December 2020 - Boost-users - lists.stage.boost.cppalliance.org

Clarification on statechart post_event vs process_event within react()
by John McCabe 10 Jan '22

10 Jan '22

Hi, Apologies if this has already been answered/clarified, but my search of the internet to try to get a definitive answer, at least one I recognised as being definitive, was unsuccessful. Below I have posted some source code, based on the stopwatch example from the tutorial, but using an 'external' object to delegate the actual handling of event stuff to and using the state, rather than simple_state base class, and some other tweaks. This is a cut down version of something that someone I work with has devised, but I just wanted to get a clear answer on whether process_event() should be getting called in cases like this. I think it probably should not since, in the code below, you change state (with a state object's destructor being executed) whilst you're executing that state's react(EvStartStop) function. If someone can confirm this is the correct interpretation that would be very much appreciated. Any questions, feel free to ask (BTW - I'm compiling for C++17) John PS - In case it's of any use, the output from running this code is: ------ Entered Active Entered Stopped Stopped - payload2 m_charValue: a Exited ~Stopped <<<--- This is the bit I don't like the look of because... Entered Running Active - payload2 m_charValue: a <<<--- ... this happens from the forward_event() call payload1 value1: 20 payload1 value2: 200.2 Exited ~Running Entered Stopped Exiting Program Exited ~Stopped Exited ~Active ------ If I swap the //*** comments to the doProcess() lines instead of doPost(), the output seems more sensible ------ Entered Active Entered Stopped Stopped - payload2 m_charValue: a Exited ~Stopped Entered Running Active - payload2 m_charValue: a payload1 value1: 20 payload1 value2: 200.2 Exited ~Running Entered Stopped Exiting Program Exited ~Stopped Exited ~Active ------ ========== code =========== #include <boost/statechart/event.hpp> #include <boost/statechart/state_machine.hpp> #include <boost/statechart/state.hpp> #include <boost/statechart/transition.hpp> #include <boost/statechart/custom_reaction.hpp> #include <boost/mpl/list.hpp> #include <iostream> #include <string> namespace sc = boost::statechart; namespace mpl = boost::mpl; // Forward declare a handler 'class' that's going to have activity delegated to it struct Handler; // Normal example start/stop and reset events struct EvStartStop : sc::event<EvStartStop> {}; struct EvReset : sc::event<EvReset> {}; // A couple of events that carry data with them struct EvPayload1 : sc::event<EvPayload1> { EvPayload1(int value1, double value2) : m_value1 { value1 }, m_value2 { value2 } {}; int m_value1 { 0 }; double m_value2 { 0.0 }; }; struct EvPayload2 : sc::event<EvPayload2> { EvPayload2(const char& charValue) : m_charValue { charValue } {}; char m_charValue { false }; }; // Back to the example bits struct Active; struct StopWatch : sc::state_machine<StopWatch, Active> { typedef sc::transition<EvReset, Active> reactions; // The extra 'handler' stuff void setHandler(Handler* handler) { m_handler = handler; }; Handler* getHandler() const { return m_handler; }; Handler* m_handler { nullptr }; // Used to make post_event() accessible from an external class...seems unpleasant, but... using sc::state_machine<StopWatch, Active>::post_event; }; // The implementation of the Handler class; created with a reference to a StopWatch object struct Handler { Handler(StopWatch& stopWatch) : m_stopWatch { stopWatch } {}; void doPost() { m_stopWatch.post_event(EvStartStop()); } void doProcess() { m_stopWatch.process_event(EvStartStop()); } StopWatch& m_stopWatch; }; struct Stopped; struct Active : sc::state<Active, StopWatch, Stopped> { Active(my_context ctx) : my_base { ctx } { std::cout << "Entered Active" << std::endl; }; ~Active() { std::cout << "Exited ~Active" << std::endl; }; typedef mpl::list<sc::custom_reaction<EvPayload1>, sc::custom_reaction<EvPayload2>, sc::transition<EvReset, Active>> reactions; sc::result react(const EvPayload1& payload1) { std::cout << "payload1 Event NOT HANDLED" << std::endl; return discard_event(); } sc::result react(const EvPayload2& payload2) { std::cout << "Active - payload2 m_charValue: " << payload2.m_charValue << std::endl; return discard_event(); } }; struct Running : sc::state<Running, Active> { Running(my_context ctx) : my_base { ctx } { std::cout << "Entered Running" << std::endl; }; ~Running() { std::cout << "Exited ~Running" << std::endl; }; typedef mpl::list<sc::custom_reaction<EvPayload1>, sc::transition<EvStartStop, Stopped>> reactions; sc::result react(const EvPayload1& payload1) { std::cout << "payload1 value1: " << payload1.m_value1 << std::endl; std::cout << "payload1 value2: " << payload1.m_value2 << std::endl; //***context<StopWatch>().getHandler()->doPost(); context<StopWatch>().getHandler()->doProcess(); return discard_event(); } }; struct Stopped : sc::state<Stopped, Active> { Stopped(my_context ctx) : my_base { ctx } { std::cout << "Entered Stopped" << std::endl; }; ~Stopped() { std::cout << "Exited ~Stopped" << std::endl; }; typedef mpl::list<sc::custom_reaction<EvPayload2>, sc::transition<EvStartStop, Running>> reactions; sc::result react(const EvPayload2& payload2) { std::cout << "Stopped - payload2 m_charValue: " << payload2.m_charValue << std::endl; //***context<StopWatch>().getHandler()->doPost(); context<StopWatch>().getHandler()->doProcess(); return forward_event(); } }; int main() { StopWatch myWatch; Handler myHandler(myWatch); myWatch.setHandler(&myHandler); myWatch.initiate(); myWatch.process_event(EvPayload2 { 'a' }); myWatch.process_event(EvPayload1 { 20, 200.2 }); std::cout << "Exiting Program" << std::endl; return 0; }

