while (0) 是什么东东？

大家有见过 while (0) 的用法吗？是不是很奇怪，如果没见过，来看一下：

/* Simple way to give quicklistEntry structs default values with one call. */
#define initEntry(e)                                                           \do {                                                                       \(e)->zi = (e)->value = NULL;                                           \(e)->longval = -123456789;                                             \(e)->quicklist = NULL;                                                 \(e)->node = NULL;                                                      \(e)->offset = 123456789;                                               \(e)->sz = 0;                                                           \} while (0)/* Create a new quicklist.* Free with quicklistRelease(). */
quicklist *quicklistCreate(void) {struct quicklist *quicklist;quicklist = zmalloc(sizeof(*quicklist));quicklist->head = quicklist->tail = NULL;quicklist->len = 0;quicklist->count = 0;quicklist->compress = 0;quicklist->fill = -2;quicklist->bookmark_count = 0;return quicklist;
}#define COMPRESS_MAX ((1 << QL_COMP_BITS)-1)
void quicklistSetCompressDepth(quicklist *quicklist, int compress) {if (compress > COMPRESS_MAX) {compress = COMPRESS_MAX;} else if (compress < 0) {compress = 0;}quicklist->compress = compress;
}#define FILL_MAX ((1 << (QL_FILL_BITS-1))-1)
void quicklistSetFill(quicklist *quicklist, int fill) {if (fill > FILL_MAX) {fill = FILL_MAX;} else if (fill < -5) {fill = -5;}quicklist->fill = fill;
}void quicklistSetOptions(quicklist *quicklist, int fill, int depth) {quicklistSetFill(quicklist, fill);quicklistSetCompressDepth(quicklist, depth);
}/* Create a new quicklist with some default parameters. */
quicklist *quicklistNew(int fill, int compress) {quicklist *quicklist = quicklistCreate();quicklistSetOptions(quicklist, fill, compress);return quicklist;
}REDIS_STATIC quicklistNode *quicklistCreateNode(void) {quicklistNode *node;node = zmalloc(sizeof(*node));node->zl = NULL;node->count = 0;node->sz = 0;node->next = node->prev = NULL;node->encoding = QUICKLIST_NODE_ENCODING_RAW;node->container = QUICKLIST_NODE_CONTAINER_ZIPLIST;node->recompress = 0;return node;
}/* Return cached quicklist count */
unsigned long quicklistCount(const quicklist *ql) { return ql->count; }/* Free entire quicklist. */
void quicklistRelease(quicklist *quicklist) {unsigned long len;quicklistNode *current, *next;current = quicklist->head;len = quicklist->len;while (len--) {next = current->next;zfree(current->zl);quicklist->count -= current->count;zfree(current);quicklist->len--;current = next;}quicklistBookmarksClear(quicklist);zfree(quicklist);
}/* Compress the ziplist in 'node' and update encoding details.* Returns 1 if ziplist compressed successfully.* Returns 0 if compression failed or if ziplist too small to compress. */
REDIS_STATIC int __quicklistCompressNode(quicklistNode *node) {
#ifdef REDIS_TESTnode->attempted_compress = 1;
#endif/* Don't bother compressing small values */if (node->sz < MIN_COMPRESS_BYTES)return 0;quicklistLZF *lzf = zmalloc(sizeof(*lzf) + node->sz);/* Cancel if compression fails or doesn't compress small enough */if (((lzf->sz = lzf_compress(node->zl, node->sz, lzf->compressed,node->sz)) == 0) ||lzf->sz + MIN_COMPRESS_IMPROVE >= node->sz) {/* lzf_compress aborts/rejects compression if value not compressable. */zfree(lzf);return 0;}lzf = zrealloc(lzf, sizeof(*lzf) + lzf->sz);zfree(node->zl);node->zl = (unsigned char *)lzf;node->encoding = QUICKLIST_NODE_ENCODING_LZF;node->recompress = 0;return 1;
}/* Compress only uncompressed nodes. */
#define quicklistCompressNode(_node)                                           \do {                                                                       \if ((_node) && (_node)->encoding == QUICKLIST_NODE_ENCODING_RAW) {     \__quicklistCompressNode((_node));                                  \}                                                                      \} while (0)/* Uncompress the ziplist in 'node' and update encoding details.* Returns 1 on successful decode, 0 on failure to decode. */
REDIS_STATIC int __quicklistDecompressNode(quicklistNode *node) {
#ifdef REDIS_TESTnode->attempted_compress = 0;
#endifvoid *decompressed = zmalloc(node->sz);quicklistLZF *lzf = (quicklistLZF *)node->zl;if (lzf_decompress(lzf->compressed, lzf->sz, decompressed, node->sz) == 0) {/* Someone requested decompress, but we can't decompress.  Not good. */zfree(decompressed);return 0;}zfree(lzf);node->zl = decompressed;node->encoding = QUICKLIST_NODE_ENCODING_RAW;return 1;
}/* Decompress only compressed nodes. */
#define quicklistDecompressNode(_node)                                         \do {                                                                       \if ((_node) && (_node)->encoding == QUICKLIST_NODE_ENCODING_LZF) {     \__quicklistDecompressNode((_node));                                \}                                                                      \} while (0)/* Force node to not be immediately re-compresable */
