#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
/* This is the iterator function. It is entered on the first call to
* swapcontext, and loops from 0 to 9. Each value is saved in i_from_iterator,
* and then swapcontext used to return to the main loop. The main loop prints
* the value and calls swapcontext to swap back into the function. When the end
* of the loop is reached, the function exits, and execution switches to the
* context pointed to by main_context1. */
void loop(
ucontext_t *loop_context,
ucontext_t *other_context,
int *i_from_iterator)
{
int i;
for (i=0; i < 10; ++i) {
/* Write the loop counter into the iterator return location. */
*i_from_iterator = i;
/* Save the loop context (this point in the code) into ''loop_context'',
* and switch to other_context. */
swapcontext(loop_context, other_context);
}
/* The function falls through to the calling context with an implicit
* ''setcontext(&loop_context->uc_link);'' */
}
int main(void)
{
/* The three contexts:
* (1) main_context1 : The point in main to which loop will return.
* (2) main_context2 : The point in main to which control from loop will
* flow by switching contexts.
* (3) loop_context : The point in loop to which control from main will
* flow by switching contexts. */
ucontext_t main_context1, main_context2, loop_context;
/* The stack for the iterator function. */
char iterator_stack[SIGSTKSZ];
/* Flag indicating that the iterator has completed. */
volatile int iterator_finished;
/* The iterator return value. */
volatile int i_from_iterator;
/* Initialise the iterator context. uc_link points to main_context1, the
* point to return to when the iterator finishes. */
loop_context.uc_link = &main_context1;
loop_context.uc_stack.ss_sp = iterator_stack;
loop_context.uc_stack.ss_size = sizeof(iterator_stack);
getcontext(&loop_context);
/* Fill in loop_context so that it makes swapcontext start loop. The
* (void (*)(void)) typecast is to avoid a compiler warning but it is
* not relevant to the behaviour of the function. */
makecontext(&loop_context, (void (*)(void)) loop,
3, &loop_context, &main_context2, &i_from_iterator);
/* Clear the finished flag. */
iterator_finished = 0;
/* Save the current context into main_context1. When loop is finished,
* control flow will return to this point. */
getcontext(&main_context1);
if (!iterator_finished) {
/* Set iterator_finished so that when the previous getcontext is
* returned to via uc_link, the above if condition is false and the
* iterator is not restarted. */
iterator_finished = 1;
while (1) {
/* Save this point into main_context2 and switch into the iterator.
* The first call will begin loop. Subsequent calls will switch to
* the swapcontext in loop. */
swapcontext(&main_context2, &loop_context);
printf("%d\n", i_from_iterator);
}
}
return 0;
}
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