Date: Mon, 14 Aug 2000 14:20:03 -0700 (PDT) From: "Daniel M. Eischen" <deischen@gdeb.com> To: freebsd-ports@FreeBSD.org Subject: Re: ports/20490: Termios timeout parameters, VMIN, VTIME, dont work with Python. Message-ID: <200008142120.OAA11140@freefall.freebsd.org>
next in thread | raw e-mail | index | archive | help
The following reply was made to PR ports/20490; it has been noted by GNATS.
From: "Daniel M. Eischen" <deischen@gdeb.com>
To: freebsd-gnats-submit@FreeBSD.org, r_chipi@yahoo.com
Cc: tg@melaten.rwth-aachen.de
Subject: Re: ports/20490: Termios timeout parameters, VMIN, VTIME, dont work with
Python.
Date: Mon, 14 Aug 2000 17:13:39 -0400
I don't think this is a libc_r problem. The threads library sets
all file descriptors to non-blocking and uses poll() to determine
when data is ready. But before poll()ing, the threads library
performs a read() on the non-blocking socket. It expects the
return value to be -1 with errno set to EAGAIN if there is no
data ready. If you set the file to non-blocking in the example
provided, read() returns 0 instead of -1.
Another problem to arise, even if this is fixed, is that our
current threads library knows nothing of VMIN/VTIME and tcsetattr().
Since the file descriptor is non-blocking, the timer will not
be effective, and the expected 20 second timeout will not occur.
I'd expect the example program to block indefinitely until the
requested (40 bytes of) data arrives. This will work correctly
when we get scheduler activations. I don't think it is worth the
time or effort to get the threads library to wrap tcsetattr() and
emulate the effects of VMIN/VTIME. If you want this to work with
our current threads library, I would suggest recoding it to use
select() or poll() to give you the needed timeout (and not use
VMIN/VTIME).
Here's the modified example:
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <fcntl.h>
#include <assert.h>
#include <errno.h>
main()
{
struct termios term;
int fd, bytes, flags;
char buf[80];
fd = open("/dev/cuaa1", O_RDWR);
if (fd < 0)
{
perror("open");
return 1;
}
tcgetattr(fd, &term);
term.c_cflag = CS8 | CREAD | CLOCAL;
term.c_oflag &= ~OPOST;
term.c_lflag = 0;
term.c_cc[VMIN] = 0;
term.c_cc[VTIME] = 200;
tcsetattr(fd, TCSANOW, &term);
assert((flags = fcntl(fd, F_GETFL, 0)) != -1);
assert(fcntl(fd, F_SETFL, flags | O_NONBLOCK) == 0);
bytes = read(fd, buf, 40);
printf("Read returned %d, errno = %d\n", bytes, errno);
buf[bytes] = 0;
puts(buf);
return 0;
}
--
Dan Eischen
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-ports" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200008142120.OAA11140>