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shutdown( ) blocks forever
by Fariya F 30 Dec '20

30 Dec '20

I have a server that accepts ssl connections on port 443. I am using the boost libraries for the server implementation. Below is the code snippet: ssl_socket socket_; { // Open the acceptor with the option to reuse the address (i.e. SO_REUSEADDR). boost::asio::ip::tcp::resolver resolver(io_service_); boost::asio::ip::tcp::resolver::query query(address, port); boost::asio::ip::tcp::endpoint endpoint = *resolver.resolve(query); acceptor_.open(endpoint.protocol()); acceptor_.set_option(boost::asio::ip::tcp::acceptor::reuse_address(true)); acceptor_.bind(endpoint); acceptor_.listen(1000); start_accept(); } void server::start_accept() { new_connection_.reset( new connection(io_service_, connection_manager_, request_handler_, context_)); acceptor_.async_accept(new_connection_->socket(), boost::bind(&server::handle_accept, this, boost::asio::placeholders::error)); } void server::handle_accept(const boost::system::error_code& e) { // Check whether the server was stopped by a signal before this completion // handler had a chance to run. if (!acceptor_.is_open()) { return; } else { start(); } start_accept(); } void server::start() { socket_.async_handshake(boost::asio::ssl::stream_base::server, boost::bind(&connection::handle_handshake, this, boost::asio::placeholders::error)); if (boost::asio::placeholders::error) { // printf("Error is there. Check it\n"); } } void connection::handle_handshake(const boost::system::error_code& error) { if (!error) { DEBUG("async read some"); socket_.async_read_some(boost::asio::buffer(buffer_), boost::bind(&connection::handle_read, shared_from_this(), boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred)); } else { std::cout << "Error number: " << error.value(); std::cout << " Error message: " << error.message(); std::cout << " Error category: " << error.category().name() << std::endl; delete this; } } When I initiate multiple clients to connect to the server, most of the time the connection succeeds i.e TCP handshake followed by SSL handshake succeeds and I am able to do data transfer. However, there are times when the TCP handshake succeeds (as seen by wireshark) but the SSL request isn't responded to by my server. I am able to see the SSL handshake request coming from the client onto my server, but the server doesn't respond, neither is my handle_accept() function gets called. When I check the output of netstat, I see a lot of data pending in the Recv-Q. Below is the output of netstat -atn command on the server during one such scenario: Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 15 0 0.0.0.0:443 0.0.0.0:* LISTEN tcp 0 213 10.110.112.1:443 10.110.112.2:52363 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52624 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52581 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52646 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52749 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52824 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52805 ESTABLISHED tcp 180 0 10.110.112.1:443 10.110.112.2:52645 ESTABLISHED tcp 0 471 10.110.112.1:443 10.110.112.2:52362 ESTABLISHED tcp 1 0 