The answer seems to be related to my problem yesterday with strcpy:
https://github.com/khowe/quicktree/issues/8
https://github.com/khowe/quicktree/blob/master/src/distancemat.c
Once I hear back from the vendor, I'll know for sure.
The github repo seems to be a for a biological algorithm. :)
The issue is more clearly a length issue in this example but the D3 issue could be that the forked process can't get access to the D3 memory. Maybe the forks aren't running as root. Not sure how forks work to be honest.