Sometimes R users need to encounter C, for example, to maintain an R package, or to speed up existing functions. It is fairly straightforward to run C code from within R. Here is how, with some easy-to-follow examples. More detailed information can be found in Writing R Extensions.
Everything should be already installed. You may need to install developer tools on Mac if they're not already there.
Install Rtools to get the necessary toold to compile code and build packages for R. Be careful if you already have Cygwin installed.
From the command line:
R CMD SHLIB foo.c
(you may need the whole path to R if it's not in your path)
To load what you just built to use in R:
file.path(path, paste("foo", .Platform$dynlib.ext, sep=""))