diff --git a/devel/wasi-libc/Makefile b/devel/wasi-libc/Makefile
index 32770ed0b0ae..b93e1e6c5e34 100644
--- a/devel/wasi-libc/Makefile
+++ b/devel/wasi-libc/Makefile
@@ -1,46 +1,47 @@
 PORTNAME=	wasi-libc
 DISTVERSIONPREFIX=	wasi-sdk-
 DISTVERSION=	21
+PORTREVISION=	1
 CATEGORIES=	devel
 
 MAINTAINER=	vishwin@FreeBSD.org
 COMMENT=	C standard library for WebAssembly System Interface
 WWW=		https://github.com/WebAssembly/wasi-libc
 
 LICENSE=	APACHE20 MIT LLVM2
 LICENSE_COMB=	multi
 LICENSE_FILE_APACHE20=	${WRKSRC}/LICENSE-APACHE
 LICENSE_FILE_MIT=	${WRKSRC}/LICENSE-MIT
 LICENSE_FILE_LLVM2=	${WRKSRC}/LICENSE-APACHE-LLVM
 LICENSE_NAME_LLVM2=	Apache License 2.0 with LLVM Exceptions
 LICENSE_PERMS_LLVM2=	dist-mirror dist-sell pkg-mirror pkg-sell auto-accept
 
 USES=		gmake llvm:min=12
 USE_GITHUB=	yes
 GH_ACCOUNT=	WebAssembly
 GH_TUPLE=	WebAssembly:WASI:59cbe14:wasi/tools/wasi-headers/WASI
 NO_ARCH=	yes
 
 ALL_TARGET=	finish
 
 # NOTE: matches the https://github.com/WebAssembly/wasi-sdk
 WASI_SYSROOT=	${PREFIX}/share/wasi-sysroot
 
-# NOTE: our llvm ports don't ship stdarg/stddef clang headers, so they're in FILESDIR
-#       and we install them to the sysroot
-
 MAKE_ENV=	INSTALL_DIR=${STAGEDIR}${WASI_SYSROOT} \
 		EXTRA_CFLAGS="-O2 -DNDEBUG -isystem ${FILESDIR}" \
 		AR=${LOCALBASE}/bin/llvm-ar${LLVM_VERSION} \
 		NM=${LOCALBASE}/bin/llvm-nm${LLVM_VERSION} \
 
 post-patch:
 # NOTE: phony targets cause make install to rebuild everything
 #       https://github.com/WebAssembly/wasi-libc/issues/156
 	${REINPLACE_CMD} -e 's|install: finish|install:|' ${WRKSRC}/Makefile
 
 post-install:
-	${INSTALL_DATA} ${FILESDIR}/stdarg.h ${STAGEDIR}${WASI_SYSROOT}/include/
-	${INSTALL_DATA} ${FILESDIR}/stddef.h ${STAGEDIR}${WASI_SYSROOT}/include/
+# NOTE: our llvm ports don't ship stdarg/stddef clang headers, so
+#       they are copied from clang/lib/Headers into FILESDIR and we install
+#       them to the sysroot
+	${INSTALL_DATA} ${FILESDIR}/*stdarg*.h ${STAGEDIR}${WASI_SYSROOT}/include/
+	${INSTALL_DATA} ${FILESDIR}/*stddef*.h ${STAGEDIR}${WASI_SYSROOT}/include/
 
