Sending the code for isready()
int isready(int fd)
{
int rc;
fd_set fds;
struct timeval tv;
tv.tv_sec = 60 ;
tv.tv_usec = 0;
FD_ZERO(&fds);
FD_SET(fd,&fds);
printf("Before select %d\n",fd);
rc = select(fd+1, &fds, NULL, NULL, &tv);
printf("Return type = %d\n",rc);
if (rc < 0)
return -1;
rc = FD_ISSET(fd,&fds) ? 1 : 0;
FD_ZERO(&fds);
printf("After select\n");
return rc;
}
On 4/30/08, Thomas Hruska <[EMAIL PROTECTED]> wrote:
>
> Sathya Raghunathan wrote:
> > I opened a named pipe fd in read/nonblocking mode, and then associated a
> > stream using fdopen
> >
> > int pipe1_fd = open("/tmp/myfirstfifo", open_mode);
> > FILE *pipe_fd= fdopen(pipe1_fd,"r");
> >
> > First time, the read gets blocked on select with a time out of 20 secs
> as
> > set in the function call(expected behaviour).
> > Once there is something in the pipe, select returns and FD_ISSET
> succeeds.
> >
> > But after the first successful read from pipe, the while loop continues
> and
> > the next select always returns 1 instead of blocking again for 20
> secs(Note:
> > I am not writing any data to the pipe at this time). I am not able to
> reset
> > the fds' value. How can i do that? What went wrong? Is it because i am
> > mixing file descriptor and the stream?
> >
> > The code is
> >
> > main()
> > {
> > fd_set fds;
> > while(1)
> > {
> > if(isready(pipe_fd1,&fds)==1)
> > // there is data to process...
> > // read using fscanf(%d%c",&length,&comma) and
> fread(pipe_fd,buffer,length);
> > else
> > // there is no data to process. need to block on select
> > }
> > }
> >
> >
> > isready(int fd, fd_set *fds)
> > {
> > FD_ZERO(fds);
> > FD_SET(pipe_fd,fds);
> > rc = select(fd+1, fds, NULL, NULL, NULL);
> > if rc<0
> > return -1
> > FD_ISSET(fd,fds)?1:0;
> > }
>
> Which mode is the pipe opened up in? Blocking or non-blocking? You
> aren't very clear on that.
>
> Your use of select() is likely wrong. select() only CHECKS for
> readability/writability/etc. There is NO GUARANTEE that the pipe is
> actually readable between the select() call and the time the pipe is
> attempted to be read. If the pipe is blocking, you can end up blocking
> during the attempted read. If the pipe is non-blocking, there might not
> be any data there during the read. Both scenarios can (and do) result
> in obscure errors.
>
> Don't worry - it took almost 7 years before someone pointed out that
> mistake to me. Using select() properly is actually much less intuitive
> than the documentation makes it out to be. Basically - if you are using
> it to poll some state of a socket/pipe/resource thinking a future call
> will succeed based on the output of select(), you are using the function
> incorrectly and the application design is flawed. Using it incorrectly
> also creates very ugly code while using it correctly creates much more
> elegant code.
>
> --
> Thomas Hruska
> CubicleSoft President
> Ph: 517-803-4197
>
> *NEW* MyTaskFocus 1.1
> Get on task. Stay on task.
>
> http://www.CubicleSoft.com/MyTaskFocus/<http://www.cubiclesoft.com/MyTaskFocus/>
>
>
>
[Non-text portions of this message have been removed]
