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Date:      Tue, 10 Apr 2012 20:52:13 +0100
From:      Ben Short <ben@benshort.co.uk>
To:        freebsd-questions@freebsd.org
Subject:   Re: Using kqueue with 2 threads
Message-ID:  <CAOiKr=tV7GioR4gVnXQmWx=HU4yzpzubAdsiycJHZcxTGJ2FKw@mail.gmail.com>
In-Reply-To: <CAOiKr=t3V=MWCc6Z_+skZYkS-jrXwXUiOkuOk7N_sBCaKWMZxQ@mail.gmail.com>
References:  <CAOiKr=t3V=MWCc6Z_+skZYkS-jrXwXUiOkuOk7N_sBCaKWMZxQ@mail.gmail.com>

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I have put together the following example and can still see the problem.
Once the file is written to extended is printed every second. If I remove
the sleep then it prints a whole lot more. So it seems that the call to
kevent in the worker_thread method blocks until the file is written to then
it returns the same event straight away on every call.

Is this expected? Am I meant to clear the event some how?

#include <cstdlib>

#include <iostream>

#include <sys/stat.h>
#include <sys/errno.h>
#include <iostream>
#include <fstream>
#include <cstring>
#include <stdio.h>

#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>
#include <stdio.h>
#include <fcntl.h>

using namespace std;

struct wrapper {
    int kq;
};

static void* worker_thread(void* obj) {

    int kq = ((wrapper*)obj)->kq;

    std::cerr << "Started worker" << std::endl;

    struct kevent ke;
    int i;

    while ( 1 ) {

        memset(&ke, 0x00, sizeof(ke));

        i = kevent(kq, NULL, 0, &ke, 1, NULL);
        if ( i == -1 ) {
            std::cerr << "kqueue produced error: " << strerror(i) <<
std::endl;
            continue; // todo is this the best thing todo?
        } else if ( i == 0 ) {
            std::cerr << "kqueue time limit expired" << std::endl;
            continue;
        }

        if ( ke.filter == EVFILT_VNODE ) {

            if ( ke.fflags & NOTE_DELETE ) {
                std::cerr << "delete" << std::endl;

                break;
            } else if ( ke.fflags & NOTE_RENAME ) {
                std::cerr << "rename" << std::endl;
            } else if ( ke.fflags & NOTE_EXTEND ) {
                std::cerr << "extended" << std::endl;
            } else {
                std::cerr << "some other fflags" << std::endl;
            }

        } else {
            std::cerr << "some other filter" << std::endl;
        }


        sleep(1);

    }

    std::cerr << "Shutting down worker" << std::endl;

    return 0;
}

void writeToFile(std::string fileName, std::string key, std::string value) {

    std::fstream registryEntry;

    registryEntry.exceptions(std::ifstream::failbit |
std::ifstream::badbit);

    try {

        registryEntry.open(fileName.c_str(), std::ios::out |
std::ios::binary);

        size_t keyLength = key.length();

        registryEntry.write((char*)&keyLength, sizeof(size_t));

        registryEntry.write(key.c_str(), key.length());

        size_t valueLength = value.length();

        registryEntry.write((char*)&valueLength, sizeof(size_t));

        registryEntry.write(value.c_str(), value.length());

        std::cout << "Set entry with key: " << key << std::endl;

    } catch (std::ifstream::failure e) {
        // todo throw exception
        std::cerr << "Failed to set registry entry. Error: " << e.what() <<
std::endl;
    }

    registryEntry.close();

}

void test2() {

    int kq = kqueue();

    if ( kq == -1 ) {
        std::cerr << "Failed to create kqueue. Error: " << strerror(errno)
<< std::endl;
        return;
    }

    pthread_t worker;

    wrapper wrap;
    wrap.kq = kq;


    if ( pthread_create(&worker, 0, worker_thread, &wrap) != 0 ) {
        std::cerr << "Failed to create listener thread. Error: " <<
strerror(errno) << std::endl;
    }

    int fd = open("/tmp/tt", O_RDONLY);
    if ( fd == -1 ) {
        std::cerr << "Failed to open file: " << "/tmp/tt" << " Error: " <<
strerror(errno) << std::endl;
        // todo throw exception
    }

    struct kevent ke;

    EV_SET(&ke, fd, EVFILT_VNODE, EV_ADD, NOTE_DELETE | NOTE_RENAME |
NOTE_EXTEND, 0, NULL);

    if (kevent(kq, &ke, 1, NULL, 0, NULL) == -1) {
        std::cerr << "kevent produced error: " << strerror(errno) <<
std::endl;
    }

    sleep(1);

    writeToFile("/tmp/tt", "key1", "value1");

    sleep(5);

}

/*
 *
 */
int main(int argc, char** argv) {

    test2();


    return 0;
}

On 10 April 2012 15:25, Ben Short <ben@benshort.co.uk> wrote:

> Hi,
>
> I'm trying to use a kqueue to listen for VNODE events on a worker thread.
> I want new events  to be registered with the kqueue by a separate thread.
>
> I have been referring to this example [URL="
> http://doc.geoffgarside.co.uk/kqueue/file.html"]
> http://doc.geoffgarside.co.uk/kqueue/file.html[/URL]
>
> Here are the relevant parts of code as it is at the moment.
>
> In my worker thread:
>
> [CODE]std::cerr << "Started worker" << std::endl;
>
>     struct kevent ke;
>     int i;
>
>     while ( !mStopRequested ) {
>
>         memset(&ke, 0x00, sizeof(kevent));
>
>          i = kevent(kq, NULL, 0, &ke, 1, NULL);
>         if ( i == -1 ) {
>             std::cerr << "kqueue produced error: " << strerror(i) <<
> std::endl;
>             continue; // todo is this the best thing todo?
>         }
>
>         std::cerr << "Beep: " << i << std::endl;
>
>     }
>
>     std::cerr << "Shutting down worker" << std::endl;[/CODE]
>
> Other thread
>
> [CODE]int fd = open(fileName.c_str(), O_RDONLY);
>     if ( fd == -1 ) {
>         std::cerr << "Failed to open file: " << fileName << " Error: " <<
> strerror(errno) << std::endl;
>         // todo throw exception
>     }
>
>     struct kevent ke;
>
>     EV_SET(&ke, fd, EVFILT_VNODE, EV_ADD, NOTE_DELETE | NOTE_RENAME |
> NOTE_EXTEND, 0, NULL);
>
>     if (kevent(kq, &ke, 1, NULL, 0, NULL) == -1) {
>         std::cerr << "kevent produced error: " << strerror(errno) <<
> std::endl;
>     }[/CODE]
>
>
> When I run my code the kevent call in the worker code doesn't block at
> all. Any ideas what I'm missing? or if what I want to do is even possible?
>
> Ben
>



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