diff --git a/CMakeLists.txt b/CMakeLists.txt
index da67e37..b65d8bf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,6 +1,10 @@
 cmake_minimum_required(VERSION 3.13)
+
+# Note: this CMakeLists.txt can be used as a top-level CMakeLists.txt for the SDK itself. For all other uses
+# it is included as a subdirectory via the pico_sdk_init() method provided by pico_sdk_init.cmake
 if (NOT TARGET _pico_sdk_inclusion_marker)
     add_library(_pico_sdk_inclusion_marker INTERFACE)
+    # This is a no-op unless we are the top-level CMakeLists.txt
     include(pico_sdk_init.cmake)
 
     project(pico_sdk C CXX ASM)
@@ -24,6 +28,7 @@ if (NOT TARGET _pico_sdk_inclusion_marker)
 
     # allow customization
     add_sub_list_dirs(PICO_SDK_PRE_LIST_DIRS)
+    add_sub_list_files(PICO_SDK_PRE_LIST_FILES)
 
     add_subdirectory(tools)
     add_subdirectory(src)
@@ -32,6 +37,7 @@ if (NOT TARGET _pico_sdk_inclusion_marker)
 
     # allow customization
     add_sub_list_dirs(PICO_SDK_POST_LIST_DIRS)
+    add_sub_list_files(PICO_SDK_POST_LIST_FILES)
 
     if (PICO_SDK_TOP_LEVEL_PROJECT AND NOT DEFINED PICO_SDK_TESTS_ENABLED)
         set(PICO_SDK_TESTS_ENABLED 1)
diff --git a/cmake/preload/toolchains/pico_arm_gcc.cmake b/cmake/preload/toolchains/pico_arm_gcc.cmake
index b0c2648..533c2a9 100644
--- a/cmake/preload/toolchains/pico_arm_gcc.cmake
+++ b/cmake/preload/toolchains/pico_arm_gcc.cmake
@@ -12,7 +12,7 @@ if (NOT PICO_GCC_TRIPLE)
         message("PICO_GCC_TRIPLE set from environment: $ENV{PICO_GCC_TRIPLE}")
     else()
         set(PICO_GCC_TRIPLE arm-none-eabi)
-        message("PICO_GCC_TRIPLE defaulted to arm-none-eabi")
+        message(DEBUG "PICO_GCC_TRIPLE defaulted to arm-none-eabi")
     endif()
 endif()
 
diff --git a/pico_sdk_init.cmake b/pico_sdk_init.cmake
index 2c501e4..879842c 100644
--- a/pico_sdk_init.cmake
+++ b/pico_sdk_init.cmake
@@ -1,54 +1,63 @@
-# Initialize the Raspberry Pi Pico SDK
+# Pre-initialize the Raspberry Pi Pico SDK, setting up the platform and toolchain and some CMake utility functions
 # This file must be included prior to the project() call
 
-if (_PICO_SDK_INIT)
-    return()
-endif ()
-set(_PICO_SDK_INIT 1)
+# Note: this file is perhaps named badly, as it provides a method pico_sdk_init which
+# the enclosing project calls LATER to actually "initialize" the SDK (by including the CMakeLists.txt from this
+# same directory)
 
-function(pico_is_top_level_project VAR)
-    string(TOLOWER ${CMAKE_CURRENT_LIST_DIR} __list_dir)
-    string(TOLOWER ${CMAKE_SOURCE_DIR} __source_dir)
-    if (__source_dir STREQUAL __list_dir)
-        set(${VAR} 1 PARENT_SCOPE)
-    else()
-        set(${VAR} 0 PARENT_SCOPE)
-    endif()
-endfunction()
+if (NOT TARGET _pico_sdk_pre_init_marker)
+    add_library(_pico_sdk_pre_init_marker INTERFACE)
 
-if (NOT PICO_SDK_PATH)
-    set(PICO_SDK_PATH ${CMAKE_CURRENT_LIST_DIR})
-endif ()
+    function(pico_is_top_level_project VAR)
+        string(TOLOWER ${CMAKE_CURRENT_LIST_DIR} __list_dir)
+        string(TOLOWER ${CMAKE_SOURCE_DIR} __source_dir)
+        if (__source_dir STREQUAL __list_dir)
+            set(${VAR} 1 PARENT_SCOPE)
+        else()
+            set(${VAR} 0 PARENT_SCOPE)
+        endif()
+    endfunction()
 
