Hello Eli,

I am new to the code base, and it was just a suggestion to check for the ioctl call for any failures and take precautions, maybe inform the user of the issue that the suspend-emacs command did not run correctly. Since the command was not printing out anything when called with 'M-: (suspend-emacs "pwd")' I was not sure what was happening.

For users that use the linux kernel 6.2+ the ioctl command does not work correctly. When the STUFFSTRING is passed to the suspend-emacs command, it does nothing. The problem with kernel 6.2+ is that it requires CAP_SYS_ADMIN capability (https://www.man7.org/linux/man-pages/man2/TIOCSTI.2const.html).

In the code at some point suspend-emacs command calls stuff_char, which uses ioctl (code snippet below, from https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). The ioctl call fails. But in the code there is no check for that, that might confuse people that use linux kernel 6.2+. I wanted to use suspend-emacs with "pwd" as the string (as shown in the example in the documentation in https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html). But the "pwd" command was not displaying anything. Because it was not being sent/stuffed to the superior process.

/* Should perhaps error if in batch mode */
#ifdef TIOCSTI
  ioctl (fileno (CURTTY()->input), TIOCSTI, &c);
#else /* no TIOCSTI */
  error ("Cannot stuff terminal input characters in this version of Unix");
#endif /* no TIOCSTI */

I have downloaded the code for emacs 29.4 and compiled it with a simple error check and the ioctl call failed on my system silently (with kernel 6.6.47).

To add a check is of course for the people who develop emacs to decide. I have seen the code today, and am not in a position to suggest anything at this stage. Maybe there would be a better way of handling this error.

Adding an error check would be one of the solutions.

The other solution might be to add a piece of information to the documentation for suspend-emacs, (and maybe to the documentation string for suspend-emacs) regarding this problem. So that people that use linux kernel 6.2+ would be aware of this issue.

The issue is fixable by just setting the dev.tty.legacy_tiocsti sysctl variable to 1. This variable was set to 0 on my system. Hence the ioctl call for TIOCSTI was failing, and no indication of the error was given. After I have read the man page for TIOCSTI, and set the sysctl variable dev.tty.legacy_tiocsti to 1 was I able to call suspend-emacs with a STUFFSTRING and it worked. It was displaying the output of the "pwd" command.

Regards
Riza


On Sat, Sep 7, 2024, 18:20 Eli Zaretskii <eliz@gnu.org> wrote:
> From: Riza Dindir <riza.dindir@gmail.com>
> Date: Sat, 7 Sep 2024 15:37:17 +0300
>
> I am running Linux with kernel 6.6.47 and am running emacs in xterm, using the -nw command line argument.
>
> I am new to emacs and was experimenting with the suspend-emacs command. Following the example on
> https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html.
>
> When following the example, I added the suspend-resume-hook to my .emacs.d/init.el file. When I run M-:
> (suspend-emacs "pwd") it does not show the current working directory. But when I do fg from the terminal that I
> got into, I see the "Resumed!" message.
>
> I asked in the libera chat about that, and also in the gnu-help-emacs list. I have been talking to wasamasa on
> libera chat (#emacs-beginners) and we pinpointed the problem to the stuff_char function (in
> https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). We came to this point from
> stuff_buffered_input (https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11963), and from
> suspend-emacs function definition (in https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11908).
>
> The stuff_char function is using ioctl with TIOCSTI. TIOCSTI requires CAP_SYS_ADMIN capability. You can set
> this capability using sysctl setting dev.tty.legacy_tiocsti to 1.
>
> Unless I had set "dev.tty.legacy_tiocsti" to 1 I could not run the suspend-emacs command with an argument
> string.
>
> Either emacs can check the return value of ioctl in stuff_char and if there return value is EPERM, then handle
> this accordingly, with a message regarding the problem.

You mean, you want suspend-emacs signal an error if it is called with
STUFFSTRING argument, but fails to stuff the string into the
terminal's input buffer?