Spar is a modular math parser that comes with a fully featured text frontend,
something like an interactive calculator. It also provides a simple but
complete API for C/C++ programmers. This API provide a complete interface to
a top down parser that supports: variables, constants, functions, modules and
a libc based math library. This math library overrides standard functions with
a "custom" version (such as. deg, rad auto conversion, infinite symbol,
function domain...). 

Spar, can be easily extended with modules. A module is a "user" extensions,
written in C/C++, to the basic features of the Simple Parser. Here you can
find the module definition: is a C structure that contains only few data
member: 
    - module name 
    - module version 
    - module description 
    - a pointer to the module main function 

The modules are loaded at run-time by the module loader (that is a high level
layer of the module manager): you can load your modules, simply, coping theirs
paths into the .spar_modules (or any other file, with the -c option).

WWW: http://spar.sourceforge.net/