Re: Syscall kill() can send signal to thread ID

Eric W. Biederman
2022-09-22T15:33:39Z

cambda@linux.alibaba.com writes:

> I found syscall kill() can send signal to a thread id, which is
> not the TGID. But the Linux manual page kill(2) said:
>
> "The kill() system call can be used to send any signal to any
> process group or process."
>
> And the Linux manual page tkill(2) said:
>
> "tgkill() sends the signal sig to the thread with the thread ID
> tid in the thread group tgid. (By contrast, kill(2) can be used
> to send a signal only to a process (i.e., thread group) as a
> whole, and the signal will be delivered to an arbitrary thread
> within that process.)"
>
> I don't know whether the meaning of this 'process' should be
> the TGID? Because I found kill(tid, 0) will return ESRCH on FreeBSD,
> while Linux sends signal to the thread group that the thread belongs
> to.
>
> If this is as expected, should we add a notice to the Linux manual
> page? Because it's a syscall and the pids not equal to tgid are not
> listed under /proc. This may be a little confusing, I guess.

How did you come across this? Were you just experimenting?

I am wondering if you were tracking a bug, or a portability problem
or something else. If the current behavior is causing problems in
some way instead of just being a detail that no one really cares about
either way it would be worth considering if we want to maintain the
current behavior.

Eric