TOMS291 is a C++ program which approximates the logarithm of the Gamma function.
The text of many ACM TOMS algorithms is available online through ACM: http://www.acm.org/pubs/calgo or NETLIB: http://www.netlib.org/toms/index.html.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
TOMS291 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA245, a C++ library which evaluates the logarithm of the Gamma function.
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POLPAK, a C++ library which evaluates a variety of special functions.
SPECFUN, a FORTRAN77 library which evaluates a variety of special functions.
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TOMS708, a FORTRAN90 library which evaluates several special functions, including the logarithm of the Gamma function.
You can go up one level to the C++ source codes.