diff --git a/float.h b/float.h
index 58b53b9..e74fb7b 100644
--- a/float.h
+++ b/float.h
@@ -12,10 +12,8 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#if !defined(__FSTD_HDRS_FLOAT_H) || defined(__FSTD_HDRS_CXX_WRAP)
-#if !defined(__FSTD_HDRS_CXX_WRAP)
+#ifndef __FSTD_HDRS_FLOAT_H
 #define __FSTD_HDRS_FLOAT_H 1
-#endif
 
 #undef FLT_ROUNDS
 #define FLT_ROUNDS 1
@@ -30,19 +28,17 @@
 #undef LDBL_MANT_DIG
 #define LDBL_MANT_DIG __LDBL_MANT_DIG__
 
+#if (defined(__cplusplus) && __cplusplus >= 201103L) \
+ || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L)
+
 #undef DECIMAL_DIG
 #define DECIMAL_DIG __DECIMAL_DIG__
 
-#undef FLT_DECIMAL_DIG
-#define FLT_DECIMAL_DIG __FLT_DECIMAL_DIG__
-#undef DBL_DECIMAL_DIG
-#define DBL_DECIMAL_DIG __DBL_DECIMAL_DIG__
-#undef LDBL_DECIMAL_DIG
-#define LDBL_DECIMAL_DIG __LDBL_DECIMAL_DIG__
-
 #undef FLT_EVAL_METHOD
 #define FLT_EVAL_METHOD __FLT_EVAL_METHOD__
 
+#endif
+
 #undef FLT_DIG
 #define FLT_DIG __FLT_DIG__
 #undef DBL_DIG
@@ -99,6 +95,16 @@
 #undef LDBL_MIN
 #define LDBL_MIN __LDBL_MIN__
 
+#if (defined(__cplusplus) && __cplusplus >= 201703L) \
+ || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
+
+#undef FLT_DECIMAL_DIG
+#define FLT_DECIMAL_DIG __FLT_DECIMAL_DIG__
+#undef DBL_DECIMAL_DIG
+#define DBL_DECIMAL_DIG __DBL_DECIMAL_DIG__
+#undef LDBL_DECIMAL_DIG
+#define LDBL_DECIMAL_DIG __LDBL_DECIMAL_DIG__
+
 #undef FLT_TRUE_MIN
 #define FLT_TRUE_MIN __FLT_DENORM_MIN__
 #undef DBL_TRUE_MIN
@@ -114,3 +120,5 @@
 #define LDBL_HAS_SUBNORM __LDBL_HAS_DENORM__
 
 #endif
+
+#endif
diff --git a/limits.h b/limits.h
index 0b503a7..5d14b49 100644
--- a/limits.h
+++ b/limits.h
@@ -12,10 +12,8 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#if !defined(__FSTD_HDRS_LIMITS_H) || defined(__FSTD_HDRS_CXX_WRAP)
-#if !defined(__FSTD_HDRS_CXX_WRAP)
+#ifndef __FSTD_HDRS_LIMITS_H
 #define __FSTD_HDRS_LIMITS_H 1
-#endif
 
 #undef CHAR_BIT
 #define CHAR_BIT __CHAR_BIT__
@@ -80,6 +78,8 @@
 #undef ULONG_MAX
 #define ULONG_MAX (LONG_MAX * 2UL + 1UL)
 
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
+
 #undef LLONG_MAX
 #define LLONG_MAX __LONG_LONG_MAX__
 #undef LLONG_MIN
@@ -89,3 +89,5 @@
 #define ULLONG_MAX (LLONG_MAX * 2ULL + 1ULL)
 
 #endif
+
+#endif
diff --git a/stdalign.h b/stdalign.h
index 28f5f88..7aab6a3 100644
--- a/stdalign.h
+++ b/stdalign.h
@@ -16,6 +16,10 @@
 #define __FSTD_HDRS_STDALIGN_H 1
 