 .include <bsd.port.mk>
diff --git a/devel/wasi-libc/files/__stdarg___gnuc_va_list.h b/devel/wasi-libc/files/__stdarg___gnuc_va_list.h
new file mode 100644
index 000000000000..2a0a7e8cc6c0
--- /dev/null
+++ b/devel/wasi-libc/files/__stdarg___gnuc_va_list.h
@@ -0,0 +1,13 @@
+/*===---- __stdarg___gnuc_va_list.h - Definition of __gnuc_va_list ---------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef __GNUC_VA_LIST
+#define __GNUC_VA_LIST
+typedef __builtin_va_list __gnuc_va_list;
+#endif
diff --git a/devel/wasi-libc/files/__stdarg___va_copy.h b/devel/wasi-libc/files/__stdarg___va_copy.h
new file mode 100644
index 000000000000..e433e18a7082
--- /dev/null
+++ b/devel/wasi-libc/files/__stdarg___va_copy.h
@@ -0,0 +1,12 @@
+/*===---- __stdarg___va_copy.h - Definition of __va_copy -------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef __va_copy
+#define __va_copy(d, s) __builtin_va_copy(d, s)
+#endif
diff --git a/devel/wasi-libc/files/__stdarg_va_arg.h b/devel/wasi-libc/files/__stdarg_va_arg.h
new file mode 100644
index 000000000000..89bd2f65d3be
--- /dev/null
+++ b/devel/wasi-libc/files/__stdarg_va_arg.h
@@ -0,0 +1,22 @@
+/*===---- __stdarg_va_arg.h - Definitions of va_start, va_arg, va_end-------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef va_arg
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+/* C23 does not require the second parameter for va_start. */
+#define va_start(ap, ...) __builtin_va_start(ap, 0)
+#else
+/* Versions before C23 do require the second parameter. */
+#define va_start(ap, param) __builtin_va_start(ap, param)
+#endif
+#define va_end(ap) __builtin_va_end(ap)
+#define va_arg(ap, type) __builtin_va_arg(ap, type)
+
+#endif
diff --git a/devel/wasi-libc/files/__stdarg_va_copy.h b/devel/wasi-libc/files/__stdarg_va_copy.h
new file mode 100644
index 000000000000..8645328c2c68
--- /dev/null
+++ b/devel/wasi-libc/files/__stdarg_va_copy.h
@@ -0,0 +1,12 @@
+/*===---- __stdarg_va_copy.h - Definition of va_copy------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef va_copy
+#define va_copy(dest, src) __builtin_va_copy(dest, src)
+#endif
diff --git a/devel/wasi-libc/files/__stdarg_va_list.h b/devel/wasi-libc/files/__stdarg_va_list.h
new file mode 100644
index 000000000000..20c2e2cad940
--- /dev/null
+++ b/devel/wasi-libc/files/__stdarg_va_list.h
@@ -0,0 +1,13 @@
+/*===---- __stdarg_va_list.h - Definition of va_list -----------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef _VA_LIST
+#define _VA_LIST
+typedef __builtin_va_list va_list;
+#endif
diff --git a/devel/wasi-libc/files/__stddef_max_align_t.h b/devel/wasi-libc/files/__stddef_max_align_t.h
new file mode 100644
index 000000000000..512606a87728
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_max_align_t.h
@@ -0,0 +1,27 @@
+/*===---- __stddef_max_align_t.h - Definition of max_align_t ---------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef __CLANG_MAX_ALIGN_T_DEFINED
+#define __CLANG_MAX_ALIGN_T_DEFINED
+
+#if defined(_MSC_VER)
+typedef double max_align_t;
+#elif defined(__APPLE__)
+typedef long double max_align_t;
+#else
+// Define 'max_align_t' to match the GCC definition.
+typedef struct {
+  long long __clang_max_align_nonce1
+      __attribute__((__aligned__(__alignof__(long long))));
+  long double __clang_max_align_nonce2
+      __attribute__((__aligned__(__alignof__(long double))));
+} max_align_t;
+#endif
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_null.h b/devel/wasi-libc/files/__stddef_null.h
new file mode 100644
index 000000000000..c10bd2d7d988
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_null.h
@@ -0,0 +1,29 @@
+/*===---- __stddef_null.h - Definition of NULL -----------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#if !defined(NULL) || !__building_module(_Builtin_stddef)
+
+/* linux/stddef.h will define NULL to 0. glibc (and other) headers then define
+ * __need_NULL and rely on stddef.h to redefine NULL to the correct value again.
+ * Modules don't support redefining macros like that, but support that pattern
+ * in the non-modules case.
+ */
+#undef NULL
+
+#ifdef __cplusplus
+#if !defined(__MINGW32__) && !defined(_MSC_VER)
+#define NULL __null
+#else
+#define NULL 0
+#endif
+#else
+#define NULL ((void*)0)
+#endif
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_nullptr_t.h b/devel/wasi-libc/files/__stddef_nullptr_t.h
new file mode 100644
index 000000000000..7f3fbe6fe0d3
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_nullptr_t.h
@@ -0,0 +1,29 @@
+/*===---- __stddef_nullptr_t.h - Definition of nullptr_t -------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(_NULLPTR_T) ||                                                    \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define _NULLPTR_T
+
+#ifdef __cplusplus
+#if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
+namespace std {
+typedef decltype(nullptr) nullptr_t;
+}
+using ::std::nullptr_t;
+#endif
+#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+typedef typeof(nullptr) nullptr_t;
+#endif
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_offsetof.h b/devel/wasi-libc/files/__stddef_offsetof.h
new file mode 100644
index 000000000000..84172c6cd273
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_offsetof.h
@@ -0,0 +1,17 @@
+/*===---- __stddef_offsetof.h - Definition of offsetof ---------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(offsetof) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define offsetof(t, d) __builtin_offsetof(t, d)
