UML is most commonly used for modelling system by using C++. In my projects C is the implementation language. I am looking for resources on UML strategies which are applicable for C. I want to use UML during design and represent the different aspects of the system.
I don't know of any existing resources that discuss using UML specifically for C. As others have mentioned, UML is language-agnostic.
Keep in mind that with UML, you can have a model for the problem domain, and another for the implementation. Try not to model the problem domain in terms of C, but rather as high-level OO. Once you understand the problem domain adequately enough, you can begin modeling an implementation.
For modeling procedural-style C implementations, the following diagrams could be useful:
Expanding on class diagrams, you can "abuse" them in the following way for procedural-style C:
Experiment, and you'll find your own conventions for abusing UML.