Add setting for SNTP server

components/settings/settings.h

@@ -136,4 +136,32 @@ size_t settings_get_timezone(char *buffer, size_t buffer_size);
  */
 void settings_add_timezone_callback(SettingsCallback callback);
 
+/**
+ * Set the SNTP server URL.
+ *
+ * The argument should be a null-terminated string, containing a valid
+ * domain name for an SNTP server. If the maximum length is exceeded,
+ * the value will still be used, but will be truncated.
+ */
+void settings_set_sntp_server(const char *sntp_server);
+
+/**
+ * Write the SNTP server URL into the given buffer.
+ *
+ * The length of the SNTP server domain is returned. If the value's
+ * size exceeds the size of the buffer, nothing will be written to the
+ * buffer but the length is still returned.
+ */
+size_t settings_get_sntp_server(char *buffer, size_t buffer_size);
+
+/**
+ * Add a callback for SNTP server URL updates.
+ *
+ * The function specified in the argument will be invoked whenever the
+ * SNTP server is updated, with the new value as its argument. The
+ * lifetime of the passed argument will be static, but the value may
+ * be modified once the callback returns.
+ */
+void settings_add_sntp_server_callback(SettingsCallback callback);
+
 #endif