[linux-dvb] Re: Help on use of DMX_OUT_TAP (II)

["Tim Wiffen" <tw99@doc.ic.ac.uk>]

I am not convinced that the poll method is better.  When I switched from
GCC 2.95 to GCC 3.2 I found that dvbtune stopped being able to read PSI
tables.  This code:

struct pollfd ufd;

** SNIP **

fd_pat = open(demuxdev[card],O_RDWR|O_NONBLOCK)

** SNIP **

ufd.fd=fd_pat;
ufd.events=POLLPRI;
if (poll(&ufd,1,2000) < 0) {
   fprintf(stderr,"TIMEOUT reading from fd_pat\n");
   close(fd_pat);
   return;
}
if (read(fd_pat,buf,3)==3) {
	
** SNIP **

} else {
  fprintf(stderr,"Nothing to read from fd_pat\n");
}

Always returns "Nothing to read from fd_pat" when compiled on my system
with GCC 3.2 but the binary compiled with GCC 2.95 works perfectly on
the same system.  The error returned was always "Resource temporarily
unavailable".  I found the following code, inspired by scan that came
with the DVB drivers, works with both versions of GCC, although I can
see it is less elegant.

time_t now = time(0);
n = -1;
while( n < 0 && time(0) < now + 10 ){
  if (read(fd_pat,buf,3)==3) {
      
	** SNIP **

  	close(fd_pat);
  	return;
  }
}
close(fd_pat);
fprintf(stderr,"Nothing to read from fd_pat\n");


I have no real explanation why one should work and the other not.

-----Original Message-----
From: linux-dvb-bounce@linuxtv.org [mailto:linux-dvb-bounce@linuxtv.org]
On Behalf Of jng_junk@greenmail.demon.co.uk
Sent: 02 June 2003 18:58
To: linux-dvb@linuxtv.org
Subject: [linux-dvb] Re: Help on use of DMX_OUT_TAP (II)

On Mon, 2 Jun 2003, Johannes Stezenbach wrote:

> jng_junk@greenmail.demon.co.uk wrote:
> ...
> >   if (type == DMX_PES_AUDIO) flags |= O_NONBLOCK;
> ...
> >   while (1) {
> >     int bytes;
> > 
> >     /* Video. */
> >     bytes = read(video_fd, buf, sizeof(buf));
> >     if (bytes < 0) {
> > 	fprintf(stderr, "Errno %d: ", errno);
> > 	perror("video_fd read");
> >     } else {
> >       fprintf(stderr, "Read %d bytes from video_fd.\n", bytes);
> >     }
> > 
> >     /* Audio. */
> >     bytes = read(audio_fd, buf, sizeof(buf));
> >     if (bytes < 0) {
> >       if (errno != EAGAIN) {
> > 	fprintf(stderr, "Errno %d: ", errno);
> > 	perror("audio_fd read");
> >       }
> >     } else {
> >       fprintf(stderr, "Read %d bytes from audio_fd.\n", bytes);
> >     }
> >   }
> 
> This can't work. You must use select() or poll() to wait for
> data to arrive, then read() from the file descriptor which is ready.

Thanks for the advice. I'll give this a try, but I'm not sure if it'll 
make any difference. I made the audio stream non-blocking, and since it
is 
a much lower data rate than the video stream, I think it should work.
But 
I agree select() it a neater way of doing it.

I'll let you know,

James.



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