+#endif
diff --git a/devel/wasi-libc/files/__stddef_ptrdiff_t.h b/devel/wasi-libc/files/__stddef_ptrdiff_t.h
new file mode 100644
index 000000000000..fd3c893c66c9
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_ptrdiff_t.h
@@ -0,0 +1,20 @@
+/*===---- __stddef_ptrdiff_t.h - Definition of ptrdiff_t -------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(_PTRDIFF_T) ||                                                    \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define _PTRDIFF_T
+
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_rsize_t.h b/devel/wasi-libc/files/__stddef_rsize_t.h
new file mode 100644
index 000000000000..dd433d40d973
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_rsize_t.h
@@ -0,0 +1,20 @@
+/*===---- __stddef_rsize_t.h - Definition of rsize_t -----------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(_RSIZE_T) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define _RSIZE_T
+
+typedef __SIZE_TYPE__ rsize_t;
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_size_t.h b/devel/wasi-libc/files/__stddef_size_t.h
new file mode 100644
index 000000000000..3dd7b1f37929
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_size_t.h
@@ -0,0 +1,20 @@
+/*===---- __stddef_size_t.h - Definition of size_t -------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(_SIZE_T) ||                                                       \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define _SIZE_T
+
+typedef __SIZE_TYPE__ size_t;
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_unreachable.h b/devel/wasi-libc/files/__stddef_unreachable.h
new file mode 100644
index 000000000000..518580c92d3f
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_unreachable.h
@@ -0,0 +1,17 @@
+/*===---- __stddef_unreachable.h - Definition of unreachable ---------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(unreachable) ||                                                   \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define unreachable() __builtin_unreachable()
+#endif
diff --git a/devel/wasi-libc/files/__stddef_wchar_t.h b/devel/wasi-libc/files/__stddef_wchar_t.h
new file mode 100644
index 000000000000..bd69f6322541
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_wchar_t.h
@@ -0,0 +1,28 @@
+/*===---- __stddef_wchar.h - Definition of wchar_t -------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#if !defined(__cplusplus) || (defined(_MSC_VER) && !_NATIVE_WCHAR_T_DEFINED)
+
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(_WCHAR_T) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
+#define _WCHAR_T
+
+#ifdef _MSC_EXTENSIONS
+#define _WCHAR_T_DEFINED
+#endif
+
+typedef __WCHAR_TYPE__ wchar_t;
+
+#endif
+
+#endif
diff --git a/devel/wasi-libc/files/__stddef_wint_t.h b/devel/wasi-libc/files/__stddef_wint_t.h
new file mode 100644
index 000000000000..0aa291507957
--- /dev/null
+++ b/devel/wasi-libc/files/__stddef_wint_t.h
@@ -0,0 +1,15 @@
+/*===---- __stddef_wint.h - Definition of wint_t ---------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef _WINT_T
+#define _WINT_T
+
+typedef __WINT_TYPE__ wint_t;
+
+#endif
diff --git a/devel/wasi-libc/files/stdarg.h b/devel/wasi-libc/files/stdarg.h
index 0bc39408c1e5..94b066566f08 100644
--- a/devel/wasi-libc/files/stdarg.h
+++ b/devel/wasi-libc/files/stdarg.h
@@ -1,35 +1,79 @@
 /*===---- stdarg.h - Variable argument handling ----------------------------===
  *
  * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  * See https://llvm.org/LICENSE.txt for license information.
  * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  *
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef __STDARG_H
-#define __STDARG_H
-
-#ifndef _VA_LIST
-typedef __builtin_va_list va_list;
-#define _VA_LIST
-#endif
-#define va_start(ap, param) __builtin_va_start(ap, param)
-#define va_end(ap)          __builtin_va_end(ap)
-#define va_arg(ap, type)    __builtin_va_arg(ap, type)
+/*
+ * This header is designed to be included multiple times. If any of the __need_
+ * macros are defined, then only that subset of interfaces are provided. This
+ * can be useful for POSIX headers that need to not expose all of stdarg.h, but
+ * need to use some of its interfaces. Otherwise this header provides all of
+ * the expected interfaces.
+ *
+ * When clang modules are enabled, this header is a textual header. It ignores
+ * its header guard so that multiple submodules can export its interfaces.
+ * Take module SM with submodules A and B, whose headers both include stdarg.h
+ * When SM.A builds, __STDARG_H will be defined. When SM.B builds, the
+ * definition from SM.A will leak when building without local submodule
+ * visibility. stdarg.h wouldn't include any of its implementation headers, and
+ * SM.B wouldn't import any of the stdarg modules, and SM.B's `export *`
+ * wouldn't export any stdarg interfaces as expected. However, since stdarg.h
+ * ignores its header guard when building with modules, it all works as
+ * expected.
+ *
+ * When clang modules are not enabled, the header guards can function in the
+ * normal simple fashion.
+ */
+#if !defined(__STDARG_H) || __has_feature(modules) ||                          \
+    defined(__need___va_list) || defined(__need_va_list) ||                    \
+    defined(__need_va_arg) || defined(__need___va_copy) ||                     \
+    defined(__need_va_copy)
 
