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#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
#include <sys/time.h>

/* Set by the signal handler. */
static volatile int expired;

/* The contexts. */
static ucontext_t uc[3];

/* We do only a certain number of switches. */
static int switches;


/* This is the function doing the work.  It is just a
   skeleton, real code has to be filled in. */
static void
f (int n)
{
  int m = 0;
  while (1)
    {
      /* This is where the work would be done. */
      if (++m % 100 == 0)
        {
          putchar ('.');
          fflush (stdout);
        }

      /* Regularly the expire variable must be checked. */
      if (expired)
        {
          /* We do not want the program to run forever. */
          if (++switches == 20)
            return;

          printf ("\nswitching from %d to %d\n", n, 3 - n);
          expired = 0;
          /* Switch to the other context, saving the current one. */
          swapcontext (&uc[n], &uc[3 - n]);
        }
    }
}

/* This is the signal handler which simply set the variable. */
void
handler (int signal)
{
  expired = 1;
}


int
main (void)
{
  struct sigaction sa;
  struct itimerval it;
  char st1[8192];
  char st2[8192];

  /* Initialize the data structures for the interval timer. */
  sa.sa_flags = SA_RESTART;
  sigfillset (&sa.sa_mask);
  sa.sa_handler = handler;
  it.it_interval.tv_sec = 0;
  it.it_interval.tv_usec = 1;
  it.it_value = it.it_interval;

  /* Install the timer and get the context we can manipulate. */
  if (sigaction (SIGPROF, &sa, NULL) < 0
      || setitimer (ITIMER_PROF, &it, NULL) < 0
      || getcontext (&uc[1]) == -1
      || getcontext (&uc[2]) == -1)
    abort ();

  /* Create a context with a separate stack which causes the
     function f to be call with the parameter 1.
     Note that the uc_link points to the main context
     which will cause the program to terminate once the function
     return. */
  uc[1].uc_link = &uc[0];
  uc[1].uc_stack.ss_sp = st1;
  uc[1].uc_stack.ss_size = sizeof st1;
  makecontext (&uc[1], (void (*) (void)) f, 1, 1);

  /* Similarly, but 2 is passed as the parameter to f. */
  uc[2].uc_link = &uc[0];
  uc[2].uc_stack.ss_sp = st2;
  uc[2].uc_stack.ss_size = sizeof st2;
  makecontext (&uc[2], (void (*) (void)) f, 1, 2);

  /* Start running. */
  swapcontext (&uc[0], &uc[1]);
  putchar ('\n');

  return 0;
}