ASA063 is a C library which evaluates the incomplete Beta function, by KL Majumder and G Bhattacharjee.
ASA063 is Applied Statistics Algorithm 63. 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.
ASA063 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version and a Python version.
ASA109, a C library which inverts the incomplete Beta function.
ASA226, a C library which evaluates the CDF of the noncentral Beta distribution.
BETA_NC, a C library which evaluates the CDF of the noncentral Beta distribution.
TEST_VALUES, a C library which stores a few values of various mathematical functions.
TOMS179, a C library which evaluates the incomplete Beta function.
Original FORTRAN77 version by KL Majumder, GP Bhattacharjee; C version by John Burkardt.
You can go up one level to the C source codes.