diff --git a/src/include/locale.h b/src/include/locale.h
index d6f3b140325666f02e36b8b455a669eb1ed9c7ee..c522f0d701bcfff7537da098ad2dbce8a0e4b683 100644
--- a/src/include/locale.h
+++ b/src/include/locale.h
@@ -7,11 +7,13 @@ extern "C" {
 
 #include <features.h>
 
+#ifndef NULL
 #ifdef __cplusplus
 #define NULL 0L
 #else
 #define NULL ((void*)0)
 #endif
+#endif /* NULL */
 
 #define LC_CTYPE    0
 #define LC_NUMERIC  1
diff --git a/src/include/time.h b/src/include/time.h
index e1c13173a0e3c61bf3bd160ece1a6915f6a04cba..dc7a860ab064fc6692b63cac2d2f47c6f7938bd7 100644
--- a/src/include/time.h
+++ b/src/include/time.h
@@ -7,11 +7,13 @@ extern "C" {
 
 #include <features.h>
 
+#ifndef NULL
 #ifdef __cplusplus
 #define NULL 0L
 #else
 #define NULL ((void*)0)
 #endif
+#endif /* NULL */
 
 typedef long    time_t;
 typedef long    clock_t;
diff --git a/src/include/wchar.h b/src/include/wchar.h
index 95e592ea7e42eeb85693944cc82d9875a9b0d65c..e6c292372c08a8977b62dd0707a6c26261132d21 100644
--- a/src/include/wchar.h
+++ b/src/include/wchar.h
@@ -18,11 +18,13 @@ typedef int mbstate_t;
 #define WCHAR_MIN (-1-0x7fffffff+L'\0')
 #endif
 
+#ifndef NULL
 #ifdef __cplusplus
 #define NULL 0L
 #else
 #define NULL ((void*)0)
 #endif
+#endif /* NULL */
 
 #undef WEOF
 #define WEOF 0xFFFFU