diff --git a/app/driver/Makefile b/app/driver/Makefile
index ebf7ac5c..dfc59989 100644
--- a/app/driver/Makefile
+++ b/app/driver/Makefile
@@ -15,6 +15,7 @@ ifndef PDIR
 GEN_LIBS = libdriver.a
 endif
 
+STD_CFLAGS=-std=gnu11 -Wimplicit
 
 #############################################################
 # Configuration i.e. compile options etc.
diff --git a/app/include/rom.h b/app/include/rom.h
index 5b8641dd..ffed887e 100644
--- a/app/include/rom.h
+++ b/app/include/rom.h
@@ -140,4 +140,7 @@ void Cache_Read_Disable(void);
 void ets_intr_lock(void);
 void ets_intr_unlock(void);
 
+void ets_install_putc1(void *routine);
+void uart_div_modify(int no, unsigned int freq);
+
 #endif
diff --git a/sdk-overrides/include/osapi.h b/sdk-overrides/include/osapi.h
index 96b836cc..374fea83 100644
--- a/sdk-overrides/include/osapi.h
+++ b/sdk-overrides/include/osapi.h
@@ -4,6 +4,9 @@
 #include "rom.h"
 void ets_timer_arm_new (ETSTimer *a, int b, int c, int isMstimer);
 
+int os_printf(const char *format, ...) __attribute__ ((format (printf, 1, 2)));
+int os_printf_plus(const char *format, ...)  __attribute__ ((format (printf, 1, 2)));
+
 #include_next "osapi.h"
 
 #endif