10.110.112.1:443 10.110.112.3:39129 CLOSE_WAIT tcp 183 0 10.110.112.1:443 10.110.112.3:39130 CLOSE_WAIT tcp 180 0 10.110.112.1:443 10.110.112.2:52595 ESTABLISHED tcp 183 0 10.110.112.1:443 10.110.112.3:39131 CLOSE_WAIT Upon further debugging, I see that the call to ssl socket member function shutdown() blocks sometimes forever and sometimes for at least 5 - 10 minutes due to which no further data is taken in and the recv-q indicates pending bytes. This is how my write handler is. Any help on where I am going wrong would be greatly appreciated: void connection::handle_write(const boost::system::error_code& e) { if (!e) { boost::system::error_code ec; socket_.shutdown(ec); //Code blocks here indefinitely } } Any reasons why the socket_.shutdown( ) blocks. How could I specify a timeout such that the shutdown( ) returns back after some time? Any other way of handling this? ( I know there is async_shutdown( ) but I prefer using shutdown( ) blocking handler) Any help would be greatly appreciated. Regards, Fariya

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[json] Using boost.json with boost.beast
by Sean Farrow 22 Dec '20

22 Dec '20

Hi, Are there any samples of using boost.json to deserialize boost.beast responses? Thanks, Sean.

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[Accumulators] How to extract P90, P99?
by Dominique Devienne 21 Dec '20

21 Dec '20

I'm already using the accumulator below to keep track of request processing times of a server. using ProcAcc = acc::accumulator_set< double, acc::stats< acc::tag::min, acc::tag::max, acc::tag::mean, acc::tag::median, acc::tag::sum > >; But now I'm interested in also extracting the P90 and P99 values, to get insight in how much of an outlier the max could be. AFAIK, the median is the P50, but it's not readily obvious to me what tag to use for the P90 or P(N) in general. Could someone help please? Thanks, --DD PS: Does accumulator allow to keep the top(N) values as well? How?

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boost::hash_value not picking up std::hash specialisations
by Marc Mutz 21 Dec '20

21 Dec '20

Hi, I'm trying to move away from Qt containers in my source code, and Qt 5.15 makes that a bit easier by providing specialisations for some its types. Now I have a struct S { QLatin1String a, b; } that I'd like to hash. I can use boost::hash_combine to implement std::hash<S>, but S is actually a nested type, and with qHash(), I could just write an overload as a hidden friend, and I figured I could do the same with hash_value() (at the cost of having to specify boost::hash<S> as a unordered_map template argument). But hash_combine seems to insist on hash_value which in turn doesn't look at std::hash at all: size_t seed = 0; boost::hash_combine(seed, key.a); // ERROR: no hash_value overload for QLatin1String boost::hash_combine(seed, key.b); // Ditto return seed; expected: calls std::hash<QLatin1String>{}::op() (if it exists). It's trivial to write hash_value(QLatin1String), of course, but what's the general strategy here? Shouldn't hash_combine() look for std::hash<T> first? And, speaking as a Qt developer: do I really need to provide qHash(), std::hash<T> _and_ hash_value(T) for every hashable T?! Thanks, Marc

