Nondeterministic errors in compiled code under macOS

I’m using: SWI-Prolog version 8.1.26
I am compiling the code with > swipl --goal=main -o tester4 -c

I want the code to:
Behave the same if executed multiple times with exactly the same input data

But what I’m getting is:
The compiled program sometimes hangs and sometimes (with exactly the same input) does not hang. (I assume it is a memory leak in the generate C-Code.)
This error occurred multiple times when using SWI-Prolog version 8.1.26 under macOS version Mojave 10.14.6. But I could not reproduce the error, when using SWI-Prolog version 8.1.26 under Parallels desktop Windows on the same Mac computer, i.e., compiled under Parallels Windows the code produces always the same correct results.

My code looks like this:

As my code contains the solutions to the assignments for my students, pIease ask for the code via email to

You can send the code in a private message. If it can be reproduced and is still relevant (the last fixup tp 8.3 fixed quite a few issues), it is worth a try. Make sure it is complete though, so I can really run it.

Hello Jan,

thank you for your fast response!

Meanwhile, I have also tested the code under opensuse, and I got the same nondeterministic hanging errors as under macOS
(which I could not yet reproduce under Parallels Windows).

Please find attached the .zip-file including the code and 2 screenshots.


Stefan Böttcher

(Attachment Code sometimes hanging under is missing)

In this thread, I can see everything (including the code that should be kept private).
Is this a private channel?


Same under Ubuntu 20.04. Turns out to be a problem with library(time) in terminating the scheduler thread that is used to signal timeouts. I think the same issue may actually cause alarms and call_with_time_limit/2 not to trigger, although I have seen no evidence that this happens.

Pushed a fix (for the clib submodule). Hope you can deal with just an update to the source?

Thank you very much for caring about the code and fixing the bugs!