A Naive C++ Eventloop (poll())

Remark: this is not a C++ course

To fight code complexity, we are now switching to C++. We’ll use conservative old-style C++ though, in a way that should still be comprehensible to those who are not so fluent in that language.

Modern C++ would probably offer functional constructs (here) for the loop registration, and lambdas (here) to implement the satellite callbacks.

Other event loop implementations

The event loop implementation that is being worked out in this section has educational value only - is not meant to be used productively. Once the concept of event loops, and the impact of using such a thing is understood, you can choose among a number of implementations out there. Here I list only some, all of which are “object oriented” in one way or the other although they might be implemented in C.

• sd-event - the libsystemd event loop: freedesktop.org documentation, Lennart Poettering: “The Event Loop API of libsystemd”

• Gtk/GLib event loop

• Qt event loop

• libevent

• Boost.Asio

Readability, And OO

See also

• Event Loop Support: signalfd()

• In Solution: Read From Two Input Sources, we react on two different file descriptors in two different ways

• Two is one representation of many

• ⟶ Complication ahead; for example …

  • signalfd(): handle SIGTERM and SIGINT to terminate gracefully (see Exercise: Graceful Termination Via SIGTERM/SIGINT)

  • signalfd(): use SIGUSR1 to commit database

  • signalfd(): use SIGUSR2 to rollback database (see Exercise: Commit/Rollback Via SIGUSR1/SIGUSR2)

  • Timers using timerfd_create()

  • More network I/O

  • …

Eventloop, And Associated Types

• Eventloop: central “loop” object

  • Replacement for while loop in Solution: Read From Two Input Sources

  • Uses poll() to watch registered file descriptors

  • Calls back associated InputHandler objects when it detects input (POLLIN) readiness (see Possible Events)

• InputHandler: handle input detected on a file descriptor

  • Polymorphic type: abstract base class, an interface

  • Usually reads from its file descriptor, and does something with the data

Re-Designing The Database Application

See also

• Solution: Read From Two Input Sources

• With the building blocks outlined above, redesign our database application

• ⟶ Abstraction is cool

• ⟶ Do not need to know everything

Re-Writing The Database Application

• Externalize building blocks from the diagram into separate files (see Building Blocks In Detail)

• Functionality is equivalent to the spaghetti implementation

• See here for the test protocol

code/main.cpp

#include "database.h"
#include "eventloop.h"
#include "udp-db.h"
#include "stdin-db.h"

int main()
{
    Eventloop loop;                                    // <-- main loop
    Database db;

    UDPToDatabase udp("0.0.0.0", 1234, db);            // <-- satellite
    StdinToDatabase stdin(db);                         // <-- satellite

    udp.hookup(loop);                                  // <-- register with loop
    stdin.hookup(loop);                                // <-- register with loop

    loop.run();                                        // <-- run until quit

    db.commit();
    return 0;
};

Building Blocks In Detail

Types	Download	View
Eventloop and InputHandler	eventloop.h eventloop.cpp	class Eventloop
UDPToDatabase	udp-db.h udp-db.cpp	class UDPToDatabase
StdinToDatabase	stdin-db.h stdin-db.cpp	class StdinToDatabase
Database (unmodified)	database.h database.cpp	class Database (And Associated)