I have gone through the help topics,do we have to compile the AUT either as an open or a closed(self-testing)application for unit testing.Is it not possible to unit test without compiling.
As I have already mentioned,my AUT gives DLL as output and is in C.Please give an example of test harness.
Please make sure you read the sections in the TestComplete help file on Unit Testing. There is a considerable amount of information there on how to set up your module, how to make TestComplete detect your tests, and how to select them in the chooser.
If the DLL has exported functions, you can call them from script by means of the DLL Access plug-in.
You can also create a separate application that loads the generated DLL and calls its functions - you will be able to compile the application in an appropriate way to do Unit Testing for and, hence, for the DLL.