【转】你真的懂select Socket模型吗？

转自：http://www.cppblog.com/xvsdf100/archive/2013/12/10/204689.html

 

 

只要接触过c/c++网路编程人都可能会知道select io 模式，网络书籍都说 fd_set {int num; SOCKET arr[64]} 有所限制，因为数组的长度只有64，那么超过64你就不能放，要么你就是用多线程分别实用select.。

一些书籍通过改定义宏 使数组的长度变长，但也不实用，不能动态的变化，我总不能定一个非常的长的长度，毕竟是在栈上。

   我就在想那么select完全只能在客户端使用，而且套接字还不能超过64。那这不就是一个软肋吗？？一直对这个有一个迷惑。。。。。

 

   后来，自己看到了libevent的源代码发现他也用的是select。。看别人说这个库，轻轻松松处理上万个套接字，我就在想select不是有限制吗？？他是怎么做到。。。。。。

看了源代码，我明白了。他只是用对上存放SOCKET的句柄。。我们看一下他的新定义结构体。

 

struct win_fd_set {

u_int fd_count;

SOCKET fd_array[1];

};

 

这个就是新定义结构体，跟原来稍微有点变化只是把64改为1，有些同学可能见多很多这样的写法，这种写法我也在一些项目使用了。这种写法可以fd_array动态变化。

win_fd_set * Set = (win_fd_set*)malloc(sizeof(win_fd_set) + sizoef(SCOEKT) * 10);

Set->fd_array 可以放11 个 SOCKET，因为我的内存大小足够放11个SOCKET。

请记住内存是没有数据格式，只要足够大小，随便你怎么放。数据格式只是方便我们管理和处理数据而已。

这样就解决64个大小限制。。我其实一直很好奇为什么中国书籍都是一样的，libevent已经出来好久了，但也没有看到有人说这一点，可能是高手们都不屑。

现在渐渐喜欢看开源的代码，不喜欢看书籍了，喜欢在代码中学习他们是怎么组织一个好项目。

有时候感慨：高手用c 写着漂亮的c++代码，而我等菜鸟却用c++ 写丑陋的c代码。。

 

我顺便把他整个定义结构和函数给大家贴出来，免大家还要自己去下libevent，不过推荐没有看过libevent同学，可以稍微看一下。

 volatile double SIGFPE_REQ = 0.0f;
 
 struct idx_info {
     int read_pos_plus1;
     int write_pos_plus1;
 };
 
 struct win32op {
     unsigned num_fds_in_fd_sets;
     int resize_out_sets;
     struct win_fd_set *readset_in;
     struct win_fd_set *writeset_in;
     struct win_fd_set *readset_out;
     struct win_fd_set *writeset_out;
     struct win_fd_set *exset_out;
     unsigned signals_are_broken : 1;
 };
 
 static void *win32_init(struct event_base *);
 static int win32_add(struct event_base *, evutil_socket_t, short old, short events, void *_idx);
 static int win32_del(struct event_base *, evutil_socket_t, short old, short events, void *_idx);
 static int win32_dispatch(struct event_base *base, struct timeval *);
 static void win32_dealloc(struct event_base *);
 
 struct eventop win32ops = {
     "win32",
     win32_init,
     win32_add,
     win32_del,
     win32_dispatch,
     win32_dealloc,
     0, /* doesn't need reinit */
     0, /* No features supported. */
     sizeof(struct idx_info),
 };
 
 #define FD_SET_ALLOC_SIZE(n) ((sizeof(struct win_fd_set) + ((n)-1)*sizeof(SOCKET)))
 
 static int
 grow_fd_sets(struct win32op *op, unsigned new_num_fds)
 {
     size_t size;
 
     EVUTIL_ASSERT(new_num_fds >= op->readset_in->fd_count &&
            new_num_fds >= op->writeset_in->fd_count);
     EVUTIL_ASSERT(new_num_fds >= 1);
 
     size = FD_SET_ALLOC_SIZE(new_num_fds);
     if (!(op->readset_in = mm_realloc(op->readset_in, size)))
         return (-1);
     if (!(op->writeset_in = mm_realloc(op->writeset_in, size)))
         return (-1);
     op->resize_out_sets = 1;
     op->num_fds_in_fd_sets = new_num_fds;
     return (0);
 }
 
