Hi Aparna,
In order to zoom in on the root cause of your problem, it would be helpful to have more information on your application. Have you tried calling your C DLL from any other programming languages? What exactly are the inputs and outputs (along with datatypes) of your DLL? Does LabVIEW give any error message when it "hangs"? I am sure that we can work together to resolve this issue.
Regards,
Casey Weltzin
Applications Engineer
National Instruments