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 intgrow_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 intdo_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;elseent->write_pos_plus1 = set->fd_count+1;return (set->fd_count++);}static intdo_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;elseent2->write_pos_plus1 = i+1;}return (0);}#define NEVENT 32void *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);}intwin32_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);}intwin32_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 voidfd_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("]\n");}*/intwin32_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);}voidwin32_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);}