diff --git a/example/README.md b/example/README.md
index 13ccd26..4aca745 100644
--- a/example/README.md
+++ b/example/README.md
@@ -13,7 +13,7 @@ This example runs on Linux.
 Clone the [Espressif IoT Development Framework](https://github.com/espressif/esp-idf):
 
 ```shell
-git clone --recurse-submodules -j6 --branch release/v3.3 git@github.com:espressif/esp-idf.git
+git clone --recurse-submodules -j6 --branch release/v4.0 git@github.com:espressif/esp-idf.git
 ./esp-idf/install.sh
 ```
 
diff --git a/script/check-esp32-build b/script/check-esp32-build
index 35f5675..8554235 100755
--- a/script/check-esp32-build
+++ b/script/check-esp32-build
@@ -31,7 +31,7 @@ set -euxo pipefail
 
 setup_esp_idf()
 {
-    git clone --branch release/v3.3 --recursive https://github.com/espressif/esp-idf.git
+    git clone --branch release/v4.0 --recursive https://github.com/espressif/esp-idf.git
     export IDF_PATH="$(pwd)"/esp-idf
 
     pip3 install wheel setuptools
diff --git a/src/alarm.c b/src/alarm.c
index 9a332d9..0fc8b29 100644
--- a/src/alarm.c
+++ b/src/alarm.c
@@ -37,6 +37,8 @@
 #include <esp_log.h>
 #include <esp_timer.h>
 
+#include <sys/time.h>
+
 #include <openthread/config.h>
 #include <openthread/platform/alarm-milli.h>
 #include <openthread/platform/diag.h>
diff --git a/src/vfs_event.c b/src/vfs_event.c
index 220cda6..8c19dbd 100644
--- a/src/vfs_event.c
+++ b/src/vfs_event.c
@@ -44,24 +44,25 @@ typedef struct Event
 {
     int     mFd;
     bool    mIsOpen;
-    bool    mCounter;
+    uint8_t mCounter;
     _lock_t mLock;
 } Event;
 
 static Event sEvent = {
     .mFd      = -1,
     .mIsOpen  = false,
-    .mCounter = false,
+    .mCounter = 0,
     .mLock    = 0, // lazy initialized
 };
 
-static SemaphoreHandle_t *sSignalSemaphore = NULL;
+static esp_vfs_select_sem_t sSignalSemaphore = {.is_sem_local = false, .sem = NULL};
 
-static esp_err_t event_start_select(int                nfds,
-                                    fd_set *           readfds,
-                                    fd_set *           writefds,
-                                    fd_set *           exceptfds,
-                                    SemaphoreHandle_t *signal_sem)
+static esp_err_t event_start_select(int                  nfds,
+                                    fd_set *             readfds,
+                                    fd_set *             writefds,
+                                    fd_set *             exceptfds,
+                                    esp_vfs_select_sem_t signal_sem,
+                                    void **              end_select_args)
 {
     esp_err_t error = ESP_OK;
 
@@ -84,9 +85,10 @@ static esp_err_t event_start_select(int                nfds,
     return error;
 }
 
-static void event_end_select()
+static esp_err_t event_end_select(void *end_select_args)
 {
-    sSignalSemaphore = NULL;
+    memset(&sSignalSemaphore, 0, sizeof sSignalSemaphore);
+    return ESP_OK;
 }
 
 static int event_open(const char *path, int flags, int mode)
@@ -123,7 +125,7 @@ static ssize_t event_write(int fd, const void *data, size_t size)
     ++sEvent.mCounter;
     _lock_release_recursive(&sEvent.mLock);
 
-    if (sSignalSemaphore != NULL)
+    if (sSignalSemaphore.sem)
     {
         esp_vfs_select_triggered(sSignalSemaphore);
     }