The NRLMSIS-00 empirical atmosphere model was developed by Mike Picone, Alan Hedin, and Doug Drob. It describes the neutral temperature and densities in Earth's atmosphere from ground to thermospheric heights.
The authors of NRLMSISE-00 have released a FORTRAN version which is available at http://uap-www.nrl.navy.mil/models_web/msis/msis_home.htm.
Based on the Official Beta Release 1.0 (NRLMSISE-00.DIST12.TXT) Dominik Brodowski wrote an implementation in C. The C release you can find here is updated to the Official Release 2.0 (NRLMSISE-00.DIST17.TXT).
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Please inform the maintainer of the C release (Dominik Brodowski - firstname.lastname@example.org) of any patches and bug-fixes you implement for NRLMSISE-00 so that this C package can be updated with these improvements.
20040322: fix gtd7d output if sw->flags is set (noted by Dr. Vasiliy Yurasov)
20041227: bugfix against memory corruption (Donald F. Linton)
20070727: bugfix concerning DFA being masked (Stacey Gage)
20100516: bugfix concerning Argon densities (Dr. Choliy Vasyl)
20131225: fix comment on switch 0, silence compiler warnings (David F. Crouse)