#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 @var{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 @code{f} to be call with the parameter @code{1}.
     Note that the @code{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 @code{2} is passed as the parameter to @code{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;
}