I have never understood 3.2.2.2 to mean that a null signal can be sent to
detect the validity of a pid *as long as you have signal priveledges on
that pid*. It seems at least that the INTENT of the null signal is to
allow any process to determine if a pid exists.
Most programs use this, for example, to see if other instances of them-
selves are running. They check for a pid file, and if it exists, they
read it. Then they send a null signal to the pid to see if such a
process really exists. While this isn't foolproof, it does provide one
extra layer of assurance.
Since 3.2.2.2 implies that this is acceptable, many people are assuming
it works, and these programs will break on Linux. In fact, Linux is the
only OS I've heard of that treats the null signal this way. We should
probably consider this broken -- and fix it.
Robey