redis_3.0.7_sds.c_sdsMakeRoomFor()

sds sdsMakeRoomFor(sds s,size_t addlen)
{
   struct sdshdr *sh,*newsh;
    sizt_t free = sdsavail(s);//sdsavail()是计算free的大小的，即空白部分的大小
    size_t len,newlen;
   if(free >= addlen) return s;
    len = sdslen(s);
    sh = (void*)(s-(sizeof(struct sdshdr)));
    newlen = (len+addlen);
    if(newlen < SDS_MAX_PREALLOC)
       newlen *= 2;
    else
        newlen += SDS_MAX_PREALLOC;
    newsh = realloc(sh,sizeof(struct sdshdr)+newlen+1);
//这里源程序是调用的zrealloc()函数，我改成了realloc()函数，只是为了让程序能跑起来。
  //zrealloc()函数的优越之处现在并不明白
    if(newsh == NULL) return NULL;
    newsh->free = newlen - len;
    return newsh->buf;
}