What happens when I try to use kill with a process id inferior to -1?

Question

I’m studying for an exam and I’m not sure at all about what happens if I use kill with a pid < -1.

Am I allowed to end processes of a group in which I’m not included?

Which processes are affected?

Answer 1

From man page, for information purposes:

If pid is less than -1, then sig is sent to every process in the process group whose ID is -pid.

The answer is generically given, but also applies to this case:

For a process to have permission to send a signal it must either be privileged (under Linux: have the CAP_KILL capability), or the real or effective user ID of the sending process must equal the real or saved set-user-ID of the target process.

In our case, it means that the calling process either has to be given the CAP_KILL capability, or the uid of the calling process has to be the same as the (set)uid of every process you want to terminate.

Sending a kill -56 will try to terminate every process of the group 56, but if one has a different uid from the calling process, it will not be terminated.

Let's say you have the process ids 9000, 9010 and 9020 in the group 56, kill -56 is the same as kill 9000 9010 9020. And if you don't have the same uid as, say, pid 9020, you won't be able to terminate it.