Correctly close file descriptor when opened through network mount

I'm currently trying to figure out how to correctly close a file descriptor when it points to a remote file and the connection is lost.

I have a simple example program which opens a file descriptor on a sshfs mount folder and start to write to the file.

I'm not able to find how to handle the case when the connection is lost.

void *write_thread(void* arg);

int main()
{
    pthread_t thread;
    int fd = -1;

    if(-1 == (fd = open("/mnt/testfile.txt", O_CREAT | O_RDWR | O_NONBLOCK, S_IRWXU)))
    {
        fprintf(stderr, "Error oppening file : %m\n");
        return EXIT_FAILURE;
    }
    else
    {
        if(0 > pthread_create(&thread, NULL, write_thread, &fd))
        {
            fprintf(stderr, "Error launching thread : %m\n");
            return EXIT_FAILURE;
        }
        fprintf(stdout, "Waiting 10 seconds before closing\n");
        sleep(10);
        if(0 > close(fd))
        {
            fprintf(stderr, "Error closing file descriptor: %m\n");
        }
    }
}

void *write_thread(void* arg)
{
    int fd = *(int*)arg;
    int ret;

    while(1)
    {
        fprintf(stdout, "Write to file\n", fd);
        if(0 > ( ret = write(fd, "Test\n", 5)))
        {
            fprintf(stderr, "Error writing to file : %m\n");
            if(errno == EBADF)
            {
                if(-1 == close(fd))
                {
                    fprintf(stderr, "Close failed : %m\n");
                }
                return NULL;
            }
        }
        else if(0 == ret)
        {
            fprintf(stderr, "Nothing happened\n");
        }
        else
        {
            fprintf(stderr, "%d bytes written\n", ret);
        }
        sleep(1);
    }
}

When the connection is lost (i.e. I unplug the ethernet cable between my boards), The close in the main thread always blocks whether I use the flag O_NONBLOCK or not.

The write call sometimes immediately fails with EBADF error or sometimes continues for a long time before failing.

My problem is that the write call doesn't always fail when the connection is lost so I can't trigger the event into the thread and I also can't trigger it from the main thread because close blocks forever.

So my question is : How to correctly handle this case in C ?

After some diggin around I found that the SSH mount could be configured to drop the connection and disconnect from server if nothing happens.

---

Setting ServerAliveInterval X on client side to disconnect if the server is unresponsive after X sec.

Setting ClientAliveCountMax X on server side to disconnect if the client is unresponsive after X sec.

ServerAliveCountMax Y and ClientAliveCountMax Y can also be used in order to retry Y times before dropping the connection.

---

With this configuration applied, the sshfs mount is automatically removed by Linux when the connection is unresponsive.

With this configuration, the write call fails with Input/output error first and then with Transport endpoint is not connected.

This is enough to detect that the connection is lost and thus cleaning up the mess before exiting.