text: // Copyright 2017 Espressif Systems (Shanghai) PTE LTD
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #include <string.h>
  #include <stdbool.h>
  #include <stdarg.h>
  #include <sys/errno.h>
  #include <sys/lock.h>
  #include <sys/fcntl.h>
  #include "esp_vfs.h"
  #include "esp_vfs_dev.h"
  #include "esp_attr.h"
  #include "soc/uart_struct.h"
  #include "lwip/sockets.h"
  #include "sdkconfig.h"
  #include "lwip/sys.h"
  
  _Static_assert(MAX_FDS >= CONFIG_LWIP_MAX_SOCKETS, "MAX_FDS < CONFIG_LWIP_MAX_SOCKETS");
  
  static void lwip_stop_socket_select()
  {
      sys_sem_signal(sys_thread_sem_get()); //socket_select will return
  }
  
  static void lwip_stop_socket_select_isr(BaseType_t *woken)
  {
      if (sys_sem_signal_isr(sys_thread_sem_get()) && woken) {
          *woken = pdTRUE;
      }
  }
  
  static int lwip_fcntl_r_wrapper(int fd, int cmd, va_list args)
  {
      return lwip_fcntl_r(fd, cmd, va_arg(args, int));
  }
  
  static int lwip_ioctl_r_wrapper(int fd, int cmd, va_list args)
  {
      return lwip_ioctl_r(fd, cmd, va_arg(args, void *));
  }
  
  void esp_vfs_lwip_sockets_register()
  {
      esp_vfs_t vfs = {
          .flags = ESP_VFS_FLAG_DEFAULT,
          .write = &lwip_write_r,
          .open = NULL,
          .fstat = NULL,
          .close = &lwip_close_r,
          .read = &lwip_read_r,
          .fcntl = &lwip_fcntl_r_wrapper,
          .ioctl = &lwip_ioctl_r_wrapper,
          .socket_select = &lwip_select,
          .stop_socket_select = &lwip_stop_socket_select,
          .stop_socket_select_isr = &lwip_stop_socket_select_isr,
      };
      /* Non-LWIP file descriptors are from 0 to (LWIP_SOCKET_OFFSET-1). LWIP
       * file descriptors are registered from LWIP_SOCKET_OFFSET to
       * MAX_FDS-1.
       */
  
      ESP_ERROR_CHECK(esp_vfs_register_fd_range(&vfs, NULL, LWIP_SOCKET_OFFSET, MAX_FDS));
  }
  