io: replace local_isatty() with a proper function __isatty_nostatus()

Replace local_isatty() inlined in libio with a proper function
__isatty_nostatus(). This allows simpler system-specific
implementations that don't need to touch errno at all.

Note: I left the prototype in include/unistd.h (the internal header
file.) It didn't much make sense to me to put it in a different header
(not-cancel.h), but perhaps someone can elucidate the need.

Add such an implementation for Linux, with a generic fallback.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/isatty_nostatus.c b/sysdeps/unix/sysv/linux/isatty_nostatus.c
new file mode 100644
index 0000000..7f110be
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/isatty_nostatus.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
+__isatty_nostatus (int fd)
+{
+  struct __kernel_termios k_termios;
+  return INTERNAL_SYSCALL_CALL (ioctl, fd, TCGETS, &k_termios) == 0;
+}