING_LAST((hp))  = APR_RING_SENTINEL((hp), elem, link);     \
 195     } while (0)
 196
 197 /**
 198  * Determine if a ring is empty
 199  * @param hp   The head of the ring
 200  * @param elem The name of the element struct
 201  * @param link The name of the APR_RING_ENTRY in the element struct
 202  * @return true or false
 203  */
 204 #define APR_RING_EMPTY(hp, elem, link)                                  \
 205     (APR_RING_FIRST((hp)) == APR_RING_SENTINEL((hp), elem, link))
 206
 207 /**
 208  * Initialize a singleton element
 209  * @param ep   The element
 210  * @param link The name of the APR_RING_ENTRY in the element struct
 211  */
 212 #define APR_RING_ELEM_INIT(ep, link) do {                               \
 213         APR_RING_NEXT((ep), link) = (ep);                               \
 214         APR_RING_PREV((ep), link) = (ep);                               \
 215     } while (0)
 216
 217
 218 /**
 219  * Splice the sequence ep1..epN into the ring before element lep
 220  *   (..lep.. becomes ..ep1..epN..lep..)
 221  * @warning This doesn't work for splicing before the first element or on
 222  *   empty rings... see APR_RING_SPLICE_HEAD for one that does
 223  * @param lep  Element in the ring to splice before
 224  * @param ep1  First element in the sequence to splice in
 225  * @param epN  Last element in the sequence to splice in
 226  * @param link The name of the APR_RING_ENTRY in the element struct
 227  */
 228 #define APR_RING_SPLICE_BEFORE(lep, ep1, epN, link) do {                \
 229         APR_RING_NEXT((epN), link) = (lep);                             \
 230         APR_RING_PREV((ep1), link) = APR_RING_PREV((lep), link);        \
 231         APR_RING_NEXT(APR_RING_PREV((lep), link), link) = (ep1);        \
 232         APR_RING_PREV((lep), link) = (epN);                             \
 233     } while (0)
 234
 235 /**
 236  * Splice the sequence ep1..epN into the ring after element lep
 237  *   (..lep.. becomes ..lep..ep1..epN..)
 238  * @warning This doesn't work for splicing after the last element or on
 239  *   empty rings... see APR_RING_SPLICE_TAIL for one that does
 240  * @param lep  Element in the ring to splice after
 241  * @param ep1  First element in the sequence to splice in
 242  * @param epN  Last element in the sequence to splice in
 243  * @param link The name of the APR_RING_ENTRY in the element struct
 244  */
 245 #define APR_RING_SPLICE_AFTER(lep, ep1, epN, link) do {                 \
 246         APR_RING_PREV((ep1), link) = (lep);                             \
 247         APR_RING_NEXT((epN), link) = APR_RING_NEXT((lep), link);        \
 248         APR_RING_PREV(APR_RING_NEXT((lep), link), link) = (epN);        \
 249         APR_RING_NEXT((lep), link) = (ep1);                             \
 250     } while (0)
 251
 252 /**
 253  * Insert the element nep into the ring before element lep
 254  *   (..lep.. becomes ..nep..lep..)
 255  * @warning This doesn't work for inserting before the first element or on
 256  *   empty rings... see APR_RING_INSERT_HEAD for one that does
 257  * @param lep  Element in the ring to insert before
 258  * @param nep  Element to insert
 259  * @param link The name of the APR_RING_ENTRY in the element struct
 260  */
 261 #define APR_RING_INSERT_BEFORE(lep, nep, link)                          \
 262         APR_RING_SPLICE_BEFORE((lep), (nep), (nep), link)
 263
 264 /**
 265  * Insert the element nep into the ring after element lep
 266  *   (..lep.. becomes ..lep..nep..)
 267  * @warning This doesn't work for inserting after the last element or on
 268  *   empty rings... see APR_RING_INSERT_TAIL for one that does
 269  * @param lep  Element in the ring to insert after
 270  * @param nep  Element to insert
 271  * @param link The name of the APR_RING_ENTRY in the element struct
 272  */
 273 #define APR_RING_INSERT_AFTER(lep, nep, link)                           \
 274         APR_RING_SPLICE_AFTER((lep), (nep), (nep), link)
 275
 276
 277 /**
 278  * Splice the sequence ep1..epN into the ring before the first element
 279  *   (..hp.. becomes ..hp..ep1..epN..)
