Thanks @kdunwoody, @CoRfr.
Sorry for the slow response - I think I finally got to the bottom of it.
So, it was a resource leak caused by my misuse of file fd IN and not another a bug in mkTools.
I setup a test to continuously upload the same file and noticed that the sent fd was always the same number but the received fd would steadily increment with each IPC API call. Things stopped working once the received fd reached 255 and subsequent attempts gave this message.
Ancillary data was discarded because it couldn't fit in our buffer.
I was only closing the open file descriptor in the sender’s address space when I (thought I was but) really should have closed it in the receiver’s address space. The IPC API takes care of closing the original fd in the sender’s address space.
The culprit was fdopen(dup(fd_in),...) in the receiver. Not sure why I was using dup() there.
Anyway, all sorted now.
Thanks again for the support.