Look for sources titled "C for Scientists" or "C for engineers". The first google hit contains answers to your questions: (http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px270/week2/week_2a.pdf).
Have also a look at (http://sccn.ucsd.edu/labinfo/computer-faq/Numerical_Recipes.html), your paths may be different.