-get_filename_component(PICO_SDK_PATH "${PICO_SDK_PATH}" REALPATH BASE_DIR "${CMAKE_BINARY_DIR}")
+    if (NOT PICO_SDK_PATH)
+        set(PICO_SDK_PATH ${CMAKE_CURRENT_LIST_DIR})
+    endif ()
 
-set(PICO_SDK_PATH ${CMAKE_CURRENT_LIST_DIR} CACHE PATH "Path to the Raspberry Pi Pico SDK" FORCE)
+    get_filename_component(PICO_SDK_PATH "${PICO_SDK_PATH}" REALPATH BASE_DIR "${CMAKE_BINARY_DIR}")
 
-list(APPEND CMAKE_MODULE_PATH ${PICO_SDK_PATH}/cmake)
+    set(PICO_SDK_PATH ${CMAKE_CURRENT_LIST_DIR} CACHE PATH "Path to the Raspberry Pi Pico SDK" FORCE)
 
-include(${CMAKE_CURRENT_LIST_DIR}/pico_sdk_version.cmake)
-include(pico_utils)
+    list(APPEND CMAKE_MODULE_PATH ${PICO_SDK_PATH}/cmake)
 
-message("PICO_SDK_PATH is ${CMAKE_CURRENT_LIST_DIR}")
+    include(${CMAKE_CURRENT_LIST_DIR}/pico_sdk_version.cmake)
+    include(pico_utils)
 
-include(pico_pre_load_platform)
+    message("PICO_SDK_PATH is ${CMAKE_CURRENT_LIST_DIR}")
 
-# todo perhaps this should be included by the platform instead?
-# We want to configure correct toolchain prior to project load
-include(pico_pre_load_toolchain)
+    include(pico_pre_load_platform)
 
-macro(pico_sdk_init)
-    if (NOT CMAKE_PROJECT_NAME)
-        message(WARNING "pico_sdk_init() should be called after the project is created (and languages added)")
-    endif()
-    add_subdirectory(${PICO_SDK_PATH} pico-sdk)
-endmacro()
+    # todo perhaps this should be included by the platform instead?
+    # We want to configure correct toolchain prior to project load
+    include(pico_pre_load_toolchain)
 
-macro(add_sub_list_dirs var)
-    foreach(LIST_DIR IN LISTS ${var})
-        get_filename_component(SHORT_NAME "${LIST_DIR}" NAME)
-        message("Including custom CMakeLists.txt ${SHORT_NAME}")
-        add_subdirectory(${LIST_DIR} ${SHORT_NAME})
-    endforeach()
-endmacro()
+    macro(pico_sdk_init)
+        if (NOT CMAKE_PROJECT_NAME)
+            message(WARNING "pico_sdk_init() should be called after the project is created (and languages added)")
+        endif()
+        add_subdirectory(${PICO_SDK_PATH} pico-sdk)
+    endmacro()
 
+    macro(add_sub_list_dirs var)
+        foreach(LIST_DIR IN LISTS ${var})
+            get_filename_component(SHORT_NAME "${LIST_DIR}" NAME)
+            message(DEBUG "Including custom CMakeLists.txt ${SHORT_NAME}")
+            add_subdirectory(${LIST_DIR} ${SHORT_NAME})
+        endforeach()
+    endmacro()
+
+    macro(add_sub_list_files var)
+        foreach(LIST_FILE IN LISTS ${var})
+            message(DEBUG "Including custom CMake file ${LIST_FILE}")
+            include(${LIST_FILE})
+        endforeach()
+    endmacro()
+endif()
diff --git a/src/common/pico_sync/sem.c b/src/common/pico_sync/sem.c
index 195cd9a..3ad9816 100644
--- a/src/common/pico_sync/sem.c
+++ b/src/common/pico_sync/sem.c
@@ -6,6 +6,7 @@
 
 #include "pico/sem.h"
 #include "pico/time.h"
+#include "sys/select.h"
 
 void sem_init(semaphore_t *sem, int16_t initial_permits, int16_t max_permits) {
     lock_init(&sem->core, next_striped_spin_lock_num());