03_wait_for.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 03 -- joining on a fiber's result.
//
// `srn_fiber_wait_for` blocks the calling fiber until a target fiber finishes,
// then returns the target's result. This is the fiber equivalent of joining a
// thread. If the target has already finished the call returns its result at
// once without blocking.
//
// A worker fiber computes a value and returns it. A waiter fiber joins on the
// worker and reads what it produced. Both fibers are created with
// `srn_fiber_make`, which leaves them NEW, so the whole graph is wired
// before `srn_fiber_schedule` starts anything and creation order carries no
// meaning. The waiter is scheduled first so it runs and parks before the
// worker finishes, which is the blocking path.
 
#include <stdio.h>
 
#include <serene/runtime.h>
 
// The worker reports a real result, so this example reads an actual payload
// out of the returned value.
static int answer;
 
// Filled in main before the run, so the waiter knows which fiber to join.
static srn_fiber_t *worker_fiber;
 
static srn_fiber_result_t worker(srn_context_t *ctx, void *arg) {
  (void)ctx;
  (void)arg;
  printf("worker: computing\n");
  return &answer;
}
 
static srn_fiber_result_t waiter(srn_context_t *ctx, void *arg) {
  (void)ctx;
  (void)arg;
  printf("waiter: blocking on the worker\n");
  srn_fiber_result_t result = srn_fiber_wait_for(worker_fiber);
  printf(
    "waiter: worker produced %s\n", result == &answer ? "the expected value" : "an unexpected value"
  );
  return nullptr;
}
 
int main(void) {
  SERENE_RUNTIME_INIT_WITH_SCHED(engine, sched, nullptr);
  srn_context_t *ctx = srn_context_make(engine);
 
  // Nothing runs until scheduled, so the join target can be wired after both
  // fibers exist. Scheduling order (not creation order) sets who runs first
  // on one worker, the waiter parks, then the worker finishes and wakes it.
  srn_fiber_t *waiting = srn_fiber_make(ctx, sched, "waiter", waiter, nullptr, 0);
  worker_fiber         = srn_fiber_make(ctx, sched, "worker", worker, nullptr, 0);
  srn_fiber_schedule(waiting);
  srn_fiber_schedule(worker_fiber);
 
  srn_sched_run(sched, 1);
 
  srn_context_release(ctx);
  SERENE_RUNTIME_SHUTDOWN(engine);
  return 0;
}