Btparse is the C component of btOOL, a pair of libraries for parsing and
processing BibTeX files.  Its primary use is as the back-end to the
Text::BibTeX library for Perl (the other half of btOOL), but there's
nothing to prevent you from writing C programs using btparse -- or from
writing extensions to other high-level languages using btparse as a
back-end.  There's even copious documentation on using the library in
the "doc" directory.