The IDL function magmax is only an interface for passing the parameters on to library routines in the files calc_lib.so and magnitude.so. The code for these files was written in C. This approach was used to safe runtime. calc_lib.so contains the functions for the calculation of the distance modulus. The IDL function magmax uses the IDL function calc_dm to get the distance modulus and calc_dm then uses the functions in calc_lib.so. For further reference on calc_dm, check it's documentation at http://panisse.lbl.gov/groupwork/documentation/cosmo/index.html