serial quad in C

quad1.c

// C translation of https://www.nag-j.co.jp/fortran/coarray/code/quad1.f90
#include <stdio.h>
#include <stdlib.h>

double rectangle_rule(double (*f)(double), double from, double to, int nstep)
{
  double integral;
  double x1, x2, y, stepval;
  int i;

  if (to <= from) {
    printf("Invalid from-to\n");
    exit(0);
  };
  stepval = (to - from) / nstep;
  x1 = from;
  for (int i = 0; i < nstep; ++i) {
    x2 = from + (i+1) * stepval;
    y = f(0.5 * (x1 + x2));
    integral += y;
    x1 = x2;
  }
  integral *= stepval;
  return integral;
}

quad1.h

double rectangle_rule(double (*f)(double), double, double, int);

serial.c

// C translation of https://www.nag-j.co.jp/fortran/coarray/code/serial.f90
#include <time.h>
#include <stdio.h>
#include <math.h>
#include "quad1.h"

double f(double x)
{
  return 1.0 / (x * x);
}

int main(void)
{
  const double a = 1.0, b = 10.0;
  const int steps = 1000000000;

  double res, tres;
  double secs;
  clock_t t1, t2;

  t1 = clock();
  res = rectangle_rule(f, a, b, steps);
  t2 = clock();
  printf("Calculated value: %e\n", res);
  tres = 1.0 - 0.1;
  printf("True value (approx): %e\n", tres);
  printf("Relative error): %e\n", fabs((res - tres)/tres));
  secs = (double)(t2 - t1) / (double)CLOCKS_PER_SEC;
  printf("Time taken %e seconds\n", secs);
  return 0;
}