diff --git a/headers/posix/string.h b/headers/posix/string.h
index d2885710e60..91009bd469c 100644
--- a/headers/posix/string.h
+++ b/headers/posix/string.h
@@ -22,7 +22,7 @@ extern void		*memccpy(void *dest, const void *source, int stopByte, size_t lengt
 extern void		*memmove(void *dest, const void *source, size_t length);
 extern void		*memset(void *dest, int value, size_t length);
 
-#ifdef __USE_GNU
+#ifdef _GNU_SOURCE
 extern void		*memrchr(const void *source, int value, size_t length);
 #endif
 
@@ -40,7 +40,7 @@ extern char		*strchr(const char *string, int character);
 extern char		*strrchr(const char *string, int character);
 extern char		*strstr(const char *string, const char *searchString);
 
-#ifdef __USE_GNU
+#ifdef _GNU_SOURCE
 extern char		*strchrnul(const char *string, int character);
 #endif
 
diff --git a/headers/posix/wchar.h b/headers/posix/wchar.h
index 6878a62faaa..2a79ebdb510 100644
--- a/headers/posix/wchar.h
+++ b/headers/posix/wchar.h
@@ -94,7 +94,7 @@ extern size_t   wcrtomb(char *dest, wchar_t wc, mbstate_t *mbState);
 extern int		wcscasecmp(const wchar_t *wcs1, const wchar_t *wcs2);
 extern wchar_t	*wcscat(wchar_t *dest, const wchar_t *src);
 extern wchar_t	*wcschr(const wchar_t *wcs, wchar_t wc);
-#ifdef __USE_GNU
+#ifdef _GNU_SOURCE
 extern wchar_t	*wcschrnul(const wchar_t *wcs, wchar_t wc);
 #endif
 extern int      wcscmp(const wchar_t *wcs1, const wchar_t *wcs2);
@@ -141,7 +141,7 @@ extern wchar_t	*wmemchr(const wchar_t *wcs, wchar_t wc, size_t n);
 extern int		wmemcmp(const wchar_t *wcs1, const wchar_t *wcs2, size_t n);
 extern wchar_t	*wmemcpy(wchar_t *dest, const wchar_t *src, size_t n);
 extern wchar_t	*wmemmove(wchar_t *dest, const wchar_t *src, size_t n);
-#ifdef __USE_GNU
+#ifdef _GNU_SOURCE
 extern wchar_t	*wmempcpy(wchar_t *dest, const wchar_t *src, size_t n);
 #endif
 extern wchar_t	*wmemset(wchar_t *dest, wchar_t wc, size_t n);