#define quicklistDecompressNodeForUse(_node)                                   \do {                                                                       \if ((_node) && (_node)->encoding == QUICKLIST_NODE_ENCODING_LZF) {     \__quicklistDecompressNode((_node));                                \(_node)->recompress = 1;                                           \}                                                                      \} while (0)/* Extract the raw LZF data from this quicklistNode.* Pointer to LZF data is assigned to '*data'.* Return value is the length of compressed LZF data. */
size_t quicklistGetLzf(const quicklistNode *node, void **data) {quicklistLZF *lzf = (quicklistLZF *)node->zl;*data = lzf->compressed;return lzf->sz;
}#define quicklistAllowsCompression(_ql) ((_ql)->compress != 0)/* Force 'quicklist' to meet compression guidelines set by compress depth.* The only way to guarantee interior nodes get compressed is to iterate* to our "interior" compress depth then compress the next node we find.* If compress depth is larger than the entire list, we return immediately. */
REDIS_STATIC void __quicklistCompress(const quicklist *quicklist,quicklistNode *node) {/* If length is less than our compress depth (from both sides),* we can't compress anything. */if (!quicklistAllowsCompression(quicklist) ||quicklist->len < (unsigned int)(quicklist->compress * 2))return;#if 0/* Optimized cases for small depth counts */if (quicklist->compress == 1) {quicklistNode *h = quicklist->head, *t = quicklist->tail;quicklistDecompressNode(h);quicklistDecompressNode(t);if (h != node && t != node)quicklistCompressNode(node);return;} else if (quicklist->compress == 2) {quicklistNode *h = quicklist->head, *hn = h->next, *hnn = hn->next;quicklistNode *t = quicklist->tail, *tp = t->prev, *tpp = tp->prev;quicklistDecompressNode(h);quicklistDecompressNode(hn);quicklistDecompressNode(t);quicklistDecompressNode(tp);if (h != node && hn != node && t != node && tp != node) {quicklistCompressNode(node);}if (hnn != t) {quicklistCompressNode(hnn);}if (tpp != h) {quicklistCompressNode(tpp);}return;}
#endif/* Iterate until we reach compress depth for both sides of the list.a* Note: because we do length checks at the *top* of this function,*       we can skip explicit null checks below. Everything exists. */quicklistNode *forward = quicklist->head;quicklistNode *reverse = quicklist->tail;int depth = 0;int in_depth = 0;while (depth++ < quicklist->compress) {quicklistDecompressNode(forward);quicklistDecompressNode(reverse);if (forward == node || reverse == node)in_depth = 1;/* We passed into compress depth of opposite side of the quicklist* so there's no need to compress anything and we can exit. */if (forward == reverse || forward->next == reverse)return;forward = forward->next;reverse = reverse->prev;}if (!in_depth)quicklistCompressNode(node);/* At this point, forward and reverse are one node beyond depth */quicklistCompressNode(forward);quicklistCompressNode(reverse);
}#define quicklistCompress(_ql, _node)                                          \do {                                                                       \if ((_node)->recompress)                                               \quicklistCompressNode((_node));                                    \else                                                                   \__quicklistCompress((_ql), (_node));                               \} while (0)/* If we previously used quicklistDecompressNodeForUse(), just recompress. */
#define quicklistRecompressOnly(_ql, _node)                                    \do {                                                                       \if ((_node)->recompress)                                               \quicklistCompressNode((_node));                                    \} while (0)/* Insert 'new_node' after 'old_node' if 'after' is 1.* Insert 'new_node' before 'old_node' if 'after' is 0.* Note: 'new_node' is *always* uncompressed, so if we assign it to*       head or tail, we do not need to uncompress it. */
REDIS_STATIC void __quicklistInsertNode(quicklist *quicklist,quicklistNode *old_node,quicklistNode *new_node, int after) {if (after) {new_node->prev = old_node;if (old_node) {new_node->next = old_node->next;if (old_node->next)old_node->next->prev = new_node;old_node->next = new_node;}if (quicklist->tail == old_node)quicklist->tail = new_node;} else {new_node->next = old_node;if (old_node) {new_node->prev = old_node->prev;if (old_node->prev)old_node->prev->next = new_node;old_node->prev = new_node;}if (quicklist->head == old_node)quicklist->head = new_node;}/* If this insert creates the only element so far, initialize head/tail. */if (quicklist->len == 0) {quicklist->head = quicklist->tail = new_node;}/* Update len first, so in __quicklistCompress we know exactly len */quicklist->len++;if (old_node)quicklistCompress(quicklist, old_node);
}

常常在宏定义中用到，可以体会一下。面试也经常考这个小知识点。

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