02_yield.c

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/* -*- C -*-
 * Serene programming language
 * Copyright (C) 2019-2026 Sameer Rahmani <[email protected]>
 *
 * 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 3 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, see <https://www.gnu.org/licenses/>.
 */
 
// Example 02 -- cooperative yielding.
//
// Fibers are cooperative. A fiber keeps the thread until it gives it up, and
// `srn_fiber_yield` is the polite way to do that: the running fiber goes to the
// back of the ready queue and the next ready fiber runs. There is no
// preemption, so a fiber that never yields and never finishes would hold the
// worker forever.
//
// Three fibers each count to three, yielding after every step. Running on a
// single worker, they take turns. The printed order makes the interleaving
// visible.
 
#include <stdio.h>
 
#include <serene/runtime.h>
 
static srn_fiber_result_t counter(srn_context_t *ctx, void *arg) {
  (void)ctx;
  // The argument carries the fiber's label, passed as a small integer.
  long id = (long)(intptr_t)arg;
  for (int step = 1; step <= 3; step++) {
    printf("fiber %ld: step %d\n", id, step);
    // Hand the worker to the next ready fiber. This fiber resumes here on its
    // next turn.
    srn_fiber_yield();
  }
  printf("fiber %ld: done\n", id);
  return nullptr;
}
 
int main(void) {
  SERENE_RUNTIME_INIT_WITH_SCHED(engine, sched, nullptr);
  srn_context_t *ctx = srn_context_make(engine);
 
  for (long id = 1; id <= 3; id++) {
    (void)srn_fiber_spawn(ctx, counter, (void *)(intptr_t)id);
  }
 
  // One worker, so the turn taking is deterministic and the output interleaves
  // the three fibers a step at a time.
  srn_sched_run(sched, 1);
 
  srn_context_release(ctx);
  SERENE_RUNTIME_SHUTDOWN(engine);
  return 0;
}