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Date: Wed, 24 Jul 2024 08:51:34 +0200
From: Greg KH <gregkh@linuxfoundation.org>
To: stsp <stsp2@yandex.ru>
Cc: linux-serial@vger.kernel.org
Subject: Re: [regression] ENOTTY returned for tty fds
Message-ID: <2024072452-pegboard-undying-4245@gregkh>
References: <b6f4aa5c-10ba-411b-994b-6dbed2bf63db@yandex.ru>
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In-Reply-To: <b6f4aa5c-10ba-411b-994b-6dbed2bf63db@yandex.ru>

On Sun, Jul 21, 2024 at 02:52:31PM +0300, stsp wrote:
> Hi!
> 
> I've noticed that one of my progs failed
> to work with serial ports. To distinguish
> between real serial ports and PTS nodes,
> it tried TIOCMBIC, checking errno for EINVAL.
> 
> Obviously that behaviour was changed
> and now ENOTTY is returned. Besides
> being backward-incompatible, I think this
> is also wrong because isatty() returns 1
> on such FDs.
> 
> Here's the test-case:
> socat PTY,link=/tmp/ttyS20,raw,echo=0,b9600
> PTY,link=/tmp/ttyS21,raw,echo=0,b9600
> 
> Then run this program:
> 
> #include <stdio.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <errno.h>
> #include <sys/ioctl.h>
> #include <unistd.h>
> 
> int main() {
>   int fd, err;
>   int data = TIOCM_DTR | TIOCM_RTS;
>   fd = open("/tmp/ttyS20", O_NONBLOCK);
>   if (fd == -1) {
>     perror("open()");
>     return 1;
>   }
>   err = ioctl(fd, TIOCMBIC, &data);
>   if (err && errno == ENOTTY && isatty(fd))
>     printf("Test FAILED: ENOTTY for tty fd\n");
>   return 0;
> }
> 
> 

What caused this change/regression?  And does any real-world programs
rely on this?  What exactly are you trying to determine with this ioctl
test?  Is there a different way to determine that?

thanks,

greg k-h