GNU libmatheval is a library which contains several procedures that make it possible to create an in-memory tree from the string representation of a mathematical function over single or multiple variables. This tree can be used later to evaluate a function for specified variable values, to create a corresponding tree for the function derivative over a specified variable, or to write a textual tree representation to a specified string. The library exposes C and Fortran 77 interfaces. This package is unmaintained upstream.