Question

Imagine that you have a Windows process A running with priority AboveNormal that starts another process B without specify the priority. Is the priority of the process B inherited from the priority of the process A? So, what will be the priority on the process B? AboveNormal, Normal, or another?

Answer 1

If not specified, the priority class is inherited, in some cases.

It occurs when the calling process is in the IDLE_PRIORITY_CLASS or BELOW_NORMAL_PRIORITY class. Otherwise, it gets NORMAL_PRIORITY_CLASS unless specified during the CreateProcess call.

MSDN says on dwCreationFlags of CreateProcess:

This parameter also controls the new process’s priority class, which
is used to determine the scheduling priorities of the process’s
threads. For a list of values, see GetPriorityClass. If none of the
priority class flags is specified, the priority class defaults to
NORMAL_PRIORITY_CLASS unless the priority class of the creating
process is IDLE_PRIORITY_CLASS or BELOW_NORMAL_PRIORITY_CLASS. In this
case, the child process receives the default priority class of the
calling process.

The answer is therefore NORMAL priority class. Basically anything that is above normal doesn’t get passed to the child.