BESSELJ is a C library which evaluates Bessel J functions of noninteger order.
The computer code and data files made available on this web page are distributed under the GNU LGPL license.
BESSELJ is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
FN, a C library which approximates elementary and special functions using Chebyshev polynomials; functions include Airy, Bessel I, J, K and Y, beta, confluent hypergeometric, error, gamma, log gamma, Pochhammer, Spence; integrals include hyperbolic cosine, cosine, Dawson, exponential, logarithmic, hyperbolic sine, sine; by Wayne Fullerton.
POLPAK, a C library which evaluates certain mathematical functions, especially some recursive polynomial families.
TEST_VALUES, a C library which stores values of many mathematical functions, and can be used for testing.
You can go up one level to the C source codes.