[PATCH RFC 0/5] kbuild: uapi: also test UAPI headers against C++ compilers

[PATCH RFC 0/5] kbuild: uapi: also test UAPI headers against C++ compilers

[Thomas Weißschuh]

C++ language requirements differ from those of C.

Also test the headers against C++ compilers to make sure no errors
creep in accidentally.

There was a previous attempt by David Howells[0], but today with the
existing UAPI testing infrastructure the implementation is a bit nicer.
This series does not derive from David's, as I found it only after
writing my patches.

[0] https://lore.kernel.org/lkml/nycvar.YFH.7.76.1809052120510.24932@n3.vanv.qr/

This depends on my other series "kbuild: uapi: remove usage of toolchain
headers", which is currently in kbuild-next-unstable.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
Thomas Weißschuh (5):
      kbuild: uapi: move some compiler arguments out of the command definition
      kbuild: uapi: move all include path flags together
      kbuild: uapi: handle UML in architecture-specific exclusion lists
      kbuild: uapi: provide a C++ compatible dummy definition of NULL
      kbuild: uapi: also test UAPI headers against C++ compilers

 usr/dummy-include/stddef.h |  5 +++++
 usr/include/Makefile       | 47 ++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 44 insertions(+), 8 deletions(-)
---
base-commit: c14d8386b0a38d45a4345f556d6491be3594cf8e
change-id: 20260223-kbuild-uapi-c-18002c9d7ca3

Best regards,
-- 
Thomas Weißschuh <linux@weissschuh.net>

[PATCH RFC 1/5] kbuild: uapi: move some compiler arguments out of the command definition

[Thomas Weißschuh]

Simplify the definition of cmd_hdrtest by moving some of it to a new
variable. This will both enable the upcoming reuse of those flags
and also the extension of cmd_hdrtest.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 usr/include/Makefile | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index 845d06b63c2b..0b1c9a4b0477 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -140,11 +140,13 @@ always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/
 target-libc = $(filter $(uses-libc), $*.h)
 target-can-compile = $(filter-out $(no-header-test), $*.h)
 
+hdrtest-flags = -fsyntax-only -Werror \
+		-nostdinc $(if $(target-libc), -I $(srctree)/usr/dummy-include)
+
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
-		$(CC) $(c_flags) -fsyntax-only -Werror -x c /dev/null \
-			-nostdinc $(if $(target-libc), -I $(srctree)/usr/dummy-include) \
+		$(CC) $(c_flags) $(hdrtest-flags) -x c /dev/null \
 			$(if $(target-can-compile), -include $< -include $<); \
 		$(PERL) $(src)/headers_check.pl $(obj) $<; \
 		touch $@

-- 
2.53.0

[PATCH RFC 2/5] kbuild: uapi: move all include path flags together

[Thomas Weißschuh]

The argument to add the root of the UAPI header tree to the include path
is separated from the other arguments concerning the include path.

Move all include path arguments together for more consistency and
balanced line lengths.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 usr/include/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index 0b1c9a4b0477..74f5ba24bb29 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -8,7 +8,7 @@
 # We cannot go as far as adding -Wpedantic since it emits too many warnings.
 UAPI_CFLAGS := -std=c90 -Werror=implicit-function-declaration
 
-override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I $(obj)
+override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile)
 
 # The following are excluded for now because they fail to build.
 #
@@ -141,7 +141,7 @@ target-libc = $(filter $(uses-libc), $*.h)
 target-can-compile = $(filter-out $(no-header-test), $*.h)
 
 hdrtest-flags = -fsyntax-only -Werror \
-		-nostdinc $(if $(target-libc), -I $(srctree)/usr/dummy-include)
+		-nostdinc -I $(obj) $(if $(target-libc), -I $(srctree)/usr/dummy-include)
 
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<

-- 
2.53.0

[PATCH RFC 3/5] kbuild: uapi: handle UML in architecture-specific exclusion lists

[Thomas Weißschuh]

When building User Mode Linux SRCARCH is set to 'um', while the actual
underlying architecture is provided in HEADER_ARCH.

Allow the exclusion lists to work on UML by comparing against
HEADER_ARCH when that is available.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 usr/include/Makefile | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index 74f5ba24bb29..edeaf9d778c4 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -7,6 +7,7 @@
 # -std=c90 (equivalent to -ansi) catches the violation of those.
 # We cannot go as far as adding -Wpedantic since it emits too many warnings.
 UAPI_CFLAGS := -std=c90 -Werror=implicit-function-declaration
+UAPI_ARCH := $(or $(HEADER_ARCH),$(SRCARCH))
 
 override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile)
 
@@ -48,11 +49,11 @@ no-header-test += xen/privcmd.h
 
 # More headers are broken in some architectures
 
-ifneq ($(filter arc openrisc xtensa nios2, $(SRCARCH)),)
+ifneq ($(filter arc openrisc xtensa nios2, $(UAPI_ARCH)),)
 no-header-test += linux/bpf_perf_event.h
 endif
 
