What if I just use userland kill(1) utility? Is it possible to kill all processes under a PGID using kill(1)? Say the PGID I get for sh is 321. If I do

What if I just use userland kill(1) utility? Is it possible to kill all processes under a PGID using kill(1)? Say the PGID I get for sh is 321. If I do kill [signal] 321 that will not kill all the processes having PGID 321? If it would not kill them, then couldn't we modify kill(1) to be able to call kill() with a negative integer as you describe? Sorry for the noob questions. I am still learning and making mistakes.