 280  * @param hp   Head of the ring
 281  * @param ep1  First element in the sequence to splice in
 282  * @param epN  Last element in the sequence to splice in
 283  * @param elem The name of the element struct
 284  * @param link The name of the APR_RING_ENTRY in the element struct
 285  */
 286 #define APR_RING_SPLICE_HEAD(hp, ep1, epN, elem, link)                  \
 287         APR_RING_SPLICE_AFTER(APR_RING_SENTINEL((hp), elem, link),      \
 288                              (ep1), (epN), link)
 289
 290 /**
 291  * Splice the sequence ep1..epN into the ring after the last element
 292  *   (..hp.. becomes ..ep1..epN..hp..)
 293  * @param hp   Head of the ring
 294  * @param ep1  First element in the sequence to splice in
 295  * @param epN  Last element in the sequence to splice in
 296  * @param elem The name of the element struct
 297  * @param link The name of the APR_RING_ENTRY in the element struct
 298  */
 299 #define APR_RING_SPLICE_TAIL(hp, ep1, epN, elem, link)                  \
 300         APR_RING_SPLICE_BEFORE(APR_RING_SENTINEL((hp), elem, link),     \
 301                              (ep1), (epN), link)
 302
 303 /**
 304  * Insert the element nep into the ring before the first element
 305  *   (..hp.. becomes ..hp..nep..)
 306  * @param hp   Head of the ring
 307  * @param nep  Element to insert
 308  * @param elem The name of the element struct
 309  * @param link The name of the APR_RING_ENTRY in the element struct
 310  */
 311 #define APR_RING_INSERT_HEAD(hp, nep, elem, link)                       \
 312         APR_RING_SPLICE_HEAD((hp), (nep), (nep), elem, link)
 313
 314 /**
 315  * Insert the element nep into the ring after the last element
 316  *   (..hp.. becomes ..nep..hp..)
 317  * @param hp   Head of the ring
 318  * @param nep  Element to insert
 319  * @param elem The name of the element struct
 320  * @param link The name of the APR_RING_ENTRY in the element struct
 321  */
 322 #define APR_RING_INSERT_TAIL(hp, nep, elem, link)                       \
 323         APR_RING_SPLICE_TAIL((hp), (nep), (nep), elem, link)
 324
 325 /**
 326  * Concatenate ring h2 onto the end of ring h1, leaving h2 empty.
 327  * @param h1   Head of the ring to concatenate onto
 328  * @param h2   Head of the ring to concatenate
 329  * @param elem The name of the element struct
 330  * @param link The name of the APR_RING_ENTRY in the element struct
 331  */
 332 #define APR_RING_CONCAT(h1, h2, elem, link) do {                        \
 333         if (!APR_RING_EMPTY((h2), elem, link)) {                        \
 334             APR_RING_SPLICE_BEFORE(APR_RING_SENTINEL((h1), elem, link), \
 335                                   APR_RING_FIRST((h2)),                 \
 336                                   APR_RING_LAST((h2)), link);           \
 337             APR_RING_INIT((h2), elem, link);                            \
 338         }                                                               \
 339     } while (0)
 340
 341 /**
 342  * Prepend ring h2 onto the beginning of ring h1, leaving h2 empty.
 343  * @param h1   Head of the ring to prepend onto
 344  * @param h2   Head of the ring to prepend
 345  * @param elem The name of the element struct
 346  * @param link The name of the APR_RING_ENTRY in the element struct
 347  */
 348 #define APR_RING_PREPEND(h1, h2, elem, link) do {                       \
 349         if (!APR_RING_EMPTY((h2), elem, link)) {                        \
 350             APR_RING_SPLICE_AFTER(APR_RING_SENTINEL((h1), elem, link),  \
 351                                   APR_RING_FIRST((h2)),                 \
 352                                   APR_RING_LAST((h2)), link);           \
 353             APR_RING_INIT((h2), elem, link);                            \
 354         }                                                               \
 355     } while (0)
 356
 357 /**
 358  * Unsplice a sequence of elements from a ring
 359  * @warning The unspliced sequence is left with dangling pointers at either end
 360  * @param ep1  First element in the sequence to unsplice
 361  * @param epN  Last element in the sequence to unsplice
 362  * @param link The name of the APR_RING_ENTRY in the element struct
 363  */
 364 #define APR_RING_UNSPLICE(ep1, epN, link) do {                          \
 365         APR_RING_NEXT(APR_RING_PREV((ep1), link), link) =               \
 366                      APR_RING_NEXT((epN), link);                        \
 367         APR_RING_PREV(APR_RING_NEXT((epN), link), link) =               \
 368                      APR_RING_PREV((ep1), link);                        \
 369     } while (0)
 370
 371 /**
 372  * Remove a single element from a ring
 373  * @warning The unspliced element is left with dangling pointers at either end
 374  * @param ep   Element to remove
 375  * @param link The name of the APR_RING_ENTRY in the element struct
 376  */
 377 #define APR_RING_REMOVE(ep, link)                                       \
 378     APR_RING_UNSPLICE((ep), (ep), link)
 379
 380
 381 /**
 382  * Iterate through a ring
 383  * @param ep The current element
 384  * @param hp The ring to iterate over
 385  * @param elem The name of the element struct
 386  * @param link The name of the APR_RING_ENTRY in the element struct
 387  * @remark This is the same as either:
 388  * <pre>
 389  *      ep = APR_RING_FIRST(hp);
 390  *      while (ep != APR_RING_SENTINEL(hp, elem, link)) {
 391  *          ...
