The Wolfram Language provides a convenient way to call functions in external C and other programs. With a .tm template specifying Wolfram Language functions corresponding to each C function, you build a WSTP-installable binary using mprep or mcc. You can exchange not only C-like data types such as integers, reals, arrays, and strings, but also arbitrary Wolfram Language expressions.
Install — load a WSTP installable executable to call from your Wolfram System session
Uninstall — close down a WSTP executable, removing its Wolfram Language functions
LinkPatterns — find functions available in a running WSTP program
mcc — complete command-line script for building WSTP programs (Unix, Mac OS X)
WSMain() — special version of main() for WSTP-installable C programs