using libev with multiple threads - c++

Question

I want to use libev with multiple threads for the handling of tcp connections. What I want to is:
The main thread listen on incoming connections, accept the
connections and forward the connection to a workerthread.
I have a pool of workerthreads. The number of threads depends on the
number of cpu's. Each worker-thread has an event loop. The worker-thread listen if I can write on the tcp socket or if
somethings available for reading.
I looked into the documentation of libev and I known this can be done with libev, but I can't find any example how I have to do that.
Does someone has an example?
I think that I have to use the ev_loop_new() api, for the worker-threads and for the main thread I have to use the ev_default_loop() ?
Regards

Answer 1

The following code can be extended to multiple threads
//This program is demo for using pthreads with libev.
//Try using Timeout values as large as 1.0 and as small as 0.000001
//and notice the difference in the output
//(c) 2009 debuguo
//(c) 2013 enthusiasticgeek for stack overflow
//Free to distribute and improve the code. Leave credits intact
#include <ev.h>
#include <stdio.h> // for puts
#include <stdlib.h>
#include <pthread.h>
pthread_mutex_t lock;
double timeout = 0.00001;
ev_timer timeout_watcher;
int timeout_count = 0;
ev_async async_watcher;
int async_count = 0;
struct ev_loop* loop2;
void* loop2thread(void* args)
{
printf("Inside loop 2"); // Here one could initiate another timeout watcher
ev_loop(loop2, 0); // similar to the main loop - call it say timeout_cb1
return NULL;
}
static void async_cb (EV_P_ ev_async *w, int revents)
{
//puts ("async ready");
pthread_mutex_lock(&lock); //Don't forget locking
++async_count;
printf("async = %d, timeout = %d \n", async_count, timeout_count);
pthread_mutex_unlock(&lock); //Don't forget unlocking
}
static void timeout_cb (EV_P_ ev_timer *w, int revents) // Timer callback function
{
//puts ("timeout");
if (ev_async_pending(&async_watcher)==false) { //the event has not yet been processed (or even noted) by the event loop? (i.e. Is it serviced? If yes then proceed to)
ev_async_send(loop2, &async_watcher); //Sends/signals/activates the given ev_async watcher, that is, feeds an EV_ASYNC event on the watcher into the event loop.
}
pthread_mutex_lock(&lock); //Don't forget locking
++timeout_count;
pthread_mutex_unlock(&lock); //Don't forget unlocking
w->repeat = timeout;
ev_timer_again(loop, &timeout_watcher); //Start the timer again.
}
int main (int argc, char** argv)
{
if (argc < 2) {
puts("Timeout value missing.\n./demo <timeout>");
return -1;
}
timeout = atof(argv[1]);
struct ev_loop *loop = EV_DEFAULT; //or ev_default_loop (0);
//Initialize pthread
pthread_mutex_init(&lock, NULL);
pthread_t thread;
// This loop sits in the pthread
loop2 = ev_loop_new(0);
//This block is specifically used pre-empting thread (i.e. temporary interruption and suspension of a task, without asking for its cooperation, with the intention to resume that task later.)
//This takes into account thread safety
ev_async_init(&async_watcher, async_cb);
ev_async_start(loop2, &async_watcher);
pthread_create(&thread, NULL, loop2thread, NULL);
ev_timer_init (&timeout_watcher, timeout_cb, timeout, 0.); // Non repeating timer. The timer starts repeating in the timeout callback function
ev_timer_start (loop, &timeout_watcher);
// now wait for events to arrive
ev_loop(loop, 0);
//Wait on threads for execution
pthread_join(thread, NULL);
pthread_mutex_destroy(&lock);
return 0;
}

Answer 2

Using libev within different threads at the same time is fine as long as each of them runs its own loop[1].
The c++ wrapper in libev (ev++.h) always uses the default loop instead of letting you specify which one you want to use. You should use the C header instead (ev.h) which allows you to specify which loop to use (e.g. ev_io_start takes a pointer to an ev_loop but the ev::io::start doesn't).
You can signal another thread's ev_loop safely through ev_async.
[1]http://doc.dvgu.ru/devel/ev.html#threads_and_coroutines