The flags parameter

Updated: October 26, 2022

The last parameter, flags, controls all kinds of things:

_NTO_INTR_FLAGS_END
Indicates that this handler should go after other handlers that may be attached to the same interrupt source.
_NTO_INTR_FLAGS_NO_UNMASK
(QNX Neutrino 6.6 or later) Indicates that the kernel should leave the interrupt masked. Normally, InterruptAttach() and InterruptAttachEvent() automatically unmask an interrupt the first time something is attached to it. If you specify _NTO_INTR_FLAGS_NO_UNMASK, the kernel leaves the interrupt masked, and you must specifically call InterruptUnmask() to enable it.
_NTO_INTR_FLAGS_PROCESS
Indicates that this handler is associated with the process rather than the thread. What this boils down to is that if you specify this flag, the interrupt handler will be automatically dissociated from the interrupt source when the process exits. If you don't specify this flag, the interrupt handler will be dissociated from the interrupt source when the thread that created the association in the first place exits.
_NTO_INTR_FLAGS_TRK_MSK
Indicates that the kernel should track the number of times the interrupt has been masked. This causes a little more work for the kernel, but is required to ensure an orderly unmasking of the interrupt source should the process or thread exit.
_NTO_INTR_FLAGS_EXCLUSIVE
(QNX Neutrino 7.0.4 or later) Indicates that the interrupt handler or event currently attached to the interrupt vector is the only one that can be attached.