How can I cancel a blocked read/recvfrom system call on vxworks or linux

Question

In TASK I:

...

while (1)
{
  if (running == false)
  break;
  ret = read(fd, buf, size); /* Or: ret = recvfrom(sock, buf, size, 0, NULL, NULL); */
  ...
 }

In task II:

...
running = true;
/* ioctl(fd, FIOCANCEL, 0); */
close(fd);   /* Or: close(sock);*/

what should do in task II to cancel the blocked task I,

In vxworks, there is a function, ioctl(fd, FIOCANCEL, 0) to cancel a blocked read or write but it can not work. Probably because the driver can not support "FIOCANCEL".

how to write task II in vxworks and linux? or are there any other way to do my task?

Answer 1

Cancelling a read/recvfrom call is impossible on linux. You cannot write these tasks using the same API. On Linux you can use epoll and O_NONBLOCK to create the semantics of canceling a read/recvfrom.

It is impossible to do this using the same code for both linux and vxworks.

Answer 2

Don't use blocking IO, this is a classic case of creating a thread with no (reachable) exit condition, which I consider to be a bug. The easiest example of how you should run your thread is as follows:

volatile bool _threadRunning = true;

void taskI()
{
    int flags = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, flags | O_NONBLOCK);

    while (_threadRunning == true)
    {
        ret = read(fd, buf, size);
        if (ret > 0)
        {
            // process buffer
        }
        else
        {
            // sleep for 1 millisecond or so...
        }
    }
    close(fd);
}


void taskII()
{
    _threadRunning = false;
    _taskI.join();
}