The UDUNITS package supports units of physical quantities. Its C library provides for arithmetic manipulation of units and for conversion of numeric values between compatible units. The package contains an extensive unit database, which is in XML format and user-extendable. The package also contains a command-line utility for investigating units and converting values.

UDUNITS is an Open Source project led by developers at the Unidata Program Center. If you're interested in contributing to the project, you might be interested in these resources:

• UDUNITS code
  in GitHub
• Git Recipes
  for developers