C语言程序中的内存结构数组

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;

}