[petsc-users] Weird handling of compiler flags by the build system

[Stefano Zampini]

Il giorno lun 1 lug 2024 alle ore 16:43 meator <meator.dev at gmail.com> ha
scritto:

> Thank you for your reply!
>
> On 7/1/24 4:14 PM, Barry Smith wrote:
> >     We have had well over a decade of debates on this issue. I would
> like to see a CFLAGS+=extra_flags option but that has been resisted.
> Instead Satish can tell you how to get what you want.
>
> This is unfortunate. I assume that patching the buildsystem or some
> other trickery will be necessary if what you're saying is true.
>
> I admit it is not so standard, but Satish always opposed this.
No need to hack the buildsystem, just use COPTFLAGS, CXXOPTFLAGS and
FOPTFLAGS


> >> Is there a way to fix the pkg-config file (apart from manually removing
> cflags_extra, cxxflags_extra, and fflags_extra from the .pc file)?
> >
> >      These are there so people can see EXACTLY what flags were used when
> PETSc was compiled. They are not intended for people using pkg-config to
> use PETSc when building their package. What is the harm in having these
> extra flags in the pkgconfig file?
>

I don't think you should use Makefile.user. That is there as a sort of
template/placeholder. The extra variables are not included in a standard
usage of pkgconfig, so I don't think this issue is "severe"

$ pkg-config --cflags PETSc.pc # standard usage

-I/Users/szampini/Devel/petsc/arch-debug/include
-I/Users/szampini/Devel/petsc/include

$ pkg-config --variable=cflags_extra PETSc.pc # non standard
 -fPIC -Wall -Wwrite-strings -Wno-unknown-pragmas -fstack-protector
-fno-stack-check -Qunused-arguments -fvisibility=hidden -g3 -O0


> Here is an excerpt from /usr/share/petsc/Makefile.user (this is a
> template Makefile supplied with PETSc for use in custom projects):
>
>  > # Additional libraries that support pkg-config can be added to the
> list of PACKAGES below.
>  > PACKAGES := $(petsc.pc)
>  >
>  > CC := $(shell pkg-config --variable=ccompiler $(PACKAGES))
>  > CXX := $(shell pkg-config --variable=cxxcompiler $(PACKAGES))
>  > FC := $(shell pkg-config --variable=fcompiler $(PACKAGES))
>  > CFLAGS_OTHER := $(shell pkg-config --cflags-only-other $(PACKAGES))
>  > CFLAGS := $(shell pkg-config --variable=cflags_extra $(PACKAGES))
> $(CFLAGS_OTHER)
>  > CXXFLAGS := $(shell pkg-config --variable=cxxflags_extra $(PACKAGES))
> $(CFLAGS_OTHER)
>  > FFLAGS := $(shell pkg-config --variable=fflags_extra $(PACKAGES))
>  > CPPFLAGS := $(shell pkg-config --cflags-only-I $(PACKAGES))
>  > LDFLAGS := $(shell pkg-config --libs-only-L --libs-only-other
> $(PACKAGES))
>  > LDFLAGS += $(patsubst -L%, $(shell pkg-config --variable=ldflag_rpath
> $(PACKAGES))%, $(shell pkg-config --libs-only-L $(PACKAGES)))
>  > LDLIBS := $(shell pkg-config --libs-only-l $(PACKAGES)) -lm
>  > CUDAC := $(shell pkg-config --variable=cudacompiler $(PACKAGES))
>  > CUDAC_FLAGS := $(shell pkg-config --variable=cudaflags_extra
> $(PACKAGES))
>  > CUDA_LIB := $(shell pkg-config --variable=cudalib $(PACKAGES))
>  > CUDA_INCLUDE := $(shell pkg-config --variable=cudainclude $(PACKAGES))
>
> CFLAGS of user projects get initialized to cflags_extra for people who
> use the official recommended Makefile template. This is not tolerable
> because the flags used for building PETSc may be incompatible with
> unrelated projects that depend on PETSc. The build environment of the
> user program may be very different from the one used to build PETSc
> itself. Users trying to build their custom programs depending on PETSc
> will likely not want flags that were used to build PETSc in a fake
> destdir in chrooted system while building the PETSc package.
>


-- 
Stefano
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