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Weight as a function of distance at source
by Amit Prakash Ambasta 18 Dec '20

18 Dec '20

Hi, Looking at BGL implementations, weight, compare and combine are used in examine edge(e) as compare(combine(old_distance, get(weight, e)), old_distance) Additionally, old_distance = m_zero to check for negative weights etc. In my implementation, edges have a bundled property struct EdgeProp { Distance weight(Distance start) { ... } }; that returns the cumulative weight on iterating over the edge given a start distance. To use this, I should be able to pass a combine function of the form []<typename BinaryFunction>(DistanceType d, BinaryFunction f) { return f(d); } However, this approach falls apart. I've tried putting together a formal example here, but not sure why this fails. (My actual use case is where weights represent nodes in a transport system and for a person arriving at a vertex at some time Tx, there is a variable weight of using the next outbound transport = waiting_time + travel_time, where waiting_time is a function of Tx #include <boost/graph/adjacency_list.hpp> #include <boost/graph/dijkstra_shortest_paths.hpp> #include <limits> struct VProp {}; struct EProp { int multiplier; int weight(int src_distance) { return src_distance * multiplier; } }; typedef boost:: adjacency_list<boost::vecS, boost::vecS, boost::directedS, VProp, EProp> Graph; template<class G> using Vertex = boost::graph_traits<G>::vertex_descriptor; template<class G> using Edge = boost::graph_traits<G>::edge_descriptor; int main() { Graph graph; auto source = boost::add_vertex(VProp{}, graph); auto target = boost::add_vertex(VProp{}, graph); boost::add_edge(source, target, EProp{ 3 }, graph); auto index_map = get(boost::vertex_index, graph); std::vector<Vertex<Graph>> predecessors(boost::num_vertices(graph)); auto predecessor_map = boost::make_iterator_property_map(predecessors.begin(), index_map); std::vector<int> distances(boost::num_vertices(graph)); auto distance_map = boost::make_iterator_property_map(distances.begin(), index_map); auto compare = [](int lhs, int rhs) -> bool { return lhs < rhs; }; auto combine = []<typename BinaryFunction>(int dst, BinaryFunction f) -> int { return f(dst); }; boost::dijkstra_shortest_paths( graph, source, predecessor_map, distance_map, boost::weight_map(boost::get(&EProp::weight, graph)), index_map, compare, combine, std::numeric_limits<int>::min(), std::numeric_limits<int>::max(), boost::default_dijkstra_visitor()); }

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boost::regex: regex meta data or AST for regex?
by Frank Bergemann 16 Dec '20

16 Dec '20

in concrete: Is it possible to not only get the #no of groups (sub-expressions) via basic_regex::mark_count(), but also their position (begin() / end() iterators) in the given regex string (c'tor argument)? more general: Can an AST be created for a the given regex string (c'tor argument)? E.g with boost::spirit?

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how to get progress indicator during phrase_parse() of boost::spirit::x3?
by Lane 15 Dec '20

15 Dec '20

I am using boost::spirit:x3, shown below. I use it to import/parse some very large files and am trying to find a way to get some sort of progress from it so that I can use for a progress bar in app, but so far I've been unable to see a way. namespace parser { namespace x3 = boost::spirit::x3; using namespace x3; auto const string = lexeme[+graph]; auto const point = rule<struct _p, ast::point> {"point"} = int_ >> double_ >> double_ >> double_; auto const element = rule<struct _e, ast::element> {"element"} = int_ >> string >> int_ >> int_ >> int_ >> eol >> *(int_ >> int_ >> int_); auto lines = [](auto p) { return *(p >> eol); }; auto const input = lines(point) >> lines(element); } And I import a file like so. { namespace x3 = boost::spirit::x3; mapped_file_source map(filePath); std::istringstream iss(map.data()); map.close(); boost::spirit::istream_iterator iter(iss >> std::noskipws), eof; bool ok = phrase_parse(iter, eof, smf::parser::input, x3::blank, types); if (iter != eof) { // ... } } Would anyone have thoughts on how to get some sort of indicator during the parsing?

