build: tolerate use of _Generic from glibc 2.43 with Clang#6233
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dscho merged 1 commit intoMay 8, 2026
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When building with `make DEVELOPER=1` we explicitly pass "-std=gnu99" to
the compiler so that we don't start leaning on features exposed by more
recent versions of the C standard. Unfortunately though, glibc 2.43
started to use type-generic expressions. This works alright with GCC,
but when compiling with Clang this leads to errors:
$ make DEVELOPER=1 CC=clang
CC daemon.o
In file included from daemon.c:3:
./git-compat-util.h:344:11: error: '_Generic' is a C11 extension [-Werror,-Wc11-extensions]
344 | return !!strchr(path, '/');
| ^
/usr/include/string.h:265:3: note: expanded from macro 'strchr'
265 | __glibc_const_generic (S, const char *, strchr (S, C))
| ^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:838:3: note: expanded from macro '__glibc_const_generic'
838 | _Generic (0 ? (PTR) : (void *) 1, \
| ^
In theory, the `__glibc_const_generic` macro does have feature gating:
#if !defined __cplusplus \
&& (__GNUC_PREREQ (4, 9) \
|| __glibc_has_extension (c_generic_selections) \
|| (!defined __GNUC__ && defined __STDC_VERSION__ \
&& __STDC_VERSION__ >= 201112L))
# define __HAVE_GENERIC_SELECTION 1
#else
# define __HAVE_GENERIC_SELECTION 0
#endif
But this feature gating isn't effective because `_has_extension()` will
always evaluate to true as C generics _are_ available as a language
extension to GNU C99 when using Clang. This would have been different if
`_has_feature()` was used instead, in which case it would have properly
evaluated to `false`.
Unfortunately, there is no easy way for us to work around the warning.
We cannot define `__HAVE_GENERIC_SELECTION` ourselves as that would lead
to a redefinition, and given that the conditions are or'd together we
cannot disable any of those, either.
Instead, work around the issue by not using -std=gnu99 with Clang when
using the Makefile and by disabling warnings about C11 extensions when
using Meson. This isn't ideal, but we at least retain the ability to
detect the (mis-)use of features from newer standards with GCC.
An alternative to this might be to simply bump the required C standard
to C11, which is 15 years old by now and should have support on most
platforms out there. But some more esoteric platforms may not have it.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
This was referenced May 6, 2026
dscho
added a commit
to microsoft/git
that referenced
this pull request
May 8, 2026
Same fix as git-for-windows#6233 in `git-for-windows/git`, cherry-picked here so the corresponding `microsoft/git` branch builds again on Ubuntu 26.04. The `ubuntu:rolling` containers used by `linux-{asan-ubsan,sha256,reftable}` now ship glibc 2.43, which puts `_Generic` into `<sys/cdefs.h>`; Clang under `-std=gnu99 -Werror` flags it as a C11 extension. Picking up Patrick Steinhardt's upstream-bound fix from https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/ ahead of its eventual merge (no list movement so far, not yet in `seen`). The diff conflicts with the `ifndef USE_MIMALLOC` wrap that this fork inherits from `git-for-windows/git`'s `fe5704a3695c "mimalloc: offer a build-time option to enable it"`; the resolution preserves that wrap on the `gcc6`-only branch surviving Patrick's patch. `meson.build` auto-merged.
dscho
added a commit
to microsoft/git
that referenced
this pull request
May 8, 2026
Same fix as git-for-windows#6233 in `git-for-windows/git`, cherry-picked here so the corresponding `microsoft/git` branch builds again on Ubuntu 26.04. The `ubuntu:rolling` containers used by `linux-{asan-ubsan,sha256,reftable}` now ship glibc 2.43, which puts `_Generic` into `<sys/cdefs.h>`; Clang under `-std=gnu99 -Werror` flags it as a C11 extension. Picking up Patrick Steinhardt's upstream-bound fix from https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/ ahead of its eventual merge (no list movement so far, not yet in `seen`). The diff conflicts with the `ifndef USE_MIMALLOC` wrap that this fork inherits from `git-for-windows/git`'s `fe5704a3695c "mimalloc: offer a build-time option to enable it"`; the resolution preserves that wrap on the `gcc6`-only branch surviving Patrick's patch. `meson.build` auto-merged.
This was referenced May 9, 2026
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 11, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 11, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 11, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 11, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
This was referenced May 12, 2026
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 12, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 12, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 13, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 14, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 15, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 18, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 18, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
This was referenced May 19, 2026
gitforwindowshelper Bot
pushed a commit
that referenced
this pull request
May 19, 2026
The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.
Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.
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The
linux-{asan-ubsan,sha256,reftable}jobs run insideubuntu:rolling, which now resolves to Ubuntu 26.04 with glibc 2.43; that pulls_Genericinto<sys/cdefs.h>and breaks our-std=gnu99 -WerrorClang builds. Concrete failure: https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.Picking up Patrick Steinhardt's fix from https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/ ahead of its upstream merge so the GfW CI goes green again. The diff conflicts with
fe5704a3695c "mimalloc: offer a build-time option to enable it", which wraps the affectedconfig.mak.devblock inifndef USE_MIMALLOC; the resolution preserves that wrap on thegcc6-only branch surviving Patrick's patch.meson.buildauto-merged.