ASA121 is a C library which evaluates the trigamma function, by BE Schneider.
ASA121 is Applied Statistics Algorithm 121. 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.
ASA121 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA103, a C library which evaluates the digamma function.
TEST_VALUES, a C library which contains sample values for a number of special functions.
Original FORTRAN77 version by BE Schneider; C version by John Burkardt.
You can go up one level to the C source codes.