-ifeq ($(SRCARCH),sparc)
+ifeq ($(UAPI_ARCH),sparc)
 no-header-test += asm/uctx.h
 no-header-test += asm/fbio.h
 endif
@@ -122,15 +123,15 @@ uses-libc += linux/wireless.h
 uses-libc += regulator/regulator.h
 uses-libc += scsi/fc/fc_els.h
 
-ifeq ($(SRCARCH),hexagon)
+ifeq ($(UAPI_ARCH),hexagon)
 uses-libc += asm/sigcontext.h
 endif
 
-ifeq ($(SRCARCH),nios2)
+ifeq ($(UAPI_ARCH),nios2)
 uses-libc += asm/ptrace.h
 endif
 
-ifeq ($(SRCARCH),s390)
+ifeq ($(UAPI_ARCH),s390)
 uses-libc += asm/chpid.h
 uses-libc += asm/chsc.h
 endif

-- 
2.53.0

[PATCH RFC 4/5] kbuild: uapi: provide a C++ compatible dummy definition of NULL

[Thomas Weißschuh]

NULL works differently in C++ compared to C.

To allow testing the UAPI headers against C++ compilers, provide
a variant of NULL which works with those.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 usr/dummy-include/stddef.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/usr/dummy-include/stddef.h b/usr/dummy-include/stddef.h
index a61703d1c896..525489daffe2 100644
--- a/usr/dummy-include/stddef.h
+++ b/usr/dummy-include/stddef.h
@@ -3,6 +3,11 @@
 #define _DUMMY_STDDEF_H
 
 #define offsetof(TYPE, MEMBER)	__builtin_offsetof(TYPE, MEMBER)
+
+#ifdef __cplusplus
+#define NULL 0
+#else
 #define NULL ((void *)0)
+#endif
 
 #endif /* _DUMMY_STDDEF_H */

-- 
2.53.0

[PATCH RFC 5/5] kbuild: uapi: also test UAPI headers against C++ compilers

[Thomas Weißschuh]

C++ language requirements differ from those of C.

Also test the headers against C++ compilers to make sure no errors
creep in accidentally.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 usr/include/Makefile | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index edeaf9d778c4..0e23aeace410 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -10,6 +10,7 @@ UAPI_CFLAGS := -std=c90 -Werror=implicit-function-declaration
 UAPI_ARCH := $(or $(HEADER_ARCH),$(SRCARCH))
 
 override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile)
+cxx_flags = $(filter-out -Wmissing-prototypes -Wstrict-prototypes -std=%, $(KBUILD_USERCFLAGS)) -std=c++98
 
 # The following are excluded for now because they fail to build.
 #
@@ -61,6 +62,27 @@ endif
 # asm-generic/*.h is used by asm/*.h, and should not be included directly
 no-header-test += asm-generic/%
 
+# The following are not compatible with C++.
+#
+# Do not add a new header to the list without legitimate reason.
+# Please consider to fix the header first.
+#
+# Sorted alphabetically.
+no-header-test-cxx += linux/auto_dev-ioctl.h
+no-header-test-cxx += linux/map_to_14segment.h
+no-header-test-cxx += linux/map_to_7segment.h
+no-header-test-cxx += linux/netfilter/xt_sctp.h
+no-header-test-cxx += linux/target_core_user.h
+no-header-test-cxx += linux/vhost.h
+no-header-test-cxx += linux/vhost_types.h
+no-header-test-cxx += linux/virtio_net.h
+no-header-test-cxx += linux/virtio_ring.h
+no-header-test-cxx += scsi/fc/fc_els.h
+
+ifeq ($(UAPI_ARCH),x86)
+no-header-test-cxx += asm/elf.h
+endif
+
 # The following are using libc header and types.
 #
 # Do not add a new header to the list without legitimate reason.
@@ -138,8 +160,12 @@ endif
 
 always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
+# $(cc-option) forces '-x c' which breaks '-x c++' detection.
+cc-can-compile-cxx := $(call try-run,$(CC) -c -x c++ /dev/null -o "$$TMP", 1)
+
 target-libc = $(filter $(uses-libc), $*.h)
 target-can-compile = $(filter-out $(no-header-test), $*.h)
+target-can-compile-cxx = $(and $(cc-can-compile-cxx), $(target-can-compile), $(filter-out $(no-header-test-cxx), $*.h))
 
 hdrtest-flags = -fsyntax-only -Werror \
 		-nostdinc -I $(obj) $(if $(target-libc), -I $(srctree)/usr/dummy-include)
@@ -149,6 +175,8 @@ quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
 		$(CC) $(c_flags) $(hdrtest-flags) -x c /dev/null \
 			$(if $(target-can-compile), -include $< -include $<); \
+		$(if $(target-can-compile-cxx), \
+			$(CC) $(cxx_flags) $(hdrtest-flags) -x c++ /dev/null -include $<;) \
 		$(PERL) $(src)/headers_check.pl $(obj) $<; \
 		touch $@
 

-- 
2.53.0

Re: [PATCH RFC 5/5] kbuild: uapi: also test UAPI headers against C++ compilers

[Nathan Chancellor]

