The Information Center File system considerations for multithreaded programming topic is clear:
The QDLS file system is not threadsafe.
You can’t use that programming to do that task in a subsystem that allows multi-threading. And you might not be able to run that program in a subsystem that doesn’t allow multiple threads. It doesn’t matter how you compile your program. The functions that you are accessing through that program are where the problem is.
And again, why are you using SNDDST? It’s not appropriate for what you’re doing. Use an e-mail function instead, and stop using QDLS.
Tom