> In my app I skipped the call to isc_commit_retaining, then the hang appeared in isc_fetch.

You have a bug in 17th line.

Comparing to this: http://www.ibase.ru/devinfo/fb1tbtech.htm, your
5Gb is next to nothing. So, every hung must be your mistake. But because
you are unable to answer elemental question, I can't help you - no
telepathy in this world, sorry.

SY, SD.