 #ifndef __cplusplus
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ > 201710L
+    /* These do not need to be defined for C23+ */
+#else
 #  undef alignas
 #  define alignas _Alignas
 #  undef alignof
@@ -28,3 +32,5 @@
 #endif
 
 #endif
+
+#endif
diff --git a/stdarg.h b/stdarg.h
index fbeeb3f..b158794 100644
--- a/stdarg.h
+++ b/stdarg.h
@@ -12,15 +12,17 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#if !defined(__FSTD_HDRS_STDARG_H) || defined(__FSTD_HDRS_CXX_WRAP)
-#if !defined(__FSTD_HDRS_CXX_WRAP)
+#ifndef __FSTD_HDRS_STDARG_H
 #define __FSTD_HDRS_STDARG_H 1
-#endif
 
 typedef __builtin_va_list va_list;
 
 #undef va_start
-#define va_start(v, l) __builtin_va_start(v, l)
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ > 201710L
+#  define va_start(v, ...) __builtin_va_start(v, 0)
+#else
+#  define va_start(v, l) __builtin_va_start(v, l)
+#endif
 #undef va_end
 #define va_end(v) __builtin_va_end(v)
 #undef va_arg
diff --git a/stdbool.h b/stdbool.h
index d109057..b0e7320 100644
--- a/stdbool.h
+++ b/stdbool.h
@@ -12,12 +12,14 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#if !defined(__FSTD_HDRS_STDBOOL_H) || defined(__FSTD_HDRS_CXX_WRAP)
-#if !defined(__FSTD_HDRS_CXX_WRAP)
+#ifndef __FSTD_HDRS_STDBOOL_H
 #define __FSTD_HDRS_STDBOOL_H 1
-#endif
 
 #ifndef __cplusplus
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ > 201710L
+    /* These do not need to be defined for C23+ */
+#else
 #  undef bool
 #  define bool _Bool
 
@@ -27,6 +29,8 @@
 #  define false 0
 #endif
 
+#endif
+
 #undef __bool_true_false_are_defined
 #define __bool_true_false_are_defined 1
 
diff --git a/stddef.h b/stddef.h
index 376eb75..adf4a32 100644
--- a/stddef.h
+++ b/stddef.h
@@ -12,10 +12,8 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#if !defined(__FSTD_HDRS_STDDEF_H) || defined(__FSTD_HDRS_CXX_WRAP)
-#if !defined(__FSTD_HDRS_CXX_WRAP)
+#ifndef __FSTD_HDRS_STDDEF_H
 #define __FSTD_HDRS_STDDEF_H 1
-#endif
 
 typedef __SIZE_TYPE__ size_t;
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
@@ -23,8 +21,7 @@ typedef __PTRDIFF_TYPE__ ptrdiff_t;
 #ifndef __cplusplus
 typedef __WCHAR_TYPE__ wchar_t;
 
-/* XXX fix to use proper __STDC_VERSION__ number later on */
-#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 202000L)
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ > 201710L
 typedef typeof(nullptr) nullptr_t;
 #endif
 
@@ -44,10 +41,11 @@ typedef decltype(nullptr) nullptr_t;
 #undef offsetof
 #define offsetof(s, m) __builtin_offsetof(s, m)
 
-/* XXX fix to use proper __STDC_VERSION__ number later on */
-#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 202000L)
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ > 201710L
 #  undef unreachable
 #  define unreachable() __builtin_unreachable()
+
+#  define __STDC_VERSION_STDDEF_H__ 202311L
 #endif
 
 #endif
diff --git a/stdint.h b/stdint.h
index 55991b6..7ff828a 100644
--- a/stdint.h
+++ b/stdint.h
@@ -12,10 +12,8 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#if !defined(__FSTD_HDRS_STDINT_H) || defined(__FSTD_HDRS_CXX_WRAP)
-#if !defined(__FSTD_HDRS_CXX_WRAP)
+#ifndef __FSTD_HDRS_STDINT_H
 #define __FSTD_HDRS_STDINT_H 1
-#endif
 
 #ifdef __UINT8_TYPE__
 typedef __UINT8_TYPE__ uint8_t;