On Sun, Mar 08, 2026 at 10:24:05PM +0100, Thomas Weißschuh wrote:
> C++ language requirements differ from those of C.
> 
> Also test the headers against C++ compilers to make sure no errors
> creep in accidentally.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  usr/include/Makefile | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/usr/include/Makefile b/usr/include/Makefile
> index edeaf9d778c4..0e23aeace410 100644
> --- a/usr/include/Makefile
> +++ b/usr/include/Makefile
> @@ -10,6 +10,7 @@ UAPI_CFLAGS := -std=c90 -Werror=implicit-function-declaration
>  UAPI_ARCH := $(or $(HEADER_ARCH),$(SRCARCH))
>  
>  override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile)
> +cxx_flags = $(filter-out -Wmissing-prototypes -Wstrict-prototypes -std=%, $(KBUILD_USERCFLAGS)) -std=c++98
>  
>  # The following are excluded for now because they fail to build.
>  #
> @@ -61,6 +62,27 @@ endif
>  # asm-generic/*.h is used by asm/*.h, and should not be included directly
>  no-header-test += asm-generic/%
>  
> +# The following are not compatible with C++.
> +#
> +# Do not add a new header to the list without legitimate reason.
> +# Please consider to fix the header first.
> +#
> +# Sorted alphabetically.
> +no-header-test-cxx += linux/auto_dev-ioctl.h
> +no-header-test-cxx += linux/map_to_14segment.h
> +no-header-test-cxx += linux/map_to_7segment.h
> +no-header-test-cxx += linux/netfilter/xt_sctp.h
> +no-header-test-cxx += linux/target_core_user.h
> +no-header-test-cxx += linux/vhost.h
> +no-header-test-cxx += linux/vhost_types.h
> +no-header-test-cxx += linux/virtio_net.h
> +no-header-test-cxx += linux/virtio_ring.h
> +no-header-test-cxx += scsi/fc/fc_els.h
> +
> +ifeq ($(UAPI_ARCH),x86)
> +no-header-test-cxx += asm/elf.h
> +endif
> +
>  # The following are using libc header and types.
>  #
>  # Do not add a new header to the list without legitimate reason.
> @@ -138,8 +160,12 @@ endif
>  
>  always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
>  
> +# $(cc-option) forces '-x c' which breaks '-x c++' detection.
> +cc-can-compile-cxx := $(call try-run,$(CC) -c -x c++ /dev/null -o "$$TMP", 1)

Include $(CLANG_FLAGS) here just in case clang cannot generate code for
the host target (I know, sounds weird, but I have seen people build
LLVM only targeting arm/arm64 with an x86_64 host...).

> +
>  target-libc = $(filter $(uses-libc), $*.h)
>  target-can-compile = $(filter-out $(no-header-test), $*.h)
> +target-can-compile-cxx = $(and $(cc-can-compile-cxx), $(target-can-compile), $(filter-out $(no-header-test-cxx), $*.h))
>  
>  hdrtest-flags = -fsyntax-only -Werror \
>  		-nostdinc -I $(obj) $(if $(target-libc), -I $(srctree)/usr/dummy-include)
> @@ -149,6 +175,8 @@ quiet_cmd_hdrtest = HDRTEST $<
>        cmd_hdrtest = \
>  		$(CC) $(c_flags) $(hdrtest-flags) -x c /dev/null \
>  			$(if $(target-can-compile), -include $< -include $<); \
> +		$(if $(target-can-compile-cxx), \
> +			$(CC) $(cxx_flags) $(hdrtest-flags) -x c++ /dev/null -include $<;) \
>  		$(PERL) $(src)/headers_check.pl $(obj) $<; \
>  		touch $@
>  
> 
> -- 
> 2.53.0
> 

Re: [PATCH RFC 0/5] kbuild: uapi: also test UAPI headers against C++ compilers

[Nathan Chancellor]

On Sun, Mar 08, 2026 at 10:24:00PM +0100, Thomas Weißschuh wrote:
> C++ language requirements differ from those of C.
> 
> Also test the headers against C++ compilers to make sure no errors
> creep in accidentally.
> 
> There was a previous attempt by David Howells[0], but today with the
> existing UAPI testing infrastructure the implementation is a bit nicer.
> This series does not derive from David's, as I found it only after
> writing my patches.
> 
> [0] https://lore.kernel.org/lkml/nycvar.YFH.7.76.1809052120510.24932@n3.vanv.qr/
> 
> This depends on my other series "kbuild: uapi: remove usage of toolchain
> headers", which is currently in kbuild-next-unstable.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
> Thomas Weißschuh (5):
>       kbuild: uapi: move some compiler arguments out of the command definition
>       kbuild: uapi: move all include path flags together
>       kbuild: uapi: handle UML in architecture-specific exclusion lists
>       kbuild: uapi: provide a C++ compatible dummy definition of NULL
>       kbuild: uapi: also test UAPI headers against C++ compilers

Aside from my comment on patch 5, this seems reasonable to me.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

Cheers,
Nathan