non-blocking I/O on serial port problems

[Michael D]

I have a board which has a fax/modem connected to the RS-232 serial port.  I have tried mgetty+fax and efax-0.9.  Both applications want some kind
of non-blocking IO on the serial port.

If I enable non-blocking io, the serial port does not always return with 
any data...

Anybody else ever see this or have any idea why?

For a quick example, below is a snippet from efaxos.c

This code never returns.

Any ideas.
Thanks,

-Mike

int tdata ( TFILE *f, int t)
{
fd_set fds ;

struct timeval timeout ;

if ( f->fd < 0 ) msg ( "Ecan't happen (faxdata)" ) ;

timeout.tv_sec  = t / 10 ;
timeout.tv_usec = ( t % 10 ) * 100000 ;

FD_ZERO ( &fds ) ;
FD_SET ( f->fd, &fds ) ;

do {
  n = select ( f->fd + 1, &fds, 0, 0, t<0 ? 0 : &timeout ) ;
  if ( n < 0 ) {
    if ( errno == EINTR ) {
      msg ( "W0  select() interrupted in tdata()" ) ;
    } else {
      err = msg ( "ES2 select() failed in tdata():" ) ;
    }
  }
} while ( n < 0 && ! err ) ;

return n ;

}