This function marks the current thread (the thread in which it is called) as uninterruptible, and/or determines whether or not any child threads, subsequently created by the current thread, will be uninterruptible.

The right argument Y is an integer whose value is the sum of the following (bit-wise) values:

• 1 : mark thread as uninterruptible
• 2 : mark its children as uninterruptible

The result R is an integer value that indicates the previous state of the thread.

In many multi-threaded applications a large proportion of the threads are used for communication mechanisms (⎕DQ on TCPSockets, Conga, isolates); but most of the "real work" is done in thread zero.

It is undesirable that a weak interrupt interrupts a seemingly random thread. The mechanism to prevent a thread from being (weak) interrupted allows an application to be configured so that only specific threads would respond to a weak interrupt.