Re: [RFC PATCH] libxfs: compile with a C++ compiler

[Sam James <sam@gentoo.org>]

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"Darrick J. Wong" <djwong@kernel.org> writes:

> From: Darrick J. Wong <djwong@kernel.org>
>
> Apparently C++ compilers don't like the implicit void* casts that go on
> in the system headers.  Compile a dummy program with the C++ compiler to
> make sure this works, so Darrick has /some/ chance of figuring these
> things out before the users do.

Thanks, this is a good idea. Double thanks for the quick fix.

1) yes, it finds the breakage:
Tested-by: Sam James <sam@gentoo.org>

2) with the fix below (CC -> CXX):
Reviewed-by: Sam James <sam@gentoo.org>

3) another thing to think about is:
* -pedantic?
* maybe do one for a bunch of standards? (I think systemd does every
possible value [1])
* doing the above for C as well

I know that sounds a bit like overkill, but systemd
does it and it's cheap to do it versus the blowup if something goes
wrong. I don't have a strong opinion on this or how far you want to go
with it.

[1] https://github.com/systemd/systemd/blob/3317aedff0901e08a8efc8346ad76b184d5d40ea/src/systemd/meson.build#L60

>
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  configure.ac         |    1 +
>  include/builddefs.in |    7 +++++++
>  libxfs/Makefile      |    8 +++++++-
>  libxfs/dummy.cpp     |   15 +++++++++++++++
>  4 files changed, 30 insertions(+), 1 deletion(-)
>  create mode 100644 libxfs/dummy.cpp
>
> diff --git a/configure.ac b/configure.ac
> index 0ffe2e5dfc53..04544f85395b 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -9,6 +9,7 @@ AC_PROG_INSTALL
>  LT_INIT
>  
>  AC_PROG_CC
> +AC_PROG_CXX
>  AC_ARG_VAR(BUILD_CC, [C compiler for build tools])
>  if test "${BUILD_CC+set}" != "set"; then
>    if test $cross_compiling = no; then
> diff --git a/include/builddefs.in b/include/builddefs.in
> index 44f95234d21b..0f312b8b88fe 100644
> --- a/include/builddefs.in
> +++ b/include/builddefs.in
> @@ -14,6 +14,7 @@ MALLOCLIB = @malloc_lib@
>  LOADERFLAGS = @LDFLAGS@
>  LTLDFLAGS = @LDFLAGS@
>  CFLAGS = @CFLAGS@ -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64 -Wno-address-of-packed-member
> +CXXFLAGS = @CXXFLAGS@ -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64 -Wno-address-of-packed-member
>  BUILD_CFLAGS = @BUILD_CFLAGS@ -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64
>  
>  # make sure we don't pick up whacky LDFLAGS from the make environment and
> @@ -234,9 +235,15 @@ ifeq ($(ENABLE_GETTEXT),yes)
>  GCFLAGS += -DENABLE_GETTEXT
>  endif
>  
> +# Override these if C++ needs other options
> +SANITIZER_CXXFLAGS = $(SANITIZER_CFLAGS)
> +GCXXFLAGS = $(GCFLAGS)
> +PCXXFLAGS = $(PCFLAGS)
> +
>  BUILD_CFLAGS += $(GCFLAGS) $(PCFLAGS)
>  # First, Sanitizer, Global, Platform, Local CFLAGS
>  CFLAGS += $(FCFLAGS) $(SANITIZER_CFLAGS) $(OPTIMIZER) $(GCFLAGS) $(PCFLAGS) $(LCFLAGS)
> +CXXFLAGS += $(FCXXFLAGS) $(SANITIZER_CXXFLAGS) $(OPTIMIZER) $(GCXXFLAGS) $(PCXXFLAGS) $(LCXXFLAGS)
>  
>  include $(TOPDIR)/include/buildmacros
>  
> diff --git a/libxfs/Makefile b/libxfs/Makefile
> index 1185a5e6cb26..bb851ab74204 100644
> --- a/libxfs/Makefile
> +++ b/libxfs/Makefile
> @@ -125,6 +125,8 @@ CFILES = buf_mem.c \
>  	xfs_trans_space.c \
>  	xfs_types.c
>  
> +LDIRT += dummy.o
> +
>  #
>  # Tracing flags:
>  # -DMEM_DEBUG		all zone memory use
> @@ -144,7 +146,11 @@ LTLIBS = $(LIBPTHREAD) $(LIBRT)
>  # don't try linking xfs_repair with a debug libxfs.
>  DEBUG = -DNDEBUG
>  
> -default: ltdepend $(LTLIBRARY)
> +default: ltdepend $(LTLIBRARY) dummy.o
> +
> +dummy.o: dummy.cpp
> +	@echo "    [CXX]    $@"
> +	$(Q)$(CC) $(CXXFLAGS) -c $<

$(CXX) ;)

>  
>  # set up include/xfs header directory
>  include $(BUILDRULES)
> diff --git a/libxfs/dummy.cpp b/libxfs/dummy.cpp
> new file mode 100644
> index 000000000000..a872c00ad84b
> --- /dev/null
> +++ b/libxfs/dummy.cpp
> @@ -0,0 +1,15 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2024 Oracle.  All Rights Reserved.
> + * Author: Darrick J. Wong <djwong@kernel.org>
> + */
> +#include "include/xfs.h"
> +#include "include/handle.h"
> +#include "include/jdm.h"
> +
> +/* Dummy program to test C++ compilation of user-exported xfs headers */
> +
> +int main(int argc, char *argv[])
> +{
> +	return 0;
> +}

cheers,
sam

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