我个人的理解：其实质，和Java里的Hash表有点类似。在C语言中是为了解决数组无法扩展的缺陷。
例子：
看 PostgreSQL对 VFD的处理：
初始化：
/* * Virtual File Descriptor array pointer and size. This grows as * needed. 'File' values are indexes into this array. * Note that VfdCache[0] is not a usable VFD, just a list header. */ static Vfd *VfdCache; static Size SizeVfdCache = 0;
/* * InitFileAccess --- initialize this module during backend startup * * This is called during either normal or standalone backend start. * It is *not* called in the postmaster. */ void InitFileAccess(void) { Assert(SizeVfdCache == 0); /* call me only once */ /* initialize cache header entry */ ///#### VfdCache = (Vfd *) malloc(sizeof(Vfd)); if (VfdCache == NULL) ereport(FATAL, (errcode(ERRCODE_OUT_OF_MEMORY), errmsg("out of memory"))); MemSet((char *) &(VfdCache[0]), 0, sizeof(Vfd)); VfdCache->fd = VFD_CLOSED; SizeVfdCache = 1; /* register proc-exit hook to ensure temp files are dropped at exit */ on_proc_exit(AtProcExit_Files, 0); }
扩展：
static File AllocateVfd(void) { Index i; File file; DO_DB(elog(LOG, "AllocateVfd. Size %lu", SizeVfdCache)); Assert(SizeVfdCache > 0); /* InitFileAccess not called? */ if (VfdCache[0].nextFree == 0) { /* * The free list is empty so it is time to increase the size of the * array. We choose to double it each time this happens. However, * there's not much point in starting *real* small. */ Size newCacheSize = SizeVfdCache * 2; Vfd *newVfdCache; if (newCacheSize < 32) newCacheSize = 32; ////#### /* * Be careful not to clobber VfdCache ptr if realloc fails. */ newVfdCache = (Vfd *) realloc(VfdCache, sizeof(Vfd) * newCacheSize); if (newVfdCache == NULL) ereport(ERROR, (errcode(ERRCODE_OUT_OF_MEMORY), errmsg("out of memory"))); VfdCache = newVfdCache; /* * Initialize the new entries and link them into the free list. */ for (i = SizeVfdCache; i < newCacheSize; i++) { MemSet((char *) &(VfdCache[i]), 0, sizeof(Vfd)); VfdCache[i].nextFree = i + 1; VfdCache[i].fd = VFD_CLOSED; } VfdCache[newCacheSize - 1].nextFree = 0; VfdCache[0].nextFree = SizeVfdCache; /* * Record the new size */ SizeVfdCache = newCacheSize; } file = VfdCache[0].nextFree; VfdCache[0].nextFree = VfdCache[file].nextFree; return file; }
