GNU libmatheval is a library (callable from C and Fortran) to parse and evaluate symbolic expressions input as text. It supports expressions in any number of variables of arbitrary names, decimal and symbolic constants, basic unary and binary operators, and elementary mathematical functions. In addition to parsing and evaluation, libmatheval can also compute symbolic derivatives and output expressions to strings.
GNU libmatheval development sources from Mercurial repository are available for browsing through corresponding Savannah Web interface. Sources could be also fetched from repository by Mercurial command:
hg clone http://hg.sv.gnu.org/hgweb/libmatheval/
For other ways to obtain GNU libmatheval, please see How to get GNU Software.
The GNU libmatheval manual is available online in a number of formats.
The GNU libmatheval Savannah page has some additional information about GNU libmatheval.
GNU libmatheval bug reports and fixes, as well as improvement suggestions, should be directed to bug-libmatheval mailing list.
GNU project incorporates some other libraries related to mathematical calculations. Most notable of these is GNU Scientific Library.