Add some log.

example/main_stack_epoll.c

@@ -168,6 +168,7 @@ int main(int argc, char * argv[])
     }
 
     epfd = epoll_create(0);
+    printf("epfd:%d\n", epfd);
     if (epfd <= 0) {
         printf("ff_epoll_create failed, errno:%d, %s\n",
             errno, strerror(errno));

example/main_stack_epoll_thread_socket.c

@@ -107,6 +107,7 @@ void *loop(void *arg)
     }
 
     epfd = epoll_create(0);
+    printf("thread %d, epfd:%d\n", thread_id, epfd);
     if (epfd <= 0) {
         printf("thread %d, ff_epoll_create failed, errno:%d, %s\n",
             thread_id, errno, strerror(errno));