 392  *          ep = APR_RING_NEXT(ep, link);
 393  *      }
 394  *   OR
 395  *      for (ep = APR_RING_FIRST(hp);
 396  *           ep != APR_RING_SENTINEL(hp, elem, link);
 397  *           ep = APR_RING_NEXT(ep, link)) {
 398  *          ...
 399  *      }
 400  * </pre>
 401  * @warning Be aware that you cannot change the value of ep within
 402  * the foreach loop, nor can you destroy the ring element it points to.
 403  * Modifying the prev and next pointers of the element is dangerous
 404  * but can be done if you're careful.  If you change ep's value or
 405  * destroy the element it points to, then APR_RING_FOREACH
 406  * will have no way to find out what element to use for its next
 407  * iteration.  The reason for this can be seen by looking closely
 408  * at the equivalent loops given in the tip above.  So, for example,
 409  * if you are writing a loop that empties out a ring one element
 410  * at a time, APR_RING_FOREACH just won't work for you.  Do it
 411  * by hand, like so:
 412  * <pre>
 413  *      while (!APR_RING_EMPTY(hp, elem, link)) {
 414  *          ep = APR_RING_FIRST(hp);
 415  *          ...
 416  *          APR_RING_REMOVE(ep, link);
 417  *      }
 418  * </pre>
 419  * @deprecated This macro causes more headaches than it's worth.  Use
 420  * one of the alternatives documented here instead; the clarity gained
 421  * in what's really going on is well worth the extra line or two of code.
 422  * This macro will be removed at some point in the future.
 423  */
 424 #define APR_RING_FOREACH(ep, hp, elem, link)                            \
 425     for ((ep)  = APR_RING_FIRST((hp));                                  \
 426          (ep) != APR_RING_SENTINEL((hp), elem, link);                   \
 427          (ep)  = APR_RING_NEXT((ep), link))
 428
 429 /**
 430  * Iterate through a ring backwards
 431  * @param ep The current element
 432  * @param hp The ring to iterate over
 433  * @param elem The name of the element struct
 434  * @param link The name of the APR_RING_ENTRY in the element struct
 435  * @see APR_RING_FOREACH
 436  */
 437 #define APR_RING_FOREACH_REVERSE(ep, hp, elem, link)                    \
 438     for ((ep)  = APR_RING_LAST((hp));                                   \
 439          (ep) != APR_RING_SENTINEL((hp), elem, link);                   \
 440          (ep)  = APR_RING_PREV((ep), link))
 441
 442
 443 /* Debugging tools: */
 444
 445 #ifdef APR_RING_DEBUG
 446 #include <stdio.h>
 447 #include <assert.h>
 448
 449 #define APR_RING_CHECK_ONE(msg, ptr)                                    \
 450         fprintf(stderr, "*** %s %p\n", msg, ptr)
 451
 452 #define APR_RING_CHECK(hp, elem, link, msg)                             \
 453         APR_RING_CHECK_ELEM(APR_RING_SENTINEL(hp, elem, link), elem, link, msg)
 454
 455 #define APR_RING_CHECK_ELEM(ep, elem, link, msg) do {                   \
 456         struct elem *start = (ep);                                      \
 457         struct elem *here = start;                                      \
 458         fprintf(stderr, "*** ring check start -- %s\n", msg);           \
 459         do {                                                            \
 460             fprintf(stderr, "\telem %p\n", here);                       \
 461             fprintf(stderr, "\telem->next %p\n",                        \
 462                     APR_RING_NEXT(here, link));                         \
 463             fprintf(stderr, "\telem->prev %p\n",                        \
 464                     APR_RING_PREV(here, link));                         \
 465             fprintf(stderr, "\telem->next->prev %p\n",                  \
 466                     APR_RING_PREV(APR_RING_NEXT(here, link), link));    \
 467             fprintf(stderr, "\telem->prev->next %p\n",                  \
 468                     APR_RING_NEXT(APR_RING_PREV(here, link), link));    \
 469             if (APR_RING_PREV(APR_RING_NEXT(here, link), link) != here) { \
 470                 fprintf(stderr, "\t*** elem->next->prev != elem\n");    \
