One easy example of Reactive Programming.

reactive_programming_sample.c

#include <stdio.h>
#include <stdlib.h>

typedef struct ReactiveInt* ReactiveInt;

struct ReactiveInt
{
  int n;
  int (*operator)(int, int);
  ReactiveInt left, right;
};

#define ReactiveNum(n)    NewReactive((n), NULL, NULL, NULL)
#define ReactiveAdd(l, r) NewReactive(0, Add, (l), (r))
#define ReactiveSub(l, r) NewReactive(0, Sub, (l), (r))
#define ReactiveMul(l, r) NewReactive(0, Mul, (l), (r))
#define ReactiveDiv(l, r) NewReactive(0, Div, (l), (r))

int Add(int a, int b) { return a + b; }
int Sub(int a, int b) { return a - b; }
int Mul(int a, int b) { return a * b; }
int Div(int a, int b) { return a / b; }

ReactiveInt NewReactive(int n, int (*operator)(int, int), ReactiveInt left, ReactiveInt right)
{
  ReactiveInt ri = malloc(sizeof(struct ReactiveInt));
  ri->n        = n;
  ri->operator = operator;
  ri->left     = left;
  ri->right    = right;
  return ri;
}

int PullValue(ReactiveInt a)
{
  if (a->operator == NULL) {
    return a->n;
  } else {
    return a->operator(PullValue(a->left), PullValue(a->right));
  }
}

void SetReactiveNum(ReactiveInt a, int n)
{
  a->n = n;
}

void main()
{
  ReactiveInt a, b, c, d, e, f, g;

  a = ReactiveNum(1);    // a = 1
  b = ReactiveNum(2);    // b = 2
  c = ReactiveNum(3);    // c = 3
  d = ReactiveNum(4);    // d = 4
  e = ReactiveAdd(a, b); // e = a + b
  f = ReactiveSub(c, d); // f = c - d
  g = ReactiveMul(e, f); // g = e * f

  printf("%d\n", PullValue(g)); // print g
  SetReactiveNum(a, 100);       // a = 100
  printf("%d\n", PullValue(g)); // print g
}