Ipopt is a popular nonlinear interior point method package for solving general nonlinear optimization problems. It is written in C++ and has a C interface available.
Visit the Ipopt site https://github.com/coin-or/Ipopt for download instructions for Ipopt. Note that Ipopt has several dependencies (in particular the HSL libraries) that need to be downloaded and put in the correct Ipopt folders before installing Ipopt. Ipopt tarballs are available at https://www.coin-or.org/download/source/Ipopt/
Refer to the Ipopt installation instructions at https://coin-or.github.io/Ipopt/INSTALL.html