+#if !defined(__need___va_list) && !defined(__need_va_list) &&                  \
+    !defined(__need_va_arg) && !defined(__need___va_copy) &&                   \
+    !defined(__need_va_copy)
+#define __STDARG_H
+#define __need___va_list
+#define __need_va_list
+#define __need_va_arg
+#define __need___va_copy
 /* GCC always defines __va_copy, but does not define va_copy unless in c99 mode
  * or -ansi is not specified, since it was not part of C90.
  */
-#define __va_copy(d,s) __builtin_va_copy(d,s)
-
-#if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L || !defined(__STRICT_ANSI__)
-#define va_copy(dest, src)  __builtin_va_copy(dest, src)
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) ||              \
+    (defined(__cplusplus) && __cplusplus >= 201103L) ||                        \
+    !defined(__STRICT_ANSI__)
+#define __need_va_copy
 #endif
-
-#ifndef __GNUC_VA_LIST
-#define __GNUC_VA_LIST 1
-typedef __builtin_va_list __gnuc_va_list;
 #endif
 
-#endif /* __STDARG_H */
+#ifdef __need___va_list
+#include <__stdarg___gnuc_va_list.h>
+#undef __need___va_list
+#endif /* defined(__need___va_list) */
+
+#ifdef __need_va_list
+#include <__stdarg_va_list.h>
+#undef __need_va_list
+#endif /* defined(__need_va_list) */
+
+#ifdef __need_va_arg
+#include <__stdarg_va_arg.h>
+#undef __need_va_arg
+#endif /* defined(__need_va_arg) */
+
+#ifdef __need___va_copy
+#include <__stdarg___va_copy.h>
+#undef __need___va_copy
+#endif /* defined(__need___va_copy) */
+
+#ifdef __need_va_copy
+#include <__stdarg_va_copy.h>
+#undef __need_va_copy
+#endif /* defined(__need_va_copy) */
+
+#endif
diff --git a/devel/wasi-libc/files/stddef.h b/devel/wasi-libc/files/stddef.h
index 15acd4427ca1..e0ad7b8d17af 100644
--- a/devel/wasi-libc/files/stddef.h
+++ b/devel/wasi-libc/files/stddef.h
@@ -1,121 +1,123 @@
 /*===---- stddef.h - Basic type definitions --------------------------------===
  *
  * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  * See https://llvm.org/LICENSE.txt for license information.
  * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  *
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(__STDDEF_H) || defined(__need_ptrdiff_t) ||                       \
-    defined(__need_size_t) || defined(__need_wchar_t) ||                       \
-    defined(__need_NULL) || defined(__need_wint_t)
+/*
+ * This header is designed to be included multiple times. If any of the __need_
+ * macros are defined, then only that subset of interfaces are provided. This
+ * can be useful for POSIX headers that need to not expose all of stddef.h, but
+ * need to use some of its interfaces. Otherwise this header provides all of
+ * the expected interfaces.
+ *
+ * When clang modules are enabled, this header is a textual header. It ignores
+ * its header guard so that multiple submodules can export its interfaces.
+ * Take module SM with submodules A and B, whose headers both include stddef.h
+ * When SM.A builds, __STDDEF_H will be defined. When SM.B builds, the
+ * definition from SM.A will leak when building without local submodule
+ * visibility. stddef.h wouldn't include any of its implementation headers, and
+ * SM.B wouldn't import any of the stddef modules, and SM.B's `export *`
+ * wouldn't export any stddef interfaces as expected. However, since stddef.h
+ * ignores its header guard when building with modules, it all works as
+ * expected.
+ *
+ * When clang modules are not enabled, the header guards can function in the
+ * normal simple fashion.
+ */
+#if !defined(__STDDEF_H) || __has_feature(modules) ||                          \
+    (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1) ||        \
+    defined(__need_ptrdiff_t) || defined(__need_size_t) ||                     \
+    defined(__need_rsize_t) || defined(__need_wchar_t) ||                      \
+    defined(__need_NULL) || defined(__need_nullptr_t) ||                       \
+    defined(__need_unreachable) || defined(__need_max_align_t) ||              \
+    defined(__need_offsetof) || defined(__need_wint_t)
 
 #if !defined(__need_ptrdiff_t) && !defined(__need_size_t) &&                   \
-    !defined(__need_wchar_t) && !defined(__need_NULL) &&                       \
-    !defined(__need_wint_t)
-/* Always define miscellaneous pieces when modules are available. */
-#if !__has_feature(modules)
+    !defined(__need_rsize_t) && !defined(__need_wchar_t) &&                    \
+    !defined(__need_NULL) && !defined(__need_nullptr_t) &&                     \
+    !defined(__need_unreachable) && !defined(__need_max_align_t) &&            \
+    !defined(__need_offsetof) && !defined(__need_wint_t)
 #define __STDDEF_H
-#endif
 #define __need_ptrdiff_t
 #define __need_size_t
+/* ISO9899:2011 7.20 (C11 Annex K): Define rsize_t if __STDC_WANT_LIB_EXT1__ is
+ * enabled. */
+#if defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1
+#define __need_rsize_t
+#endif
 #define __need_wchar_t
 #define __need_NULL
-#define __need_STDDEF_H_misc
-/* __need_wint_t is intentionally not defined here. */
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) ||              \
+    defined(__cplusplus)
+#define __need_nullptr_t
 #endif
-
-#if defined(__need_ptrdiff_t)
-#if !defined(_PTRDIFF_T) || __has_feature(modules)
-/* Always define ptrdiff_t when modules are available. */
-#if !__has_feature(modules)
-#define _PTRDIFF_T
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#define __need_unreachable
+#endif
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) ||              \
+    (defined(__cplusplus) && __cplusplus >= 201103L)
+#define __need_max_align_t
 #endif
-typedef __PTRDIFF_TYPE__ ptrdiff_t;
+#define __need_offsetof
+/* wint_t is provided by <wchar.h> and not <stddef.h>. It's here
+ * for compatibility, but must be explicitly requested. Therefore
+ * __need_wint_t is intentionally not defined here. */
 #endif
+
+#if defined(__need_ptrdiff_t)
+#include <__stddef_ptrdiff_t.h>
 #undef __need_ptrdiff_t
 #endif /* defined(__need_ptrdiff_t) */
 
