Trouble with using non-system compiler with custom triplet/toolchain

[tuero]

The default compiler on my Ubuntu system is GCC13, but I have GCC15 in /usr/local/bin which I would like to use to build my dependencies and project code (so I can do among other things enable LTO). I'm running into trouble with a dependency failing to build install this way, while it works if I do not specify a triplet/toolchain (which makes me think that I have setup my triplet/toolchain incorrectly).

My project directory looks as such:

cmake/
  | -- toolchains/
  |       | -- gcc-15-toolchain.cmake
  | -- triplets/
  |       | -- x64-linux-gcc-15.cmake
CMakeLists.txt
CMakePresets.json
main.cpp
vcpkg-configuration.json
vcpkg.json

This is the contents of gcc-15-toolchain.cmake:

set(CMAKE_C_COMPILER "/usr/local/bin/gcc-15.1")
set(CMAKE_CXX_COMPILER "/usr/local/bin/g++-15.1")
message("gcc-15 toolchain CMAKE_C_COMPILER = ${CMAKE_C_COMPILER}")
message("gcc-15 toolchain CMAKE_CXX_COMPILER = ${CMAKE_CXX_COMPILER}")

This is the contents of x64-linux-gcc-15.cmake:

set(VCPKG_TARGET_ARCHITECTURE x64)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE static)
set(VCPKG_CMAKE_SYSTEM_NAME Linux)
set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE ${CMAKE_CURRENT_LIST_DIR}/../toolchains/gcc-15-toolchain.cmake)
message("gcc-15 triplet CMAKE_C_COMPILER = ${CMAKE_C_COMPILER}")
message("gcc-15 triplet CMAKE_CXX_COMPILER = ${CMAKE_CXX_COMPILER}")

And finally the contents of my CMakePresets.json:

{
  "version": 4,
  "configurePresets": [
    {
      "name": "vcpkg",
      "binaryDir": "${sourceDir}/build",
      "cacheVariables": {
        "CMAKE_TOOLCHAIN_FILE": "$env{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake",
        "VCPKG_OVERLAY_TRIPLETS": "${sourceDir}/cmake/triplets",
        "VCPKG_TARGET_TRIPLET": "x64-linux-gcc-15",
        "VCPKG_CHAINLOAD_TOOLCHAIN_FILE": "${sourceDir}/cmake/toolchains/gcc-15-toolchain.cmake",
        "CMAKE_POSITION_INDEPENDENT_CODE": "ON",
        "CMAKE_C_COMPILER": "gcc-15.1",
        "CMAKE_CXX_COMPILER": "g++-15.1"
      }
    }
  ],
  "buildPresets": [
    {
      "name": "vcpkg",
      "displayName": "vcpkg",
      "configurePreset": "vcpkg"
    }
  ]
}

The troubling package in questions is igraph, specifically when it pulls its openblas dependency. From the output of my cmake --preset=vcpkg, I can see that the message from the toolchain file has the compiler set, but further down when printing from the triplet file its empty.

gcc-15 toolchain CMAKE_C_COMPILER = /usr/local/bin/gcc-15.1
gcc-15 toolchain CMAKE_CXX_COMPILER = /usr/local/bin/g++-15.1

... # skipping several lines of successful output

Installing 6/11 openblas:x64-linux@0.3.29...
Building openblas:x64-linux@0.3.29...
/home/tuero/.cache/vcpkg/registries/git-trees/3d3d198cfb372ccd328a36248c4c12fb7c6b3bb6: info: installing from git registry git+https://github.com/microsoft/vcpkg@3d3d198cfb372ccd328a36248c4c12fb7c6b3bb6
-- Using cached OpenMathLib-OpenBLAS-v0.3.29.tar.gz
-- Extracting source /home/tuero/vcpkg/downloads/OpenMathLib-OpenBLAS-v0.3.29.tar.gz
-- Applying patch disable-testing.diff
-- Applying patch getarch.diff
-- Applying patch system-check-msvc.diff
-- Applying patch win32-uwp.diff
-- Using source at /home/tuero/vcpkg/buildtrees/openblas/src/v0.3.29-abfa9cf6a4.clean
-- OpenBLAS native build
-- Configuring x64-linux
-- Building x64-linux-dbg
-- Building x64-linux-rel
-- Fixing pkgconfig file: /home/tuero/vcpkg/packages/openblas_x64-linux/lib/pkgconfig/openblas.pc
-- Fixing pkgconfig file: /home/tuero/vcpkg/packages/openblas_x64-linux/debug/lib/pkgconfig/openblas.pc
-- Installing: /home/tuero/vcpkg/packages/openblas_x64-linux/share/openblas/copyright
-- Adjusted RPATH of '/home/tuero/vcpkg/packages/openblas_x64-linux/manual-tools/openblas/Linux_x64/getarch' (From '' -> To '$ORIGIN:$ORIGIN/../../../lib')
-- Adjusted RPATH of '/home/tuero/vcpkg/packages/openblas_x64-linux/manual-tools/openblas/Linux_x64/getarch_2nd' (From '' -> To '$ORIGIN:$ORIGIN/../../../lib')
-- Performing post-build validation
Starting submission of openblas:x64-linux@0.3.29 to 1 binary cache(s) in the background
Elapsed time to handle openblas:x64-linux: 31 s
openblas:x64-linux package ABI: 94077d0655f652ed9982836f700fd7fc59e9989105db2344fac7dd60f6fa2652
Completed submission of libxml2[core,iconv,zlib]:x64-linux-gcc-15@2.15.0 to 1 binary cache(s) in 361 ms

