The third edition of Numerical Recipes moved from C to C++. You can find legal, purchasable source code, but pirated copies do exist on GitHub. The official code is available from the Numerical Recipes website, not GitHub.
Missing header includes, implicit function declaration warnings. Add #include <stdlib.h> and #include <stdio.h> where missing. numerical recipes in c github
Compute an FFT of a sine wave or invert a small matrix. Compare with output from a trusted tool like Octave or Python NumPy. The third edition of Numerical Recipes moved from C to C++