evfd_sample.cpp

#include <stdio.h>
#include <pthread.h>
#include <sys/eventfd.h>
#include <unistd.h>

struct workset {
  int tid_;
  int efd_;
};

void *worker(void *arg) {
  pthread_t tid;
  workset* ids = (workset*)arg;

  eventfd_read(ids->efd_, NULL);
  printf("worker(%d) finish\n", ids->tid_);
}

int main() {
  pthread_t tid[10];
  workset ids[10];
  int efd;

  efd = eventfd(0, EFD_SEMAPHORE);
  printf("main: efd=%d\n", efd);
  for(int i = 0; i < 10; i++) {
    ids[i].tid_ = i;
    ids[i].efd_ = efd;
    pthread_create(&tid[i], NULL, worker, &ids[i]);
    printf("main: spawn worker(%d)\n", i);
    sleep(1);
  }

  sleep(1);
  eventfd_write(efd, 5);

  sleep(1);
  eventfd_write(efd, 5);

  for(int i = 0; i < 10; i++)
    pthread_join(tid[i], NULL);

  return 0;
}

output.txt

main: efd=3
main: spawn worker(0)
main: spawn worker(1)
main: spawn worker(2)
main: spawn worker(3)
main: spawn worker(4)
main: spawn worker(5)
main: spawn worker(6)
main: spawn worker(7)
main: spawn worker(8)
main: spawn worker(9)
worker(5) finish
worker(0) finish
worker(8) finish
worker(1) finish
worker(7) finish
worker(6) finish
worker(9) finish
worker(2) finish
worker(3) finish
worker(4) finish