 #if defined(__need_size_t)
-#if !defined(_SIZE_T) || __has_feature(modules)
-/* Always define size_t when modules are available. */
-#if !__has_feature(modules)
-#define _SIZE_T
-#endif
-typedef __SIZE_TYPE__ size_t;
-#endif
+#include <__stddef_size_t.h>
 #undef __need_size_t
 #endif /*defined(__need_size_t) */
 
-#if defined(__need_STDDEF_H_misc)
-/* ISO9899:2011 7.20 (C11 Annex K): Define rsize_t if __STDC_WANT_LIB_EXT1__ is
- * enabled. */
-#if (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1 && \
-     !defined(_RSIZE_T)) || __has_feature(modules)
-/* Always define rsize_t when modules are available. */
-#if !__has_feature(modules)
-#define _RSIZE_T
-#endif
-typedef __SIZE_TYPE__ rsize_t;
-#endif
-#endif /* defined(__need_STDDEF_H_misc) */
+#if defined(__need_rsize_t)
+#include <__stddef_rsize_t.h>
+#undef __need_rsize_t
+#endif /* defined(__need_rsize_t) */
 
 #if defined(__need_wchar_t)
-#ifndef __cplusplus
-/* Always define wchar_t when modules are available. */
-#if !defined(_WCHAR_T) || __has_feature(modules)
-#if !__has_feature(modules)
-#define _WCHAR_T
-#if defined(_MSC_EXTENSIONS)
-#define _WCHAR_T_DEFINED
-#endif
-#endif
-typedef __WCHAR_TYPE__ wchar_t;
-#endif
-#endif
+#include <__stddef_wchar_t.h>
 #undef __need_wchar_t
 #endif /* defined(__need_wchar_t) */
 
 #if defined(__need_NULL)
-#undef NULL
-#ifdef __cplusplus
-#  if !defined(__MINGW32__) && !defined(_MSC_VER)
-#    define NULL __null
-#  else
-#    define NULL 0
-#  endif
-#else
-#  define NULL ((void*)0)
-#endif
-#ifdef __cplusplus
-#if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
-namespace std { typedef decltype(nullptr) nullptr_t; }
-using ::std::nullptr_t;
-#endif
-#endif
+#include <__stddef_null.h>
 #undef __need_NULL
 #endif /* defined(__need_NULL) */
 
-#if defined(__need_STDDEF_H_misc)
-#if __STDC_VERSION__ >= 201112L || __cplusplus >= 201103L
-#include "__stddef_max_align_t.h"
-#endif
-#define offsetof(t, d) __builtin_offsetof(t, d)
-#undef __need_STDDEF_H_misc
-#endif  /* defined(__need_STDDEF_H_misc) */
+#if defined(__need_nullptr_t)
+#include <__stddef_nullptr_t.h>
+#undef __need_nullptr_t
+#endif /* defined(__need_nullptr_t) */
+
+#if defined(__need_unreachable)
+#include <__stddef_unreachable.h>
+#undef __need_unreachable
+#endif /* defined(__need_unreachable) */
+
+#if defined(__need_max_align_t)
+#include <__stddef_max_align_t.h>
+#undef __need_max_align_t
+#endif /* defined(__need_max_align_t) */
+
+#if defined(__need_offsetof)
+#include <__stddef_offsetof.h>
+#undef __need_offsetof
+#endif /* defined(__need_offsetof) */
 
 /* Some C libraries expect to see a wint_t here. Others (notably MinGW) will use
 __WINT_TYPE__ directly; accommodate both by requiring __need_wint_t */
 #if defined(__need_wint_t)
-/* Always define wint_t when modules are available. */
-#if !defined(_WINT_T) || __has_feature(modules)
-#if !__has_feature(modules)
-#define _WINT_T
-#endif
-typedef __WINT_TYPE__ wint_t;
-#endif
+#include <__stddef_wint_t.h>
 #undef __need_wint_t
 #endif /* __need_wint_t */
 
