[FFmpeg-devel] [PATCH v2 1/2] configure: don't force specific C++ standard library linking
Kacper Michajlow
kasper93 at gmail.com
Wed Sep 6 19:54:36 EEST 2023
On Wed, 6 Sept 2023 at 12:15, Timo Rothenpieler <timo at rothenpieler.org> wrote:
>
> On 06/09/2023 01:26, Kacper Michajłow wrote:
> > Other C++ standard libraries exist. Also, this is not a proper way to
> > link the standard library anyway. Instead when a C++ dependency is
> > detected, switch to the C++ compiler driver to properly link everything.
> >
> > Signed-off-by: Kacper Michajłow <kasper93 at gmail.com>
> > ---
> > configure | 26 ++++++++++++++++++--------
> > 1 file changed, 18 insertions(+), 8 deletions(-)
> >
> > diff --git a/configure b/configure
> > index bd7f7697c8..f3ff48586a 100755
> > --- a/configure
> > +++ b/configure
> > @@ -3584,11 +3584,9 @@ bktr_indev_deps_any="dev_bktr_ioctl_bt848_h machine_ioctl_bt848_h dev_video_bktr
> > caca_outdev_deps="libcaca"
> > decklink_deps_any="libdl LoadLibrary"
> > decklink_indev_deps="decklink threads"
> > -decklink_indev_extralibs="-lstdc++"
> > decklink_indev_suggest="libzvbi"
> > decklink_outdev_deps="decklink threads"
> > decklink_outdev_suggest="libklvanc"
> > -decklink_outdev_extralibs="-lstdc++"
> > dshow_indev_deps="IBaseFilter"
> > dshow_indev_extralibs="-lpsapi -lole32 -lstrmiids -luuid -loleaut32 -lshlwapi"
> > fbdev_indev_deps="linux_fb_h"
> > @@ -4984,6 +4982,18 @@ set_ccvars HOSTCC
> > test -n "$cc_type" && enable $cc_type ||
> > warn "Unknown C compiler $cc, unable to select optimal CFLAGS"
> >
> > +cxx_deps=(
> > + decklink
> > + libglslang
> > + libgme
> > + libopenmpt
> > + librubberband
> > + libsnappy
> > +)
>
> Can't say I'm a fan of maintaining a random list in the middle of
> configure, which manually contains all libs we think need libstdc++.
>
> This could for one change any time. But also, those dependenies might
> have other dependencies, which when linking statically might need libstdc++.
>
> But in general, this seems extremely brittle and a maintenance nightmare.
Yes, that's the main problem. Linking static dependencies is an
unsolved problem. We can discuss that, but wouldn't you agree that the
proposed patch is an improvement over the current status quo? Forcing
`-lstdc++` unconditionally, when some of the dependencies are enabled
is way worse. The logic of maintaining the list does not change, it is
just centralized in one place and properly switches linking language
to C++ when needed, instead side-loading stdc++.
Good solution that would not require any maintenance is to switch to
CXX always for linking.
> > +for l in ${cxx_deps[@]}; do
> > + enabled $l && ld_default=$cxx
> > +done
> > +
> > : ${as_default:=$cc}
> > : ${objcc_default:=$cc}
> > : ${dep_cc_default:=$cc}
> > @@ -6706,12 +6716,12 @@ enabled libfribidi && require_pkg_config libfribidi fribidi fribidi.h fri
> > enabled libharfbuzz && require_pkg_config libharfbuzz harfbuzz hb.h hb_buffer_create
> > enabled libglslang && { check_lib spirv_compiler glslang/Include/glslang_c_interface.h glslang_initialize_process \
> > -lglslang -lMachineIndependent -lOSDependent -lHLSL -lOGLCompiler -lGenericCodeGen \
> > - -lSPVRemapper -lSPIRV -lSPIRV-Tools-opt -lSPIRV-Tools -lpthread -lstdc++ -lm ||
> > + -lSPVRemapper -lSPIRV -lSPIRV-Tools-opt -lSPIRV-Tools -lpthread -lm ||
> > require spirv_compiler glslang/Include/glslang_c_interface.h glslang_initialize_process \
> > -lglslang -lOSDependent -lHLSL -lOGLCompiler \
> > - -lSPVRemapper -lSPIRV -lSPIRV-Tools-opt -lSPIRV-Tools -lpthread -lstdc++ -lm; }
> > + -lSPVRemapper -lSPIRV -lSPIRV-Tools-opt -lSPIRV-Tools -lpthread -lm; }
> > enabled libgme && { check_pkg_config libgme libgme gme/gme.h gme_new_emu ||
> > - require libgme gme/gme.h gme_new_emu -lgme -lstdc++; }
> > + require libgme gme/gme.h gme_new_emu -lgme; }
> > enabled libgsm && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do
> > check_lib libgsm "${gsm_hdr}" gsm_create -lgsm && break;
> > done || die "ERROR: libgsm not found"; }
> > @@ -6770,7 +6780,7 @@ enabled libopencv && { check_headers opencv2/core/core_c.h &&
> > enabled libopenh264 && require_pkg_config libopenh264 openh264 wels/codec_api.h WelsGetCodecVersion