 static int
 do_fd_set(struct win32op *op, struct idx_info *ent, evutil_socket_t s, int read)
 {
     struct win_fd_set *set = read ? op->readset_in : op->writeset_in;
     if (read) {
         if (ent->read_pos_plus1 > 0)
             return (0);
     } else {
         if (ent->write_pos_plus1 > 0)
             return (0);
     }
     if (set->fd_count == op->num_fds_in_fd_sets) {
         if (grow_fd_sets(op, op->num_fds_in_fd_sets*2))
             return (-1);
         /* set pointer will have changed and needs reiniting! */
         set = read ? op->readset_in : op->writeset_in;
     }
     set->fd_array[set->fd_count] = s;
     if (read)
         ent->read_pos_plus1 = set->fd_count+1;
     else
         ent->write_pos_plus1 = set->fd_count+1;
     return (set->fd_count++);
 }
 
 static int
 do_fd_clear(struct event_base *base,
             struct win32op *op, struct idx_info *ent, int read)
 {
     int i;
     struct win_fd_set *set = read ? op->readset_in : op->writeset_in;
     if (read) {
         i = ent->read_pos_plus1 - 1;
         ent->read_pos_plus1 = 0;
     } else {
         i = ent->write_pos_plus1 - 1;
         ent->write_pos_plus1 = 0;
     }
     if (i < 0)
         return (0);
     if (--set->fd_count != (unsigned)i) {
         struct idx_info *ent2;
         SOCKET s2;
         s2 = set->fd_array[i] = set->fd_array[set->fd_count];
 
         ent2 = evmap_io_get_fdinfo(&base->io, s2);
 
         if (!ent2) /* This indicates a bug. */
             return (0);
         if (read)
             ent2->read_pos_plus1 = i+1;
         else
             ent2->write_pos_plus1 = i+1;
     }
     return (0);
 }
 
 #define NEVENT 32
 void *
 win32_init(struct event_base *_base)
 {
     struct win32op *winop;
     size_t size;
     if (!(winop = mm_calloc(1, sizeof(struct win32op))))
         return NULL;
     winop->num_fds_in_fd_sets = NEVENT;
     size = FD_SET_ALLOC_SIZE(NEVENT);
     if (!(winop->readset_in = mm_malloc(size)))
         goto err;
     if (!(winop->writeset_in = mm_malloc(size)))
         goto err;
     if (!(winop->readset_out = mm_malloc(size)))
         goto err;
     if (!(winop->writeset_out = mm_malloc(size)))
         goto err;
     if (!(winop->exset_out = mm_malloc(size)))
         goto err;
     winop->readset_in->fd_count = winop->writeset_in->fd_count = 0;
     winop->readset_out->fd_count = winop->writeset_out->fd_count
         = winop->exset_out->fd_count = 0;
 
     if (evsig_init(_base) < 0)
         winop->signals_are_broken = 1;
 
     return (winop);
  err:
     XFREE(winop->readset_in);
     XFREE(winop->writeset_in);
     XFREE(winop->readset_out);
     XFREE(winop->writeset_out);
     XFREE(winop->exset_out);
     XFREE(winop);
     return (NULL);
 }
 
 int
 win32_add(struct event_base *base, evutil_socket_t fd,
              short old, short events, void *_idx)
 {
     struct win32op *win32op = base->evbase;
     struct idx_info *idx = _idx;
 
     if ((events & EV_SIGNAL) && win32op->signals_are_broken)
         return (-1);
 
     if (!(events & (EV_READ|EV_WRITE)))
         return (0);
 
     event_debug(("%s: adding event for %d", __func__, (int)fd));
     if (events & EV_READ) {
         if (do_fd_set(win32op, idx, fd, 1)<0)
             return (-1);
     }
     if (events & EV_WRITE) {
         if (do_fd_set(win32op, idx, fd, 0)<0)
             return (-1);
     }
     return (0);
 }
 
 int
 win32_del(struct event_base *base, evutil_socket_t fd, short old, short events,
           void *_idx)
 {
     struct win32op *win32op = base->evbase;
     struct idx_info *idx = _idx;
 
     event_debug(("%s: Removing event for "EV_SOCK_FMT,
         __func__, EV_SOCK_ARG(fd)));
     if (events & EV_READ)
         do_fd_clear(base, win32op, idx, 1);
     if (events & EV_WRITE)
         do_fd_clear(base, win32op, idx, 0);
 
     return 0;
 }
 
 static void
 fd_set_copy(struct win_fd_set *out, const struct win_fd_set *in)
 {
     out->fd_count = in->fd_count;
     memcpy(out->fd_array, in->fd_array, in->fd_count * (sizeof(SOCKET)));
 }
 
