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num_probes_active == 0 in HostScanStats::completed. The reason for this is fairly subtle and I didn't realize it at first: We have to make sure there are no active probes because once in the completed list, probes don't time out. Probes that are active stay active in the count. If the congestion window ever falls below the number of these active probes, the program will hang waiting for them to time out. We could get away with this in the case of up hosts, because we call HostScanStats::destroyAllOutstandingProbes in that case. We could do that in the down case too, but that would prohibit a down host from being found up later on. That's currently a matter of some luck; we don't keep sending probes after a host is down but will accept replies to any other probes that have already been sent.
Here is some documentation for Nmap, but these files are much less comprehensive than what you'll find at the actual Nmap documentation site ( http://nmap.org ).