pcap_breakloop() sets a flag that will force
pcap_loop() to return rather than looping; they will return the number of packets
that have been processed so far, or -2 if no packets have been
processed so far.
This routine is safe to use inside a signal handler on UNIX or a console
control handler on Windows, as it merely sets a flag that is checked
within the loop.
The flag is checked in loops reading packets from the OS - a signal by
itself will not necessarily terminate those loops - as well as in loops
processing a set of packets returned by the OS.
Note that if you are catching signals on UNIX systems that support
restarting system calls after a signal, and calling pcap_breakloop()
in the signal handler, you must specify, when catching those signals,
that system calls should NOT be restarted by that signal. Otherwise,
if the signal interrupted a call reading packets in a live capture,
when your signal handler returns after calling pcap_breakloop(), the
call will be restarted, and the loop will not terminate until more
packets arrive and the call completes.
Note also that, in a multi-threaded application, if one thread is
blocked in pcap_dispatch(), pcap_loop(), pcap_next(), or pcap_next_ex(),
a call to pcap_breakloop() in a different thread will not unblock that
thread; you will need to use whatever mechanism the OS provides for
breaking a thread out of blocking calls in order to unblock the thread,
such as thread cancellation in systems that support POSIX threads.
pcap_next_ex() will, on some platforms, loop reading packets from the OS; that loop
will not necessarily be terminated by a signal, so
pcap_breakloop() should be used to terminate packet processing even if
pcap_next_ex() is being used.
pcap_breakloop() does not guarantee that no further packets will be processed by
pcap_loop() after it is called; at most one more packet might be processed.
If -2 is returned from
pcap_loop(), the flag is cleared, so a subsequent call will resume reading packets.
If a positive number is returned, the flag is not cleared, so a
subsequent call will return -2 and clear the flag.