diff --git a/include/ctype.h b/include/ctype.h
index 6c47c31..7c2a010 100644
--- a/include/ctype.h
+++ b/include/ctype.h
@@ -18,6 +18,7 @@ int isupper(int);
 int isxdigit(int);
 
 /* POSIX extensions */
+#ifdef _POSIX_C_SOURCE
 int isalnum_l(int, locale_t);
 int isalpha_l(int, locale_t);
 int isblank_l(int, locale_t);
@@ -30,18 +31,25 @@ int ispunct_l(int, locale_t);
 int isspace_l(int, locale_t);
 int isupper_l(int, locale_t);
 int isxdigit_l(int, locale_t);
+#endif
 
 /* C standard stuff */
-int toascii(int);
 int tolower(int);
 int toupper(int);
 
 /* POSIX extensions */
+#ifdef _POSIX_C_SOURCE
 int tolower_l(int, locale_t);
 int toupper_l(int, locale_t);
+#endif
 
+#ifdef _XOPEN_SOURCE
+/* Obsolete XSI stuff */
+int isascii(int);
+int toascii(int);
 /* Obsolete XSI macros */
 #define _toupper(x) toupper(x)
 #define _tolower(x) tolower(x)
+#endif
 
 #endif