diff --git a/src/serial.c b/src/serial.c
index b1aef45..e24ebbf 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -62,7 +62,7 @@ static int get_macos_serial(uint8_t *out) {
 
 #pragma comment(lib, "wbemuuid.lib")
 
-int get_system_uuid(char *out) {
+static int get_system_uuid(char *out) {
     char serial[64] = {0};
     HRESULT hr;
     IWbemLocator *locator = NULL;
@@ -184,7 +184,7 @@ static int append_field(char *out, size_t out_len, const char *prefix,const char
     return 0;
 }
 
-int get_linux_hardware_id(char *out) {
+static int get_linux_hardware_id(char *out) {
     if (!out) {
         return -1;
     }
diff --git a/src/serial.h b/src/serial.h
index f8e2b0f..8be352c 100644
--- a/src/serial.h
+++ b/src/serial.h
@@ -21,7 +21,7 @@
 #include <stdint.h>
 
 #if !defined (PICO_PLATFORM)
-#if __APPLE__
+#if defined(__APPLE__) || defined(_MSC_VER) || defined(__linux__)
 #define PICO_UNIQUE_BOARD_ID_SIZE_BYTES 16
 #else
 #define PICO_UNIQUE_BOARD_ID_SIZE_BYTES 8