> > enabled libopenjpeg && { check_pkg_config libopenjpeg "libopenjp2 >= 2.1.0" openjpeg.h opj_version ||
> > { require_pkg_config libopenjpeg "libopenjp2 >= 2.1.0" openjpeg.h opj_version -DOPJ_STATIC && add_cppflags -DOPJ_STATIC; } }
> > -enabled libopenmpt && require_pkg_config libopenmpt "libopenmpt >= 0.2.6557" libopenmpt/libopenmpt.h openmpt_module_create -lstdc++ && append libopenmpt_extralibs "-lstdc++"
> > +enabled libopenmpt && require_pkg_config libopenmpt "libopenmpt >= 0.2.6557" libopenmpt/libopenmpt.h openmpt_module_create
> > enabled libopenvino && { { check_pkg_config libopenvino openvino openvino/c/openvino.h ov_core_create && enable openvino2; } ||
> > { check_pkg_config libopenvino openvino c_api/ie_c_api.h ie_c_api_version ||
> > require libopenvino c_api/ie_c_api.h ie_c_api_version -linference_engine_c_api; } }
> > @@ -6789,12 +6799,12 @@ enabled librav1e && require_pkg_config librav1e "rav1e >= 0.5.0" rav1e.
> > enabled librist && require_pkg_config librist "librist >= 0.2.7" librist/librist.h rist_receiver_create
> > enabled librsvg && require_pkg_config librsvg librsvg-2.0 librsvg-2.0/librsvg/rsvg.h rsvg_handle_render_cairo
> > enabled librtmp && require_pkg_config librtmp librtmp librtmp/rtmp.h RTMP_Socket
> > -enabled librubberband && require_pkg_config librubberband "rubberband >= 1.8.1" rubberband/rubberband-c.h rubberband_new -lstdc++ && append librubberband_extralibs "-lstdc++"
> > +enabled librubberband && require_pkg_config librubberband "rubberband >= 1.8.1" rubberband/rubberband-c.h rubberband_new
> > enabled libshaderc && require_pkg_config spirv_compiler "shaderc >= 2019.1" shaderc/shaderc.h shaderc_compiler_initialize
> > enabled libshine && require_pkg_config libshine shine shine/layer3.h shine_encode_buffer
> > enabled libsmbclient && { check_pkg_config libsmbclient smbclient libsmbclient.h smbc_init ||
> > require libsmbclient libsmbclient.h smbc_init -lsmbclient; }
> > -enabled libsnappy && require libsnappy snappy-c.h snappy_compress -lsnappy -lstdc++
> > +enabled libsnappy && require libsnappy snappy-c.h snappy_compress -lsnappy
> > enabled libsoxr && require libsoxr soxr.h soxr_create -lsoxr
> > enabled libssh && require_pkg_config libssh libssh libssh/sftp.h sftp_init
> > enabled libspeex && require_pkg_config libspeex speex speex/speex.h speex_decoder_init
>
> The only correct way I see to deal with this would be to fully rely on
> pkg-config to contain the correct library.
Using pkg-config is simply a workaround for an unresolved problem, and
it "works" only because there is one blessed GNU C++ Library.
Listing implicit stdlibs in .pc is not a good solution and in practice
is not feasible to do so in a sensible way. Simply because your
buildsystem doesn't know which implicit libraries the toolchain will
link, making generating .pc files non-trivial and not portable. Of
course, users of said library also lack this knowledge, which is why,
when linking static libraries, it is generally recommended to use the
same toolchain and the correct linking language mode. This is
precisely what this patch accomplishes.
Furthermore, the selection of stdlib is not done by `-l`, but rather
by `-stdlib=` and the appropriate compiler driver (e.g., clang++
instead of clang). While -stdlib= can be included in the flags in .pc
files, it's essential to use a C++ compiler explicitly. See
https://libcxx.llvm.org/UsingLibcxx.html It is possible to use
`-nostdinc++ -nodefaultlibs` and manually list everything, but this is
not a task for pkg-config and could potentially disrupt the linking of
other components.
In general, pkg-config is better suited for shared library linking. If
you don't have control over the build parameters of the static
library, you shouldn't be using them anyway.
A more appropriate solution would be to enhance pkg-config to include
information about the required linking language for a given library.
In summary, I don't believe pkg-config is suitable for listing
implicit standard libraries. It's not straightforward to implement
this on the library side, and it could likely cause issues for its
users.
- Kacper
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