[scripts] Set default PKG_CONFIG_PATH to help cross-compilation on Linux

[vserdyuk]

This fixes qtbase:arm64-linux on x64-linux host. Tested on Ubuntu 24.04.

Based on https://wiki.qt.io/Cross-Compile_Qt_6_for_Raspberry_Pi

[dg0yt]

This fixes qtbase:arm64-linux on x64-linux host. Tested on Ubuntu 24.04.

No, it makes things worse: It adds host (x64) directories which are incompatible with target (arm64) binaries.

What vcpkg would need is using a pkgconfig tool pre-configured for the target prefix/sysroot when building for the target (but not when building for the host!). It is probably possible to inject this via the triplet file. Some environment would provide a <autotools-target-triplet>-pkg-config wrapper script.

[vserdyuk]

No, it makes things worse: It adds host (x64) directories which are incompatible with target (arm64) binaries.

How is that? It adds /usr/lib/pkgconfig/ and /usr/lib/aarch64-linux-gnu/pkgconfig/ (installed on the x64 host with e.g. apt install libxkbcommon-dev:arm64). It does not add /usr/lib/x86_64-linux-gnu/pkgconfig/.

The Qt binaries built with vcpkg are then used to cross-compile my project and it runs fine on target arm64 Ubuntu 24.04.

Though maybe I'm missing some other cases for which this patch would actually make things worse.

[dg0yt]

This is from /usr/lib/pkgconfig, but not arm64:

$ cat /usr/lib/pkgconfig/opencl-12.9.pc 
cudaroot=/usr/local/cuda-12.9
libdir=${cudaroot}/targets/x86_64-linux/lib
includedir=${cudaroot}/targets/x86_64-linux/include

Name: opencl
Description: CUDA OpenCL Library
Version: 12.9
Libs: -L${libdir} -lopencl
Cflags: -I${includedir}

(I do believe that it works for your Qt binaries. But IMO the suggested implementation is too specific for a particular case.)

[vserdyuk]

Now I see. On my system /usr/lib/pkgconfig was just empty. So I think we can just remove that.

[dg0yt]

/usr/share/pkgconfig isn't much safer. While those files should not directly reference platform-specific artifacts (-l, -L), they still expose dependencies (Requires:) which may break resolving modules.

IMO the customization point is the triplet file (via "overlay triplets"). But things get interesting when you want the same in a top-level project and in manifest-mode dependencies.

[BillyONeal]

I agree with @dg0yt that makes it too easy to accidentally depend on host bits.

[vserdyuk]

Looks like setting only PKG_CONFIG_PATH is enough.

CI building qtbase failed last time probably because of missing arm64 system packages. The following works for me:

sudo apt install bison flex libtool autoconf automake autoconf-archive python3-distutils python3-jinja2 '^libxcb.*-dev' libx11-xcb-dev libglu1-mesa-dev libxrender-dev libxi-dev libxkbcommon-dev libxkbcommon-x11-dev libegl1-mesa-dev libsm-dev

sudo apt install '^libxcb.*-dev':arm64 libx11-xcb-dev:arm64 libglu1-mesa-dev:arm64 libxrender-dev:arm64 libxi-dev:arm64 libxkbcommon-dev:arm64 libxkbcommon-x11-dev:arm64 libegl1-mesa-dev:arm64 libxrandr-dev:arm64 libsm-dev:arm64 python3-venv

[dg0yt]

... and it only reaches ports which use the CMake toolchain (CMake project mode).

[BillyONeal]

After discussion with @vicroms we have decided to close this.