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[Beast] Client connection times out after 5s. How to avoid?
by Dominique Devienne 15 Dec '20

15 Dec '20

Hi. Continuing my explorations of Boost.Beast. So now I have: * a small Boost.Beast HTTP server (based on async_http_server example), and * a small Boost.Beast HTTP client (based on sync_http_client example). This works, in that I can start the client (wrapped in a helper class), make several requests, and close the client. But until now, the client was short-lived, so no problem. (the server was started too of course, and long lived as expected). Now I'm embedding that HTTP client into another (WebSocket) server, which uses that same wrapper class above, successfully connects to the HTTP server on startup, and now waits for WebSocket connections, at which time it's supposed to make HTTP requests to the HTTP server for various reasons. Problem is, the HTTP client times out after 5s. While I want it to be long lived. I've read Vinnie's answer in https://stackoverflow.com/a/56880415/103724 about sync operation not having timeouts, but I don't want to control timeouts of individual requests here (for now at least), I want to not have the underlying socket not timeout on its own, the same way a WebSocket does not timeout either. Is that possible? Who controls that 5s timeout? Would switching to an async HTTP client make any difference? Sounds to me that it would allow me to control timeouts of requests, but would it make any difference to the socket timeout described above? Thanks, --DD

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[Beast] Slow HTTP connect on Windows. Normal?
by Dominique Devienne 14 Dec '20

14 Dec '20

Hi. I'm writing a small interaction between: 1) a Boost.Beast-based HTTP server (based on http_server_async.cpp example) 2) a Boost.Beast-based HTTP client (based on http_client_sync.cpp example) When I log the time it takes to resolve the server address and connect to it, then issue my HTTP request, it takes over 1 second (Win10, VS2019, C++17, Release, localhost for both client and server): 2020-11-25T09:46:37.473936 Prologue in 0.001s 2020-11-25T09:46:37.482470 Resolved in 0.008s 2020-11-25T09:46:38.506180 Connected in 1.023s 2020-11-25T09:46:38.509607 OK: Authenticated; in 0.003s While the same on Linux (RH7.5) is just over 2ms: 2020-11-25T09:45:45.515926 Prologue in 0.000s 2020-11-25T09:45:45.517083 Resolved in 0.001s 2020-11-25T09:45:45.517550 Connected in 0.000s 2020-11-25T09:45:45.518010 OK: Authenticated; in 0.000s That's a huge difference! Almost 500x... And when I contact the same HTTP server on Windows, but from Chrome this time, it takes about 300ms, and if I hit reload rapidly, the time jumps around to as low as 3ms, and as high as 300ms (the same initial delay), with ~50ms and ~100ms in between. (I also see several different connections being established, for some reason...) I've seen similar differences between Windows and Linux connection times, but with WebSocketPP-based client and server this time, also based on Boost.ASIO. Q1: Am I doing anything wrong? I.e. is this "normal" somehow? Q2: How come Chrome, on Windows too, is 3x faster than ASIO-based clients, with the same Beast-based server? Given the high connect time on Windows, I thought I'd try to keep the connection open on the client, and issue several send-request/read-response pairs, using the Beast-based (sync) HTTP client, but only the first one works correctly, the 2nd errors out with: Error: An established connection was aborted by the software in your host machine I suspect it is a user-error, with the server closing the connection, despite the Keep-Alive HTTP header being present server-side on the request. Q3: Given the Beast HTTP examples I'm based off, would anyone have suggestions on what changes are necessary to allow the client issuing several (non-overlapping) requests to the server, on the same connection? (since connecting is so expensive). Q4: And for better performance, what about overlapping request/response pairs, with HTTP pipelining. How to set that up on the client and server with Beast? Thanks for any help on this. --DD PS: Great examples in Boost.Beast BTW. Thanks Vinnie.

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