-1

我正在使用 pcap 库捕获 IEEE802.11 数据包。到目前为止，我使用 pcap_loop 和一个回调函数来接收然后处理数据包。但现在我已经切换了设备定期监听的 wifi 频道。不幸的是， pcap_loop 被阻塞了，所以我不能使用超时来调用我的函数。然后我读到了 pcap_dispatch，但我真的不知道它应该如何异步工作，因为做类似的事情

while(1) {
    int cnt = pcap_dispatch(handle, -1, callback, null);
}

不会解决任何事情。

那么任何人都可以向我解释如何使 pcap 捕获数据包异步/基于事件，或者换句话说如何解决我的困境？

4

1 回答 1

0

引用pcap_get_selectable_fd()手册页（这是来自主干的版本）：

DESCRIPTION

   pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for
   a  file descriptor on which one can do a select() or poll() to wait for
   it to be possible to read packets without blocking, if such a  descrip‐
   tor  exists, or −1, if no such descriptor exists.  Some network devices
   opened   with    pcap_create()    and    pcap_activate(),    or    with
   pcap_open_live(), do not support select() or poll() (for example, regu‐
   lar network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so
   −1 is returned for those devices.

   Note that in:

          FreeBSD prior to FreeBSD 4.6;

          NetBSD prior to NetBSD 3.0;

          OpenBSD prior to OpenBSD 2.4;

          Mac OS X prior to Mac OS X 10.7;

   select()   and   poll()   do   not   work  correctly  on  BPF  devices;
   pcap_get_selectable_fd() will return a file descriptor on most of those
   versions  (the  exceptions  being  FreeBSD  4.3  and 4.4), but a simple
   select() or poll() will not indicate that the  descriptor  is  readable
   until  a  full  buffer’s worth of packets is received, even if the read
   timeout expires before then.  To work around this, an application  that
   uses  select()  or  poll()  to  wait for packets to arrive must put the
   pcap_t in non‐blocking mode, and must  arrange  that  the  select()  or
   poll()  have a timeout less than or equal to the read timeout, and must
   try to read packets after that timeout expires, regardless  of  whether
   select() or poll() indicated that the file descriptor for the pcap_t is
   ready to be read or not.  (That workaround will not work in FreeBSD 4.3
   and  later; however, in FreeBSD 4.6 and later, select() and poll() work
   correctly on BPF devices, so the workaround isn’t  necessary,  although
   it does no harm.)

   Note  also that poll() doesn’t work on character special files, includ‐
   ing BPF devices, in Mac OS X 10.4 and 10.5, so, while select()  can  be
   used  on  the  descriptor  returned by pcap_get_selectable_fd(), poll()
   cannot be used on it those versions of Mac OS X.   Kqueues  also  don’t
   work  on  that  descriptor.   poll(),  but  not  kqueues,  work on that
   descriptor in Mac OS X releases prior to 10.4; poll() and kqueues  work
   on that descriptor in Mac OS X 10.6 and later.

   pcap_get_selectable_fd() is not available on Windows.

此外，由于 10.6 的 BPF 中的一个错误（作为修复另一个错误的一部分而引入），该解决方法在 Mac OS X 10.6 上不起作用；解决方法将适用于 10.7 及更高版本，但不是必需的。

select()因此，如果您有一个基于/// kqueue poll()/ etc. 的事件循环，则可以使其基于epoll()事件 - 将您从中获得的描述符添加pcap_get_selectable_fd()为在该事件循环中选择的文件描述符之一。

于 2013-01-15T10:01:11.977 回答