[v2] meson: Do not force libc++ when using clang

25621 diff mbox series

From: Khem Raj <raj.khem@gmail.com>

Currently the meson scripts force the use of libc++ when using clang
as the compiler. This behaviour cannot be overridden by the user, and
it is suboptimal as it means that a clang build cannot reliably use
system qt, gtest, etc since those might use libstdc++.

To fix that, simply do not force the use of any particular standard
library, and detect the currently used one based on predefined macros.
This is exactly what meson does internally, although the result is
not readily available for meson scripts[0][1]; so the test needs
to be largely replicated.

[0]: https://github.com/mesonbuild/meson/commit/675b47b0692131fae974298829ba807d730ab098
[1]: https://stackoverflow.com/a/31658120

Closes: https://gitlab.freedesktop.org/camera/libcamera/-/issues/226
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
---
changes in v2:
  * use `has_header_symbol()`
  * do not set `-stdlib=` when using clang
  * reword commit message

v1: https://patchwork.libcamera.org/patch/24811/
---
 meson.build | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

--
2.52.0

On Mon, Jan 05, 2026 at 10:31:19AM +0100, Barnabás Pőcze wrote:
> From: Khem Raj <raj.khem@gmail.com>
> 
> Currently the meson scripts force the use of libc++ when using clang
> as the compiler. This behaviour cannot be overridden by the user, and
> it is suboptimal as it means that a clang build cannot reliably use
> system qt, gtest, etc since those might use libstdc++.
> 
> To fix that, simply do not force the use of any particular standard
> library, and detect the currently used one based on predefined macros.
> This is exactly what meson does internally, although the result is
> not readily available for meson scripts[0][1]; so the test needs
> to be largely replicated.
> 
> [0]: https://github.com/mesonbuild/meson/commit/675b47b0692131fae974298829ba807d730ab098
> [1]: https://stackoverflow.com/a/31658120
> 
> Closes: https://gitlab.freedesktop.org/camera/libcamera/-/issues/226
> Signed-off-by: Khem Raj <raj.khem@gmail.com>
> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
> ---
> changes in v2:
>   * use `has_header_symbol()`
>   * do not set `-stdlib=` when using clang
>   * reword commit message
> 
> v1: https://patchwork.libcamera.org/patch/24811/
> ---
>  meson.build | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/meson.build b/meson.build
> index fa6487f65..4aa7ecc96 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -118,7 +118,16 @@ cpp_arguments = [
>      '-Wnon-virtual-dtor',
>  ]
> 
> -cxx_stdlib = 'libstdc++'
> +cxx_stdlib = ''
> +
> +# \todo Switch to `version` when moving to C++20 as `ciso646` has been removed in C++20.
> +if cxx.has_header_symbol('ciso646', '_LIBCPP_VERSION')
> +    cxx_stdlib = 'libc++'
> +elif cxx.has_header_symbol('ciso646', '__GLIBCXX__')
> +    cxx_stdlib = 'libstdc++'

else
  error(...)

would be a nice addition. With that,

> +endif
> +
> +message('Detected C++ standard library: ' + cxx_stdlib)
> 
>  if cc.get_id() == 'clang'
>      if cc.version().version_compare('<9')
> @@ -139,15 +148,6 @@ if cc.get_id() == 'clang'
>          endif
>      endif
> 
> -    # Use libc++ by default if available instead of libstdc++ when compiling
> -    # with clang.
> -    if cc.find_library('c++', required : false).found()
> -        cpp_arguments += [
> -            '-stdlib=libc++',
> -        ]
> -        cxx_stdlib = 'libc++'

cxx_stdlib is used in test/py/meson.build only. Should the above be
moved there ?

> -    endif
> -
>      cpp_arguments += [
>          '-Wextra-semi',
>          '-Wthread-safety',

