#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>/* 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 iterator return value. */
volatile int i_from_iterator;/* 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 stack for the iterator function. */char iterator_stack[SIGSTKSZ];/* Flag indicating that the iterator has completed. */volatile int iterator_finished;getcontext(&loop_context);/* 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);/* 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;
}