Very simple 'yield' demo in C for DES

yield.c

#include <stdio.h>
#include <stdlib.h>
#include <setjmp.h>
#include <math.h>
#include <time.h>

typedef struct process {
  double *now;
  double (*run)(struct process *);
  jmp_buf buf;
} process_t;

#define YIELD(t) self->run = &reentry; if (!setjmp(self->buf)) return t;
double reentry(process_t *self) { longjmp(self->buf, 1); }
double rexp(double lambda);

double customer(process_t *self) {
  printf("customer started at: %.3f\n", *self->now);
  YIELD(rexp(1));
  printf("customer finished at: %.3f\n", *self->now);
  return -1;
}

int main(int argc, char *argv[]) {
  double now = 0, delay;
  process_t process = { &now, &customer };
  srand((unsigned)time(NULL));

  while ((delay = process.run(&process)) >= 0)
    now += delay;

  return 0;
}

double rexp(double lambda) {
  double u = rand() / (RAND_MAX + 1.0);
  return -log(1.0 - u) / lambda;
}