 #endif
diff --git a/devel/wasi-libc/pkg-plist b/devel/wasi-libc/pkg-plist
index 6b7902c5d833..1a52ec4c7996 100644
--- a/devel/wasi-libc/pkg-plist
+++ b/devel/wasi-libc/pkg-plist
@@ -1,223 +1,238 @@
 share/wasi-sysroot/include/__errno.h
 share/wasi-sysroot/include/__errno_values.h
 share/wasi-sysroot/include/__fd_set.h
 share/wasi-sysroot/include/__function___isatty.h
 share/wasi-sysroot/include/__functions_malloc.h
 share/wasi-sysroot/include/__functions_memcpy.h
 share/wasi-sysroot/include/__header_dirent.h
 share/wasi-sysroot/include/__header_fcntl.h
 share/wasi-sysroot/include/__header_inttypes.h
 share/wasi-sysroot/include/__header_netinet_in.h
 share/wasi-sysroot/include/__header_poll.h
 share/wasi-sysroot/include/__header_stdlib.h
 share/wasi-sysroot/include/__header_string.h
 share/wasi-sysroot/include/__header_sys_ioctl.h
 share/wasi-sysroot/include/__header_sys_resource.h
 share/wasi-sysroot/include/__header_sys_socket.h
 share/wasi-sysroot/include/__header_sys_stat.h
 share/wasi-sysroot/include/__header_time.h
 share/wasi-sysroot/include/__header_unistd.h
 share/wasi-sysroot/include/__macro_FD_SETSIZE.h
 share/wasi-sysroot/include/__macro_PAGESIZE.h
 share/wasi-sysroot/include/__mode_t.h
 share/wasi-sysroot/include/__seek.h
+share/wasi-sysroot/include/__stdarg___gnuc_va_list.h
+share/wasi-sysroot/include/__stdarg___va_copy.h
+share/wasi-sysroot/include/__stdarg_va_arg.h
+share/wasi-sysroot/include/__stdarg_va_copy.h
+share/wasi-sysroot/include/__stdarg_va_list.h
+share/wasi-sysroot/include/__stddef_max_align_t.h
+share/wasi-sysroot/include/__stddef_null.h
+share/wasi-sysroot/include/__stddef_nullptr_t.h
+share/wasi-sysroot/include/__stddef_offsetof.h
+share/wasi-sysroot/include/__stddef_ptrdiff_t.h
+share/wasi-sysroot/include/__stddef_rsize_t.h
+share/wasi-sysroot/include/__stddef_size_t.h
+share/wasi-sysroot/include/__stddef_unreachable.h
+share/wasi-sysroot/include/__stddef_wchar_t.h
+share/wasi-sysroot/include/__stddef_wint_t.h
 share/wasi-sysroot/include/__struct_dirent.h
 share/wasi-sysroot/include/__struct_in6_addr.h
 share/wasi-sysroot/include/__struct_in_addr.h
 share/wasi-sysroot/include/__struct_iovec.h
 share/wasi-sysroot/include/__struct_msghdr.h
 share/wasi-sysroot/include/__struct_pollfd.h
 share/wasi-sysroot/include/__struct_rusage.h
 share/wasi-sysroot/include/__struct_sockaddr.h
 share/wasi-sysroot/include/__struct_sockaddr_in.h
 share/wasi-sysroot/include/__struct_sockaddr_in6.h
 share/wasi-sysroot/include/__struct_sockaddr_storage.h
 share/wasi-sysroot/include/__struct_sockaddr_un.h
 share/wasi-sysroot/include/__struct_stat.h
 share/wasi-sysroot/include/__struct_timespec.h
 share/wasi-sysroot/include/__struct_timeval.h
 share/wasi-sysroot/include/__struct_tm.h
 share/wasi-sysroot/include/__struct_tms.h
 share/wasi-sysroot/include/__typedef_DIR.h
 share/wasi-sysroot/include/__typedef_blkcnt_t.h
 share/wasi-sysroot/include/__typedef_blksize_t.h
 share/wasi-sysroot/include/__typedef_clock_t.h
 share/wasi-sysroot/include/__typedef_clockid_t.h
 share/wasi-sysroot/include/__typedef_dev_t.h
 share/wasi-sysroot/include/__typedef_fd_set.h
 share/wasi-sysroot/include/__typedef_gid_t.h
 share/wasi-sysroot/include/__typedef_in_addr_t.h
 share/wasi-sysroot/include/__typedef_in_port_t.h
 share/wasi-sysroot/include/__typedef_ino_t.h
 share/wasi-sysroot/include/__typedef_mode_t.h
 share/wasi-sysroot/include/__typedef_nfds_t.h
 share/wasi-sysroot/include/__typedef_nlink_t.h
 share/wasi-sysroot/include/__typedef_off_t.h