 471                 break;                                                  \
 472             }                                                           \
 473             if (APR_RING_NEXT(APR_RING_PREV(here, link), link) != here) { \
 474                 fprintf(stderr, "\t*** elem->prev->next != elem\n");    \
 475                 break;                                                  \
 476             }                                                           \
 477             here = APR_RING_NEXT(here, link);                           \
 478         } while (here != start);                                        \
 479         fprintf(stderr, "*** ring check end\n");                        \
 480     } while (0)
 481
 482 #define APR_RING_CHECK_CONSISTENCY(hp, elem, link)                      \
 483         APR_RING_CHECK_ELEM_CONSISTENCY(APR_RING_SENTINEL(hp, elem, link),\
 484                                         elem, link)
 485
 486 #define APR_RING_CHECK_ELEM_CONSISTENCY(ep, elem, link) do {            \
 487         struct elem *start = (ep);                                      \
 488         struct elem *here = start;                                      \
 489         do {                                                            \
 490             assert(APR_RING_PREV(APR_RING_NEXT(here, link), link) == here); \
 491             assert(APR_RING_NEXT(APR_RING_PREV(here, link), link) == here); \
 492             here = APR_RING_NEXT(here, link);                           \
 493         } while (here != start);                                        \
 494     } while (0)
 495
 496 #else
 497 /**
 498  * Print a single pointer value to STDERR
 499  *   (This is a no-op unless APR_RING_DEBUG is defined.)
 500  * @param msg Descriptive message
 501  * @param ptr Pointer value to print
 502  */
 503 #define APR_RING_CHECK_ONE(msg, ptr)
 504 /**
 505  * Dump all ring pointers to STDERR, starting with the head and looping all
 506  * the way around the ring back to the head.  Aborts if an inconsistency
 507  * is found.
 508  *   (This is a no-op unless APR_RING_DEBUG is defined.)
 509  * @param hp   Head of the ring
 510  * @param elem The name of the element struct
 511  * @param link The name of the APR_RING_ENTRY in the element struct
 512  * @param msg  Descriptive message
 513  */
 514 #define APR_RING_CHECK(hp, elem, link, msg)
 515 /**
 516  * Loops around a ring and checks all the pointers for consistency.  Pops
 517  * an assertion if any inconsistency is found.  Same idea as APR_RING_CHECK()
 518  * except that it's silent if all is well.
 519  *   (This is a no-op unless APR_RING_DEBUG is defined.)
 520  * @param hp   Head of the ring
 521  * @param elem The name of the element struct
 522  * @param link The name of the APR_RING_ENTRY in the element struct
 523  */
 524 #define APR_RING_CHECK_CONSISTENCY(hp, elem, link)
 525 /**
 526  * Dump all ring pointers to STDERR, starting with the given element and
 527  * looping all the way around the ring back to that element.  Aborts if
 528  * an inconsistency is found.
 529  *   (This is a no-op unless APR_RING_DEBUG is defined.)
 530  * @param ep   The element
 531  * @param elem The name of the element struct
 532  * @param link The name of the APR_RING_ENTRY in the element struct
 533  * @param msg  Descriptive message
 534  */
 535 #define APR_RING_CHECK_ELEM(ep, elem, link, msg)
 536 /**
 537  * Loops around a ring, starting with the given element, and checks all
 538  * the pointers for consistency.  Pops an assertion if any inconsistency
 539  * is found.  Same idea as APR_RING_CHECK_ELEM() except that it's silent
 540  * if all is well.
 541  *   (This is a no-op unless APR_RING_DEBUG is defined.)
 542  * @param ep   The element
 543  * @param elem The name of the element struct
 544  * @param link The name of the APR_RING_ENTRY in the element struct
 545  */
 546 #define APR_RING_CHECK_ELEM_CONSISTENCY(ep, elem, link)
 547 #endif
 548
 549 /** @} */
 550
 551 #endif /* !APR_RING_H */