Portabilty of poll trick

[Bastien Roucariès]

Hi,

I want to assess the safety and portability of the following trick, for getting 
outside poll. Replacing by using dup2 a read poll object by a writtable one. I 
think to use this for stopping polling in multithread program
It work really well, and could for instance allow me to tear down eventfd of 
other event like file that does not support shutdown

CC also   Michael Kerrisk (mtk.manpa...@gmail.com) for comments on this trick

#include <errno.h>
#include <poll.h>
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

static void *run_poll (void *arg)
{
    struct pollfd   fds[1];
    int             rc;

    fds[0].fd = ((int *)arg)[0];
    fds[0].events = POLLERR | POLLHUP | POLLNVAL | POLLIN;
 
            //
            // Poll indefinitely
            //

    printf("starting poll\n");
    fflush(stdout);
    rc = poll((struct pollfd *) &fds, 1, -1);

    if ( rc == -1 )
    {
            printf("POLL returned error %d: %s\n", errno, strerror(errno));
    }
    else
    {
            printf("POLL returned %d (revents = 0x%08x): %s %s %s %s %s\n",
                   rc,
                   fds[0].revents,
                   ( ( fds[0].revents & POLLIN  ) ? "pollin" : "noin" ),
                   ( ( fds[0].revents & POLLOUT  ) ? "pollout" : "noout" ),
                   ( ( fds[0].revents & POLLERR  ) ? "pollerr" : "noerr" ),
                   ( ( fds[0].revents & POLLHUP  ) ? "pollhup" : "nohup" ),
                   ( ( fds[0].revents & POLLNVAL ) ? "pollnval" : "nonval" ));
    }

    return  NULL;
}

int main (int argc, char **argv)
{
    pthread_t       thread;
    int             rc;

    int fdst[2];
    pipe(fdst);
    rc = pthread_create(&thread, NULL, run_poll, &fdst);

    sleep(3);
    printf("removing access\n");
    dup2(fdst[0],fdst[1]);
    sleep(3);

    return  0;
}

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