2026. 01. 05. 11:21 keltezéssel, Laurent Pinchart írta:
> On Mon, Jan 05, 2026 at 10:31:19AM +0100, Barnabás Pőcze wrote:
>> From: Khem Raj <raj.khem@gmail.com>
>>
>> Currently the meson scripts force the use of libc++ when using clang
>> as the compiler. This behaviour cannot be overridden by the user, and
>> it is suboptimal as it means that a clang build cannot reliably use
>> system qt, gtest, etc since those might use libstdc++.
>>
>> To fix that, simply do not force the use of any particular standard
>> library, and detect the currently used one based on predefined macros.
>> This is exactly what meson does internally, although the result is
>> not readily available for meson scripts[0][1]; so the test needs
>> to be largely replicated.
>>
>> [0]: https://github.com/mesonbuild/meson/commit/675b47b0692131fae974298829ba807d730ab098
>> [1]: https://stackoverflow.com/a/31658120
>>
>> Closes: https://gitlab.freedesktop.org/camera/libcamera/-/issues/226
>> Signed-off-by: Khem Raj <raj.khem@gmail.com>
>> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
>> ---
>> changes in v2:
>>    * use `has_header_symbol()`
>>    * do not set `-stdlib=` when using clang
>>    * reword commit message
>>
>> v1: https://patchwork.libcamera.org/patch/24811/
>> ---
>>   meson.build | 20 ++++++++++----------
>>   1 file changed, 10 insertions(+), 10 deletions(-)
>>
>> diff --git a/meson.build b/meson.build
>> index fa6487f65..4aa7ecc96 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -118,7 +118,16 @@ cpp_arguments = [
>>       '-Wnon-virtual-dtor',
>>   ]
>>
>> -cxx_stdlib = 'libstdc++'
>> +cxx_stdlib = ''
>> +
>> +# \todo Switch to `version` when moving to C++20 as `ciso646` has been removed in C++20.
>> +if cxx.has_header_symbol('ciso646', '_LIBCPP_VERSION')
>> +    cxx_stdlib = 'libc++'
>> +elif cxx.has_header_symbol('ciso646', '__GLIBCXX__')
>> +    cxx_stdlib = 'libstdc++'
> 
> else
>    error(...)

Okay.


> 
> would be a nice addition. With that,
> 

I have to assume something is missing here?


>> +endif
>> +
>> +message('Detected C++ standard library: ' + cxx_stdlib)
>>
>>   if cc.get_id() == 'clang'
>>       if cc.version().version_compare('<9')
>> @@ -139,15 +148,6 @@ if cc.get_id() == 'clang'
>>           endif
>>       endif
>>
>> -    # Use libc++ by default if available instead of libstdc++ when compiling
>> -    # with clang.
>> -    if cc.find_library('c++', required : false).found()
>> -        cpp_arguments += [
>> -            '-stdlib=libc++',
>> -        ]
>> -        cxx_stdlib = 'libc++'
> 
> cxx_stdlib is used in test/py/meson.build only. Should the above be
> moved there ?

I would keep it here.


> 
>> -    endif
>> -
>>       cpp_arguments += [
>>           '-Wextra-semi',
>>           '-Wthread-safety',
>

On Mon, Jan 05, 2026 at 11:28:30AM +0100, Barnabás Pőcze wrote:
> 2026. 01. 05. 11:21 keltezéssel, Laurent Pinchart írta:
> > On Mon, Jan 05, 2026 at 10:31:19AM +0100, Barnabás Pőcze wrote:
> >> From: Khem Raj <raj.khem@gmail.com>
> >>
> >> Currently the meson scripts force the use of libc++ when using clang
> >> as the compiler. This behaviour cannot be overridden by the user, and
> >> it is suboptimal as it means that a clang build cannot reliably use
> >> system qt, gtest, etc since those might use libstdc++.
> >>
> >> To fix that, simply do not force the use of any particular standard
> >> library, and detect the currently used one based on predefined macros.
> >> This is exactly what meson does internally, although the result is
> >> not readily available for meson scripts[0][1]; so the test needs
> >> to be largely replicated.
> >>
> >> [0]: https://github.com/mesonbuild/meson/commit/675b47b0692131fae974298829ba807d730ab098
> >> [1]: https://stackoverflow.com/a/31658120
> >>
> >> Closes: https://gitlab.freedesktop.org/camera/libcamera/-/issues/226
> >> Signed-off-by: Khem Raj <raj.khem@gmail.com>
> >> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
> >> ---
> >> changes in v2:
> >>    * use `has_header_symbol()`
> >>    * do not set `-stdlib=` when using clang
> >>    * reword commit message
> >>
> >> v1: https://patchwork.libcamera.org/patch/24811/
> >> ---
> >>   meson.build | 20 ++++++++++----------
> >>   1 file changed, 10 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/meson.build b/meson.build
> >> index fa6487f65..4aa7ecc96 100644
> >> --- a/meson.build
> >> +++ b/meson.build
> >> @@ -118,7 +118,16 @@ cpp_arguments = [
> >>       '-Wnon-virtual-dtor',
> >>   ]
> >>
> >> -cxx_stdlib = 'libstdc++'
> >> +cxx_stdlib = ''
> >> +
> >> +# \todo Switch to `version` when moving to C++20 as `ciso646` has been removed in C++20.
> >> +if cxx.has_header_symbol('ciso646', '_LIBCPP_VERSION')
> >> +    cxx_stdlib = 'libc++'
> >> +elif cxx.has_header_symbol('ciso646', '__GLIBCXX__')
> >> +    cxx_stdlib = 'libstdc++'
> > 
> > else
> >    error(...)
> 
> Okay.
> 
> > would be a nice addition. With that,
> 
> I have to assume something is missing here?

