Comment by CamperBob2

     if (sscanf(string,"%lf",&foo) != 1) 
        { 
        bad_float(); 
        }

This has the benefit of handling scientific notation, too, which some of the other CRTL functions inexplicably do not.

For extra reassurance, initialize foo to NaN or DBL_MAX or something, and verify that it is no longer equal to that value.