We may need to guess `CMAKE_FIND_ROOT` when cross-compiling

[illwieckz]

When cross-compiling, CMake lines like this don't work:

find_library(LIBRT rt)

We need an explicit CMake option like -DCMAKE_FIND_ROOT_PATH=/usr/aarch64-linux-gnu.

For some reasons on some distributions or architectures it is not required to explicitly pass -lrt but on some, it is required to do it explicitly.

We may guess that root path in some ways, when it is not set. For example using <compiler> -dumpmachine.

[slipher]

Using find_library with rt seems weird. Probably we should just be using target_link_libraries(<target> rt). Then CMake will just pass in -lrt and let the linker find it itself.

But I suppose there are plenty of other libs, say SDL, that we might legitimately want to find sometimes.

[slipher]

What distro or toolchain does find_library not work by default on?

[illwieckz]

We should expect all of them? If find_library would find arm64 library on Linux amd64 by default it would be wrong, I guess?

[illwieckz]

We don't need that patch in the dockered build script because we rebuild almost all the dependencies for the target platform, so find_library finds the library in external_deps, but outside of that scenario, it probably never worked.

But to make it easier and faster for me to test, I build arm64 binaries in a native amd64 Debian chroot with arm64 Debian packages, and run the produced binaries on an arm64 system running the exact same Debian version with exact same arm64 packages.

So find_library running on the amd64 build machine will never return arm64 libraries without telling CMake to explicitely look for them in the arm64 folder, and it is expected I guess (find_library returning arm64 libraries on an amd64 system by default would be a bug).

[slipher]

It works on my Debian with just installing the per-architecture apt packages. For example if I configure a Daemon build with CC=aarch64-linux-gnu-gcc CXX=aarch64-linux-gnu-g++ cmake ... I get LIBRT:FILEPATH=/usr/lib/aarch64-linux-gnu/librt.a, OPENGL_gl_LIBRARY:FILEPATH=/usr/lib/aarch64-linux-gnu/libGL.so, ZLIB_LIBRARY_RELEASE:FILEPATH=/usr/lib/aarch64-linux-gnu/libz.so etc. in the cmake -LA output. The only shortcoming is it uses the system headers instead of arch-specific ones. It seems that each library is installed in at least 3 places:

• /usr/lib/aarch64-linux-gnu/ paths that I get above
• /lib/aarch64-linux-gnu/ which contains symlinks into the former
• /usr/aarch64-linux-gnu/lib which contains another full copy of the file