 share/wasi-sysroot/include/__typedef_sa_family_t.h
 share/wasi-sysroot/include/__typedef_sigset_t.h
 share/wasi-sysroot/include/__typedef_socklen_t.h
 share/wasi-sysroot/include/__typedef_ssize_t.h
 share/wasi-sysroot/include/__typedef_suseconds_t.h
 share/wasi-sysroot/include/__typedef_time_t.h
 share/wasi-sysroot/include/__typedef_uid_t.h
 share/wasi-sysroot/include/alloca.h
 share/wasi-sysroot/include/ar.h
 share/wasi-sysroot/include/arpa/ftp.h
 share/wasi-sysroot/include/arpa/inet.h
 share/wasi-sysroot/include/arpa/nameser.h
 share/wasi-sysroot/include/arpa/nameser_compat.h
 share/wasi-sysroot/include/arpa/telnet.h
 share/wasi-sysroot/include/arpa/tftp.h
 share/wasi-sysroot/include/assert.h
 share/wasi-sysroot/include/bits/alltypes.h
 share/wasi-sysroot/include/bits/dirent.h
 share/wasi-sysroot/include/bits/fcntl.h
 share/wasi-sysroot/include/bits/fenv.h
 share/wasi-sysroot/include/bits/float.h
 share/wasi-sysroot/include/bits/hwcap.h
 share/wasi-sysroot/include/bits/io.h
 share/wasi-sysroot/include/bits/ioctl.h
 share/wasi-sysroot/include/bits/ioctl_fix.h
 share/wasi-sysroot/include/bits/ipcstat.h
 share/wasi-sysroot/include/bits/limits.h
 share/wasi-sysroot/include/bits/mman.h
 share/wasi-sysroot/include/bits/poll.h
 share/wasi-sysroot/include/bits/posix.h
 share/wasi-sysroot/include/bits/reg.h
 share/wasi-sysroot/include/bits/resource.h
 share/wasi-sysroot/include/bits/signal.h
 share/wasi-sysroot/include/bits/socket.h
 share/wasi-sysroot/include/bits/stat.h
 share/wasi-sysroot/include/bits/stdint.h
 share/wasi-sysroot/include/byteswap.h
 share/wasi-sysroot/include/complex.h
 share/wasi-sysroot/include/cpio.h
 share/wasi-sysroot/include/crypt.h
 share/wasi-sysroot/include/ctype.h
 share/wasi-sysroot/include/dirent.h
 share/wasi-sysroot/include/dlfcn.h
 share/wasi-sysroot/include/endian.h
 share/wasi-sysroot/include/err.h
 share/wasi-sysroot/include/errno.h
 share/wasi-sysroot/include/fcntl.h
 share/wasi-sysroot/include/features.h
 share/wasi-sysroot/include/fenv.h
 share/wasi-sysroot/include/float.h
 share/wasi-sysroot/include/fmtmsg.h
 share/wasi-sysroot/include/fnmatch.h
 share/wasi-sysroot/include/ftw.h
 share/wasi-sysroot/include/getopt.h
 share/wasi-sysroot/include/glob.h
 share/wasi-sysroot/include/iconv.h
 share/wasi-sysroot/include/ifaddrs.h
 share/wasi-sysroot/include/inttypes.h
 share/wasi-sysroot/include/iso646.h
 share/wasi-sysroot/include/langinfo.h
 share/wasi-sysroot/include/libgen.h
 share/wasi-sysroot/include/limits.h
 share/wasi-sysroot/include/locale.h
 share/wasi-sysroot/include/malloc.h
 share/wasi-sysroot/include/math.h
 share/wasi-sysroot/include/memory.h
 share/wasi-sysroot/include/monetary.h
 share/wasi-sysroot/include/mqueue.h
 share/wasi-sysroot/include/netinet/icmp6.h
 share/wasi-sysroot/include/netinet/igmp.h
 share/wasi-sysroot/include/netinet/in.h
 share/wasi-sysroot/include/netinet/in_systm.h
 share/wasi-sysroot/include/netinet/ip.h
 share/wasi-sysroot/include/netinet/ip6.h
 share/wasi-sysroot/include/netinet/ip_icmp.h
 share/wasi-sysroot/include/netinet/tcp.h
 share/wasi-sysroot/include/netinet/udp.h
 share/wasi-sysroot/include/netpacket/packet.h
 share/wasi-sysroot/include/nl_types.h
 share/wasi-sysroot/include/poll.h
 share/wasi-sysroot/include/regex.h
 share/wasi-sysroot/include/sched.h
 share/wasi-sysroot/include/search.h
 share/wasi-sysroot/include/semaphore.h
 share/wasi-sysroot/include/signal.h
 share/wasi-sysroot/include/stdalign.h
 share/wasi-sysroot/include/stdarg.h
