如何检测其他程序关闭的eventfd文件描述符

Question

I have a client/server communicate through eventfd. If either client or server call close(fd) I would like the other end to find out (like file descriptor is closed now). I tried to use select with non-zero timeout, it always return 0 which is timeout. I saw people suggesting use fcntl it doesn't seems to be working either. Any suggestions?

Addtion Details (omitted non important part code, you can see here for how to exchange file descriptor detail code : It is multi processes application. Server process created eventfd by calling

 struct msghdr control_message;
 int fd = eventfd(0,0);
 *CMSG_DATA(control_message) = fd;
 message.msg_control = &control_message;
 sendmsg(socket_fd, & message,0); //send this to client

From client side:

 recvmsg(socket_fd, & message,0);
 //loop using CMSG_NXTHDR(&message, control_message)
 int fd = *(int *) CMSG_DATA(contro_message);

Then on server side:

 close(fd);

On Client side: int rc; rc = dup2(fd,fd);

rc is never invalid.

Answer 1

Checking for a closed file descriptor? How about this?

#include <errno.h>
#include <stdio.h>
#include <string.h>

static void checkit ()
{
    int     rc;

    rc = dup2(2, 2);

    if ( rc == -1 )
            printf("error %d on dup2(2, 2): %s\n", errno, strerror(errno));
    else
            printf("dup2 successful\n");

    write(2, "still working\n", 14);
}

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

    checkit();

    close(2);

    checkit();

    return  0;
}

Running it generates this output:

dup2 successful
still working
error 9 on dup2(2, 2): Bad file descriptor

If this is a multi-threaded application using poll and you want poll to return when the file descriptor is closed by another thread, POLLERR, POLLHUP, or POLLNVAL might help.

Multi-Threaded Version using Poll

And here's a sample that shows how to detect a closed fd with poll (POLLNVAL is the event) in a multi-threaded program:

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

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

    fds[0].fd = 2;
    fds[0].events = POLLERR | POLLHUP | POLLNVAL;

            //
            // 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\n",
                   rc,
                   fds[0].revents,
                   ( ( 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;

    rc = pthread_create(&thread, NULL, run_poll, NULL);

    usleep(100);
    printf("closing stderr\n");
    close(2);
    usleep(100);

    return  0;
}

This generates the output

starting poll
closing stderr
POLL returned 1 (revents = 0x00000020): noerr nohup pollnval