manual/examples/swapcontext.c

   1 /* Complete Context Control
   2    Copyright (C) 1991-2017 Free Software Foundation, Inc.
   3
   4    This program is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU General Public License
   6    as published by the Free Software Foundation; either version 2
   7    of the License, or (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program; if not, if not, see <http://www.gnu.org/licenses/>.
  16 */
  17
  18 #include <signal.h>
  19 #include <stdio.h>
  20 #include <stdlib.h>
  21 #include <ucontext.h>
  22 #include <sys/time.h>
  23
  24 /* Set by the signal handler.  */
  25 static volatile int expired;
  26
  27 /* The contexts.  */
  28 static ucontext_t uc[3];
  29
  30 /* We do only a certain number of switches.  */
  31 static int switches;
  32
  33
  34 /* This is the function doing the work.  It is just a
  35    skeleton, real code has to be filled in.  */
  36 static void
  37 f (int n)
  38 {
  39   int m = 0;
  40   while (1)
  41     {
  42       /* This is where the work would be done.  */
  43       if (++m % 100 == 0)
  44         {
  45           putchar ('.');
  46           fflush (stdout);
  47         }
  48
  49       /* Regularly the @var{expire} variable must be checked.  */
  50       if (expired)
  51         {
  52           /* We do not want the program to run forever.  */
  53           if (++switches == 20)
  54             return;
  55
  56           printf ("\nswitching from %d to %d\n", n, 3 - n);
  57           expired = 0;
  58           /* Switch to the other context, saving the current one.  */
  59           swapcontext (&uc[n], &uc[3 - n]);
  60         }
  61     }
  62 }
  63
  64 /* This is the signal handler which simply set the variable.  */
  65 void
  66 handler (int signal)
  67 {
  68   expired = 1;
  69 }
  70
  71
  72 int
  73 main (void)
  74 {
  75   struct sigaction sa;
  76   struct itimerval it;
  77   char st1[8192];
  78   char st2[8192];
  79
  80   /* Initialize the data structures for the interval timer.  */
  81   sa.sa_flags = SA_RESTART;
  82   sigfillset (&sa.sa_mask);
  83   sa.sa_handler = handler;
  84   it.it_interval.tv_sec = 0;
  85   it.it_interval.tv_usec = 1;
  86   it.it_value = it.it_interval;
  87
  88   /* Install the timer and get the context we can manipulate.  */
  89   if (sigaction (SIGPROF, &sa, NULL) < 0
  90       || setitimer (ITIMER_PROF, &it, NULL) < 0
  91       || getcontext (&uc[1]) == -1
  92       || getcontext (&uc[2]) == -1)
  93     abort ();
  94
  95   /* Create a context with a separate stack which causes the
  96      function @code{f} to be call with the parameter @code{1}.
  97      Note that the @code{uc_link} points to the main context
  98      which will cause the program to terminate once the function
  99      return.  */
 100   uc[1].uc_link = &uc[0];
 101   uc[1].uc_stack.ss_sp = st1;
 102   uc[1].uc_stack.ss_size = sizeof st1;
 103   makecontext (&uc[1], (void (*) (void)) f, 1, 1);
 104
 105   /* Similarly, but @code{2} is passed as the parameter to @code{f}.  */
 106   uc[2].uc_link = &uc[0];
 107   uc[2].uc_stack.ss_sp = st2;
 108   uc[2].uc_stack.ss_size = sizeof st2;
 109   makecontext (&uc[2], (void (*) (void)) f, 1, 2);
 110
 111   /* Start running.  */
 112   swapcontext (&uc[0], &uc[1]);
 113   putchar ('\n');
 114
 115   return 0;
 116 }