To use the program, put it into a directory and type

  gcc ising.c -o ising.out -lm

in order to compile the code. Then typing

  ./ising.out specificHeat 1.2214 1

should produce the output

  0.097650

In place of specificHeat you can say freeEnergy or internalEnergy. The
two numerical arguments are x and y. You can also say

  ./ising.out eigenvalue 10 1.2214 1

which should produce

  1548.898450

In this case, the first numerical argument is the dimension, the second
and third are x and y. Thanks to the numerics, you can go pretty far
with the dimension. Even 25 takes just a minute or so.