implement first order hold

What are you trying to Do?!?

A first-order hold is most useful to model the action of a DAC at the interface of a discrete-time digital control system with the outside world. In fact, I think of it as a mathematical construct which takes impulses as input and outputs rectangular pulses that just happen to be at the system sampling rate -- as such it is doomed to forever be a mathematical construct, because impulses do not exist in reality.

The closest thing that I can think of to a first-order hold would be a sample-and-hold block, whose behavior in system-modeling language would be a sampler followed by a hold. A sample-and-hold would be an easy thing to implement in C -- maintain a state variable for it, and only update the state on the samples that you're supposed to remember.