Hi, I am reasonably new to Boost.Asio. Below is a short test program to catch CNTR C. It nearly works as i would have expected. #include <iostream> #include #define BOOST_ASIO_ENABLE_HANDLER_TRACKING #include namespace asio = boost::asio; #define GCC_VERSION (__GNUC__*10000 + __GNUC_MINOR__*100 + __GNUC_PATCHLEVEL__ ) struct Sig_handler { Sig_handler(int& count, asio::signal_set& signals) : count_(count), signals_(signals) {} void operator()(const boost::system::error_code& ec, int signal_number) { if (!ec) { count_++; std::cout << "signal: " << signal_number << std::endl; if (count_ < 3) signals_.async_wait( Sig_handler(count_, signals_)); } else { std::cout << "Sig_handler error" << std::endl; } } int& count_; asio::signal_set& signals_; }; int main() { std::cout << "test_asio_sig starting " << std::endl; std::cout << "GCC VERSION " << GCC_VERSION << std::endl; std::cout << "BOOST VERSION " << BOOST_VERSION << std::endl; asio::io_service io_service; asio::signal_set signals(io_service, SIGINT); int count = 0; std::cout << "count " << count << std::endl; signals.async_wait( Sig_handler(count, signals) ); size_t num_handlers = io_service.run(); // size_t num_handlers = io_service.run_one(); // <<== NOTE std::cout << "num handlers " << num_handlers << std::endl; std::cout << "count " << count << std::endl; std::cout << "test_asio_sig finishing " << std::endl; } Here is the output after pressing CNTR C 3 times [robert@localhost misc]$ ./test_asio_sig test_asio_sig starting GCC VERSION 40902 BOOST VERSION 105700 count 0 @asio|1425176527.462503|0*1|signal_set@0x7fffcd9139b8.async_wait ^C@asio|1425176530.453067|>1|ec=system:0,signal_number=2 signal: 2 @asio|1425176530.453092|1*2|signal_set@0x7fffcd9139b8.async_wait @asio|1425176530.453102|<1| ^C@asio|1425176533.749159|>2|ec=system:0,signal_number=2 signal: 2 @asio|1425176533.749187|2*3|signal_set@0x7fffcd9139b8.async_wait @asio|1425176533.749191|<2| ^C@asio|1425176548.261061|>3|ec=system:0,signal_number=2 signal: 2 @asio|1425176548.261092|<3| num handlers 6 count 3 test_asio_sig finishing @asio|1425176548.261116|0|signal_set@0x7fffcd9139b8.cancel [robert@localhost misc]$ I don't understand why io_service.run() says 6 handlers where invoked but otherwise it seems OK. Now if i replace io_service.run() with io_service.run_one() i expected the handler to be invoked once when CNTR C is pressed. Here is the output [robert@localhost misc]$ ./test_asio_sig test_asio_sig starting GCC VERSION 40902 BOOST VERSION 105700 count 0 @asio|1425176928.548934|0*1|signal_set@0x7fffe64b8398.async_wait ^Cnum handlers 1 count 0 test_asio_sig finishing @asio|1425176930.187377|0|signal_set@0x7fffe64b8398.cancel @asio|1425176930.187411|~1| [robert@localhost misc]$ This suggests one handler was invoked which is what i might have expected. This does not have a line with >1 indicating entering the handler and there is no output from the handler and count remains at 0. Could someone help me to understand this. [ I am using Fedora 21, gcc 4.9.2, x86_64, boost 1.57 ] kind regards