C program that calculates the normal cumulative distribution function values

normal_table.c

// compilation: gcc -Wall -std=c99 -lm normal_table.c -o normal_table
#include <stdio.h>
#include <time.h>
#include <math.h>

#define pi 3.14159265359

float valor(double [], double);

int main(void) {
  unsigned short int a = 16807;
  unsigned long int m = 2147483647, x = time(NULL), t = time(NULL);
  double u[250000];

  for (int i = 0; i < 250000; i++) {
    x = (a * x) % m;
    u[i] = (double) x / m;
  }

  FILE *arq = fopen("/tmp/table", "w");

  for (int i = 0; i < 10; i++)
    fprintf(arq, "\t%.2f", (float) i / 100);

  for (int i = 0; i < 30; i++) {
    fprintf(arq, "\n%.1f", (float) i / 10);
    for (int j = 0; j < 10; j++)
      fprintf(arq, "\t%.4f", valor(u, (float) i / 10 + (float) j / 100));
  }

  printf("Table generation time: %ldmin %lds\n", (time(NULL) - t) / 60,
    (time(NULL) - t) % 60);

  return 0;
}

float valor(double y[], double b) {
  double *p, s = 0;

  for (p = &y[0]; p < &y[250000]; p++)
    s += exp(-pow((*p * b), 2) / 2) * b / sqrt(2 * pi);

  s /= 250000;

  return s;
}