 /*
   static void dump_fd_set(struct win_fd_set *s)
   {
   unsigned int i;
   printf("[ ");
   for(i=0;i<s->fd_count;++i)
   printf("%d ",(int)s->fd_array[i]);
   printf("]
");
   }
 */
 
 int
 win32_dispatch(struct event_base *base, struct timeval *tv)
 {
     struct win32op *win32op = base->evbase;
     int res = 0;
     unsigned j, i;
     int fd_count;
     SOCKET s;
 
     if (win32op->resize_out_sets) {
         size_t size = FD_SET_ALLOC_SIZE(win32op->num_fds_in_fd_sets);
         if (!(win32op->readset_out = mm_realloc(win32op->readset_out, size)))
             return (-1);
         if (!(win32op->exset_out = mm_realloc(win32op->exset_out, size)))
             return (-1);
         if (!(win32op->writeset_out = mm_realloc(win32op->writeset_out, size)))
             return (-1);
         win32op->resize_out_sets = 0;
     }
 
     fd_set_copy(win32op->readset_out, win32op->readset_in);
     fd_set_copy(win32op->exset_out, win32op->writeset_in);
     fd_set_copy(win32op->writeset_out, win32op->writeset_in);
 
     fd_count =
         (win32op->readset_out->fd_count > win32op->writeset_out->fd_count) ?
         win32op->readset_out->fd_count : win32op->writeset_out->fd_count;
 
     if (!fd_count) {
         long msec = tv ? evutil_tv_to_msec(tv) : LONG_MAX;
         /* Sleep's DWORD argument is unsigned long */
         if (msec < 0)
             msec = LONG_MAX;
         /* Windows doesn't like you to call select() with no sockets */
         Sleep(msec);
         return (0);
     }
 
     EVBASE_RELEASE_LOCK(base, th_base_lock);
 
     res = select(fd_count,
              (struct fd_set*)win32op->readset_out,
              (struct fd_set*)win32op->writeset_out,
              (struct fd_set*)win32op->exset_out, tv);
 
     EVBASE_ACQUIRE_LOCK(base, th_base_lock);
 
     event_debug(("%s: select returned %d", __func__, res));
 
     if (res <= 0) {
         return res;
     }
 
     if (win32op->readset_out->fd_count) {
         i = rand() % win32op->readset_out->fd_count;
         for (j=0; j<win32op->readset_out->fd_count; ++j) {
             if (++i >= win32op->readset_out->fd_count)
                 i = 0;
             s = win32op->readset_out->fd_array[i];
             evmap_io_active(base, s, EV_READ);
         }
     }
     if (win32op->exset_out->fd_count) {
         i = rand() % win32op->exset_out->fd_count;
         for (j=0; j<win32op->exset_out->fd_count; ++j) {
             if (++i >= win32op->exset_out->fd_count)
                 i = 0;
             s = win32op->exset_out->fd_array[i];
             evmap_io_active(base, s, EV_WRITE);
         }
     }
     if (win32op->writeset_out->fd_count) {
         SOCKET s;
         i = rand() % win32op->writeset_out->fd_count;
         for (j=0; j<win32op->writeset_out->fd_count; ++j) {
             if (++i >= win32op->writeset_out->fd_count)
                 i = 0;
             s = win32op->writeset_out->fd_array[i];
             evmap_io_active(base, s, EV_WRITE);
         }
     }
     return (0);
 }
 
 void
 win32_dealloc(struct event_base *_base)
 {
     struct win32op *win32op = _base->evbase;
 
     evsig_dealloc(_base);
     if (win32op->readset_in)
         mm_free(win32op->readset_in);
     if (win32op->writeset_in)
         mm_free(win32op->writeset_in);
     if (win32op->readset_out)
         mm_free(win32op->readset_out);
     if (win32op->writeset_out)
         mm_free(win32op->writeset_out);
     if (win32op->exset_out)
         mm_free(win32op->exset_out);
     /* XXXXX free the tree. */
 
     memset(win32op, 0, sizeof(win32op));
     mm_free(win32op);
 }