 share/wasi-sysroot/include/stdbool.h
 share/wasi-sysroot/include/stdc-predef.h
 share/wasi-sysroot/include/stddef.h
 share/wasi-sysroot/include/stdint.h
 share/wasi-sysroot/include/stdio.h
 share/wasi-sysroot/include/stdio_ext.h
 share/wasi-sysroot/include/stdlib.h
 share/wasi-sysroot/include/stdnoreturn.h
 share/wasi-sysroot/include/string.h
 share/wasi-sysroot/include/strings.h
 share/wasi-sysroot/include/stropts.h
 share/wasi-sysroot/include/sys/dir.h
 share/wasi-sysroot/include/sys/errno.h
 share/wasi-sysroot/include/sys/eventfd.h
 share/wasi-sysroot/include/sys/fcntl.h
 share/wasi-sysroot/include/sys/file.h
 share/wasi-sysroot/include/sys/ioctl.h
 share/wasi-sysroot/include/sys/mman.h
 share/wasi-sysroot/include/sys/param.h
 share/wasi-sysroot/include/sys/poll.h
 share/wasi-sysroot/include/sys/random.h
 share/wasi-sysroot/include/sys/reg.h
 share/wasi-sysroot/include/sys/resource.h
 share/wasi-sysroot/include/sys/select.h
 share/wasi-sysroot/include/sys/signal.h
 share/wasi-sysroot/include/sys/socket.h
 share/wasi-sysroot/include/sys/stat.h
 share/wasi-sysroot/include/sys/stropts.h
 share/wasi-sysroot/include/sys/syscall.h
 share/wasi-sysroot/include/sys/sysinfo.h
 share/wasi-sysroot/include/sys/time.h
 share/wasi-sysroot/include/sys/timeb.h
 share/wasi-sysroot/include/sys/times.h
 share/wasi-sysroot/include/sys/timex.h
 share/wasi-sysroot/include/sys/ttydefaults.h
 share/wasi-sysroot/include/sys/types.h
 share/wasi-sysroot/include/sys/uio.h
 share/wasi-sysroot/include/sys/un.h
 share/wasi-sysroot/include/sys/utsname.h
 share/wasi-sysroot/include/syscall.h
 share/wasi-sysroot/include/sysexits.h
 share/wasi-sysroot/include/tar.h
 share/wasi-sysroot/include/tgmath.h
 share/wasi-sysroot/include/threads.h
 share/wasi-sysroot/include/time.h
 share/wasi-sysroot/include/uchar.h
 share/wasi-sysroot/include/unistd.h
 share/wasi-sysroot/include/utime.h
 share/wasi-sysroot/include/values.h
 share/wasi-sysroot/include/wasi/api.h
 share/wasi-sysroot/include/wasi/libc-environ.h
 share/wasi-sysroot/include/wasi/libc-find-relpath.h
 share/wasi-sysroot/include/wasi/libc-nocwd.h
 share/wasi-sysroot/include/wasi/libc.h
 share/wasi-sysroot/include/wchar.h
 share/wasi-sysroot/include/wctype.h
 share/wasi-sysroot/lib/wasm32-wasi/crt1-command.o
 share/wasi-sysroot/lib/wasm32-wasi/crt1-reactor.o
 share/wasi-sysroot/lib/wasm32-wasi/crt1.o
 share/wasi-sysroot/lib/wasm32-wasi/libc-printscan-long-double.a
 share/wasi-sysroot/lib/wasm32-wasi/libc-printscan-no-floating-point.a
 share/wasi-sysroot/lib/wasm32-wasi/libc.a
 share/wasi-sysroot/lib/wasm32-wasi/libc.imports
 share/wasi-sysroot/lib/wasm32-wasi/libcrypt.a
 share/wasi-sysroot/lib/wasm32-wasi/libdl.a
 share/wasi-sysroot/lib/wasm32-wasi/libm.a
 share/wasi-sysroot/lib/wasm32-wasi/libpthread.a
 share/wasi-sysroot/lib/wasm32-wasi/libresolv.a
 share/wasi-sysroot/lib/wasm32-wasi/librt.a
 share/wasi-sysroot/lib/wasm32-wasi/libutil.a
 share/wasi-sysroot/lib/wasm32-wasi/libwasi-emulated-getpid.a
 share/wasi-sysroot/lib/wasm32-wasi/libwasi-emulated-mman.a
 share/wasi-sysroot/lib/wasm32-wasi/libwasi-emulated-process-clocks.a
 share/wasi-sysroot/lib/wasm32-wasi/libwasi-emulated-signal.a
 share/wasi-sysroot/lib/wasm32-wasi/libxnet.a
 share/wasi-sysroot/share/wasm32-wasi/defined-symbols.txt
 share/wasi-sysroot/share/wasm32-wasi/include-all.c
 share/wasi-sysroot/share/wasm32-wasi/predefined-macros.txt
 share/wasi-sysroot/share/wasm32-wasi/undefined-symbols.txt
 @dir share/wasi-sysroot/include/net
 @dir share/wasi-sysroot/include/scsi