/* Complete Context Control

   Copyright (C) 1991-2018 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or

   modify it under the terms of the GNU General Public License

   as published by the Free Software Foundation; either version 2

   of the License, or (at your option) any later version.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program; if not, if not, see <http://www.gnu.org/licenses/>.

*/

#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;

}