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The flows

Flows are captured by stringing together filters as explained in Section 2.2. The packets received in a flow can be read by the application in different ways. The simplest way is to read continuously from the buffer whenever a packet is available, e.g., using the filter_getnext_pkt() operation. Doing so, however, keeps the application polling constantly. From a CPU usage and context switching point of view, packets may be read more efficiently by blocking, e.g., until a certain number of packets has been received. FFPF offers two flavours of blocking: (a) wait_for_n_pkts(n), a blocking call that only returns after $n$ packets are received, and (b) installing a filter_callback() which is non-blocking itself and results in a callback of a registered callback function whenever $n$ packets are received. At callback registration time, users specify how long the callback should remain active. Of course, even with filter_getnext_pkt() an application may block explicitly, e.g. by calling sleep(10) to process every 10 seconds all packets that were received in that period.



Herbert Bos 2004-10-06