Ignore the "With that," :-)

> >> +endif
> >> +
> >> +message('Detected C++ standard library: ' + cxx_stdlib)
> >>
> >>   if cc.get_id() == 'clang'
> >>       if cc.version().version_compare('<9')
> >> @@ -139,15 +148,6 @@ if cc.get_id() == 'clang'
> >>           endif
> >>       endif
> >>
> >> -    # Use libc++ by default if available instead of libstdc++ when compiling
> >> -    # with clang.
> >> -    if cc.find_library('c++', required : false).found()
> >> -        cpp_arguments += [
> >> -            '-stdlib=libc++',
> >> -        ]
> >> -        cxx_stdlib = 'libc++'
> > 
> > cxx_stdlib is used in test/py/meson.build only. Should the above be
> > moved there ?
> 
> I would keep it here.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> >> -    endif
> >> -
> >>       cpp_arguments += [
> >>           '-Wextra-semi',
> >>           '-Wthread-safety',

Quoting Laurent Pinchart (2026-01-05 18:04:40)
> On Mon, Jan 05, 2026 at 11:28:30AM +0100, Barnabás Pőcze wrote:
> > 2026. 01. 05. 11:21 keltezéssel, Laurent Pinchart írta:
> > > On Mon, Jan 05, 2026 at 10:31:19AM +0100, Barnabás Pőcze wrote:
> > >> From: Khem Raj <raj.khem@gmail.com>
> > >>
> > >> Currently the meson scripts force the use of libc++ when using clang
> > >> as the compiler. This behaviour cannot be overridden by the user, and
> > >> it is suboptimal as it means that a clang build cannot reliably use
> > >> system qt, gtest, etc since those might use libstdc++.
> > >>
> > >> To fix that, simply do not force the use of any particular standard
> > >> library, and detect the currently used one based on predefined macros.
> > >> This is exactly what meson does internally, although the result is
> > >> not readily available for meson scripts[0][1]; so the test needs
> > >> to be largely replicated.
> > >>
> > >> [0]: https://github.com/mesonbuild/meson/commit/675b47b0692131fae974298829ba807d730ab098
> > >> [1]: https://stackoverflow.com/a/31658120
> > >>
> > >> Closes: https://gitlab.freedesktop.org/camera/libcamera/-/issues/226
> > >> Signed-off-by: Khem Raj <raj.khem@gmail.com>
> > >> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
> > >> ---
> > >> changes in v2:
> > >>    * use `has_header_symbol()`
> > >>    * do not set `-stdlib=` when using clang
> > >>    * reword commit message
> > >>
> > >> v1: https://patchwork.libcamera.org/patch/24811/
> > >> ---
> > >>   meson.build | 20 ++++++++++----------
> > >>   1 file changed, 10 insertions(+), 10 deletions(-)
> > >>
> > >> diff --git a/meson.build b/meson.build
> > >> index fa6487f65..4aa7ecc96 100644
> > >> --- a/meson.build
> > >> +++ b/meson.build
> > >> @@ -118,7 +118,16 @@ cpp_arguments = [
> > >>       '-Wnon-virtual-dtor',
> > >>   ]
> > >>
> > >> -cxx_stdlib = 'libstdc++'
> > >> +cxx_stdlib = ''
> > >> +
> > >> +# \todo Switch to `version` when moving to C++20 as `ciso646` has been removed in C++20.
> > >> +if cxx.has_header_symbol('ciso646', '_LIBCPP_VERSION')
> > >> +    cxx_stdlib = 'libc++'
> > >> +elif cxx.has_header_symbol('ciso646', '__GLIBCXX__')
> > >> +    cxx_stdlib = 'libstdc++'
> > > 
> > > else
> > >    error(...)
> > 
> > Okay.
> > 
> > > would be a nice addition. With that,
> > 
> > I have to assume something is missing here?
> 
> Ignore the "With that," :-)
> 
> > >> +endif
> > >> +
> > >> +message('Detected C++ standard library: ' + cxx_stdlib)

Any value in adding this to the summary? But a message is probably fine
enough on it's own too.


Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

> > >>
> > >>   if cc.get_id() == 'clang'
> > >>       if cc.version().version_compare('<9')
> > >> @@ -139,15 +148,6 @@ if cc.get_id() == 'clang'
> > >>           endif
> > >>       endif
> > >>
> > >> -    # Use libc++ by default if available instead of libstdc++ when compiling
> > >> -    # with clang.
> > >> -    if cc.find_library('c++', required : false).found()
> > >> -        cpp_arguments += [
> > >> -            '-stdlib=libc++',
> > >> -        ]
> > >> -        cxx_stdlib = 'libc++'
> > > 
> > > cxx_stdlib is used in test/py/meson.build only. Should the above be
> > > moved there ?
> > 
> > I would keep it here.
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> 
> > >> -    endif
> > >> -
> > >>       cpp_arguments += [
> > >>           '-Wextra-semi',
> > >>           '-Wthread-safety',
> 
> -- 
> Regards,
> 
> Laurent Pinchart