Syscall kill() can send signal to thread ID

cambda
2022-09-22T09:11:18Z

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.

Regards,
Cambda