context switching using ucontext_t and context family functions

context_switching.c

#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
#include <sys/time.h>
#include <stdbool.h>

void *xmalloc(size_t size) {
  void *ptr = malloc(size);

  if (ptr == NULL) {
    fprintf(stderr, "[Fatal Error] Failed to allocate memory\n");
    exit(EXIT_FAILURE);
  }

  return ptr;
}

#define WORKER_STACK_SIZE 8192
#define WORKERS 4

ucontext_t main_ctx;
ucontext_t worker_ctxs[WORKERS];

bool worker_finished[WORKERS];

static void worker(int id) {
  printf("worker<id: %d> <1>\n", id);
  // back to main ctx
  swapcontext(&worker_ctxs[id], &main_ctx);

  printf("worker<id: %d> <2>\n", id);
  // back to main ctx
  swapcontext(&worker_ctxs[id], &main_ctx);

  printf("worker<id: %d> <3>\n", id);
  // back to main ctx
  swapcontext(&worker_ctxs[id], &main_ctx);

  worker_finished[id] = true;
}

bool all_worker_finished(void) {
  bool ret = true;

  for (int id = 0; id < WORKERS; id++) {
    ret &= worker_finished[id];
  }

  return ret;
}

int main(int argc, char const* argv[]) {
  for (int id = 0; id < WORKERS; id++) {
    worker_finished[id] = false;

    if (getcontext(&worker_ctxs[id]) == 1) {
      abort();
    }

    worker_ctxs[id].uc_link = &main_ctx;
    worker_ctxs[id].uc_stack.ss_sp = xmalloc(WORKER_STACK_SIZE);
    worker_ctxs[id].uc_stack.ss_size = WORKER_STACK_SIZE;
    makecontext(&worker_ctxs[id], (void (*)(void))worker, 1, id);
  }

  int id = 0;

  while (!all_worker_finished()) {
    swapcontext(&main_ctx, &worker_ctxs[id]);
    id++;
    id %= WORKERS;
  }

  return 0;
}