manual/examples/swapcontext.c

   1 #include <signal.h>
   2 #include <stdio.h>
   3 #include <stdlib.h>
   4 #include <ucontext.h>
   5 #include <sys/time.h>
   6
   7 /* Set by the signal handler.  */
   8 static volatile int expired;
   9
  10 /* The contexts.  */
  11 static ucontext_t uc[3];
  12
  13 /* We do only a certain number of switches.  */
  14 static int switches;
  15
  16
  17 /* This is the function doing the work.  It is just a
  18    skeleton, real code has to be filled in.  */
  19 static void
  20 f (int n)
  21 {
  22   int m = 0;
  23   while (1)
  24     {
  25       /* This is where the work would be done.  */
  26       if (++m % 100 == 0)
  27         {
  28           putchar ('.');
  29           fflush (stdout);
  30         }
  31
  32       /* Regularly the @var{expire} variable must be checked.  */
  33       if (expired)
  34         {
  35           /* We do not want the program to run forever.  */
  36           if (++switches == 20)
  37             return;
  38
  39           printf ("\nswitching from %d to %d\n", n, 3 - n);
  40           expired = 0;
  41           /* Switch to the other context, saving the current one.  */
  42           swapcontext (&uc[n], &uc[3 - n]);
  43         }
  44     }
  45 }
  46
  47 /* This is the signal handler which simply set the variable.  */
  48 void
  49 handler (int signal)
  50 {
  51   expired = 1;
  52 }
  53
  54
  55 int
  56 main (void)
  57 {
  58   struct sigaction sa;
  59   struct itimerval it;
  60   char st1[8192];
  61   char st2[8192];
  62
  63   /* Initialize the data structures for the interval timer.  */
  64   sa.sa_flags = SA_RESTART;
  65   sigfillset (&sa.sa_mask);
  66   sa.sa_handler = handler;
  67   it.it_interval.tv_sec = 0;
  68   it.it_interval.tv_usec = 1;
  69   it.it_value = it.it_interval;
  70
  71   /* Install the timer and get the context we can manipulate.  */
  72   if (sigaction (SIGPROF, &sa, NULL) < 0
  73       || setitimer (ITIMER_PROF, &it, NULL) < 0
  74       || getcontext (&uc[1]) == -1
  75       || getcontext (&uc[2]) == -1)
  76     abort ();
  77
  78   /* Create a context with a separate stack which causes the
  79      function @code{f} to be call with the parameter @code{1}.
  80      Note that the @code{uc_link} points to the main context
  81      which will cause the program to terminate once the function
  82      return.  */
  83   uc[1].uc_link = &uc[0];
  84   uc[1].uc_stack.ss_sp = st1;
  85   uc[1].uc_stack.ss_size = sizeof st1;
  86   makecontext (&uc[1], (void (*) (void)) f, 1, 1);
  87
  88   /* Similarly, but @code{2} is passed as the parameter to @code{f}.  */
  89   uc[2].uc_link = &uc[0];
  90   uc[2].uc_stack.ss_sp = st2;
  91   uc[2].uc_stack.ss_size = sizeof st2;
  92   makecontext (&uc[2], (void (*) (void)) f, 1, 2);
  93
  94   /* Start running.  */
  95   swapcontext (&uc[0], &uc[1]);
  96   putchar ('\n');
  97
  98   return 0;
  99 }