网络编程之你真了解select模型？？_c++ select 长度限制-优快云博客

本文探讨了Select模型在网络编程中的局限性及其解决方案。通过分析libevent源码，介绍了一种利用动态数组来突破传统fd_set限制的方法，实现了高效处理大量套接字。

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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("]\n");
}
*/
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);
}