ASA032 is a C library which evaluates the incomplete Gamma function, by G Bhattacharjee.
ASA032 is Applied Statistics Algorithm 32. Source code for many Applied Statistics Algorithms is available through STATLIB.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
ASA032 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA147, a C library which evaluates the incomplete Gamma function.
ASA239, a C library which evaluates the incomplete Gamma function.
PROB, a C library which evaluates and inverts a number of probabilistic distributions.
TEST_VALUES, a C library which contains sample values for a number of distributions.
Original FORTRAN77 version by G Bhattacharjee; C version by John Burkardt.
You can go up one level to the C source codes.