ACE_Stream中提供recv_n函数，该函数要求输入参数len表示要接受的字节数。

当代码中调用该函数时，程序会一直阻塞在recv_n函数调用的地方，只到recv_n对应的描叙符接收到了len个字节或者连接断开。

recv_n代码：

ACE::recv_n_i (ACE_HANDLE handle,
void *buf,
size_t len,
int flags,
size_t *bt)
{
size_t temp;
size_t &bytes_transferred = bt == 0 temp : *bt;
ssize_t n;

for (bytes_transferred = 0;
bytes_transferred < len;
bytes_transferred += n)
{
// Try to transfer as much of the remaining data as possible.
n = ACE_OS::recv (handle,
static_cast (buf) + bytes_transferred,
len - bytes_transferred,
flags);
// Check EOF.
if (n == 0)
return 0;

// Check for other errors.
if (n == -1)
{
// Check for possible blocking.
if (errno == EWOULDBLOCK)
{
// Wait for the blocking to subside.
int const result = ACE::handle_read_ready (handle, 0);

// Did select() succeed
if (result != -1)
{
// Blocking subsided. Continue data transfer.
n = 0;
continue;
}
}

// Other data transfer or select() failures.
return -1;
}
}

return static_cast (bytes_transferred);
}