[PATCH RFC v3 2/8] io: make __local_isatty () a function; stub should return 0

[Adhemerval Zanella Netto]

On 12/05/25 19:04, H. Peter Anvin wrote:
> On 5/8/25 10:39, Adhemerval Zanella Netto wrote:
>>>
>>> diff --git a/include/unistd.h b/include/unistd.h
>>> index e241603b8131..18d56bc4ee0d 100644
>>> --- a/include/unistd.h
>>> +++ b/include/unistd.h
>>> @@ -152,6 +152,7 @@ libc_hidden_proto (__ttyname_r)
>>>   extern __pid_t _Fork (void);
>>>   libc_hidden_proto (_Fork);
>>>   extern int __isatty (int __fd) attribute_hidden;
>>> +extern int __local_isatty (int __fd) attribute_hidden;
>>
>> For other internal implementation that do not set errno, usually they
>> are name as __symbol_nocancel (like the ones in not-cancel.h).
>>
> 
> Wouldn't that imply additional properties? Specifically, what would be required to prevent a cancellation point from existing there? And should there be?

Sorry, I meant __symbol_nostatus instead of __symbol_nocancel. And although
not-cancel.h header was initially used to handle cancellation, recently it
is also used for wrapper to syscall to avoid errno handling and/or return
status. I might rename it to something more representative.

> 
>>
>> The idea of stub implementation is to future pots, although it is unlikely
>> we will see any in the future.  So ENOSYS is to advertise that if you deploy
>> glibc and programs are calling isatty, you will need to proper implement it
>> instead of handling a ENOTTY.
>>
>> So I think this change is not required.
> 
> OK. I guess it would make sense to, if the need ever comes up, have an explicit opt-in for a no-tty platform (which perhaps should inline isatty() et al) and not trigger a stub warning.

Although we did have support for such platforms in the past (NaCl), I really
think this is highly unlikely we will require support for this in the
future.

> 
>>> diff --git a/sysdeps/unix/sysv/linux/local_isatty.c b/sysdeps/unix/sysv/linux/local_isatty.c
>>> new file mode 100644
>>> index 000000000000..3bd5f67ea7da
>>> --- /dev/null
>>> +++ b/sysdeps/unix/sysv/linux/local_isatty.c
>>> @@ -0,0 +1,29 @@
>>> +/* Copyright (C) 1991-2025 Free Software Foundation, Inc.
>>> +   This file is part of the GNU C Library.
>>> +
>>> +   The GNU C Library is free software; you can redistribute it and/or
>>> +   modify it under the terms of the GNU Lesser General Public
>>> +   License as published by the Free Software Foundation; either
>>> +   version 2.1 of the License, or (at your option) any later version.
>>> +
>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>> +   Lesser General Public License for more details.
>>> +
>>> +   You should have received a copy of the GNU Lesser General Public
>>> +   License along with the GNU C Library; if not, see
>>> +   <https://www.gnu.org/licenses/>.  */
>>> +
>>> +#include <sysdep.h>
>>> +#include <termios.h>
>>> +#include <kernel_termios.h>
>>> +#include <sys/ioctl.h>
>>> +
>>> +/* Return 1 if FD is a terminal, 0 if not, without changing errno  */
>>> +int
>>> +__local_isatty (int fd)
>>> +{
>>> +  struct __kernel_termios k_termios;
>>> +  return INTERNAL_SYSCALL (ioctl, 3, fd, TCGETS, &k_termios) == 0;
>>
>> Use INTERNAL_SYSCALL_CALL macro here, to avoid the need to pass the
>> number of arguments.
>>
> 
> OK. Also see previous question :)
> 
>     -hpa
>