Installing 7/11 openblas:x64-linux-gcc-15@0.3.29...
Building openblas:x64-linux-gcc-15@0.3.29...
/home/tuero/Documents/test/test_vcpkg/cmake/triplets/x64-linux-gcc-15.cmake: info: loaded overlay triplet from here
/home/tuero/.cache/vcpkg/registries/git-trees/3d3d198cfb372ccd328a36248c4c12fb7c6b3bb6: info: installing from git registry git+https://github.com/microsoft/vcpkg@3d3d198cfb372ccd328a36248c4c12fb7c6b3bb6
gcc-15 triplet CMAKE_C_COMPILER =
gcc-15 triplet CMAKE_CXX_COMPILER =
-- Using cached OpenMathLib-OpenBLAS-v0.3.29.tar.gz
-- Cleaning sources at /home/tuero/vcpkg/buildtrees/openblas/src/v0.3.29-abfa9cf6a4.clean. Use --editable to skip cleaning for the packages you specify.
-- Extracting source /home/tuero/vcpkg/downloads/OpenMathLib-OpenBLAS-v0.3.29.tar.gz
-- Applying patch disable-testing.diff
-- Applying patch getarch.diff
-- Applying patch system-check-msvc.diff
-- Applying patch win32-uwp.diff
-- Using source at /home/tuero/vcpkg/buildtrees/openblas/src/v0.3.29-abfa9cf6a4.clean
-- OpenBLAS cross build, but may use openblas:x64-linux getarch
-- Configuring x64-linux-gcc-15
CMake Error at scripts/cmake/vcpkg_execute_required_process.cmake:127 (message):
    Command failed: /home/tuero/vcpkg/downloads/tools/ninja/1.13.1-linux/ninja -v
    Working Directory: /home/tuero/vcpkg/buildtrees/openblas/x64-linux-gcc-15-rel/vcpkg-parallel-configure
    Error code: 1
    See logs for more information:
      /home/tuero/vcpkg/buildtrees/openblas/config-x64-linux-gcc-15-dbg-CMakeCache.txt.log
      /home/tuero/vcpkg/buildtrees/openblas/config-x64-linux-gcc-15-rel-CMakeCache.txt.log
      /home/tuero/vcpkg/buildtrees/openblas/config-x64-linux-gcc-15-dbg-CMakeConfigureLog.yaml.log
      /home/tuero/vcpkg/buildtrees/openblas/config-x64-linux-gcc-15-rel-CMakeConfigureLog.yaml.log
      /home/tuero/vcpkg/buildtrees/openblas/config-x64-linux-gcc-15-out.log

Call Stack (most recent call first):
  /home/tuero/Documents/test/test_vcpkg/build/vcpkg_installed/x64-linux/share/vcpkg-cmake/vcpkg_cmake_configure.cmake:269 (vcpkg_execute_required_process)
  /home/tuero/.cache/vcpkg/registries/git-trees/3d3d198cfb372ccd328a36248c4c12fb7c6b3bb6/portfile.cmake:47 (vcpkg_cmake_configure)
  scripts/ports.cmake:206 (include)


error: building openblas:x64-linux-gcc-15 failed with: BUILD_FAILED

Its building both a x64-linux and x64-linux-gcc-15 openblas, which I'm not sure means something is not setup correctly? The defautl triplet one builds just fine, but not the triplet I'm trying to setup. If I look into the error log from ~/vcpkg/buildtrees/openblas, I can see the following error for the triplet I'm trying to build.

... 
gcc-15 toolchain CMAKE_C_COMPILER = /usr/local/bin/gcc-15.1
gcc-15 toolchain CMAKE_CXX_COMPILER = /usr/local/bin/g++-15.1
gcc-15 toolchain CMAKE_C_COMPILER = /usr/local/bin/gcc-15.1
gcc-15 toolchain CMAKE_CXX_COMPILER = /usr/local/bin/g++-15.1
-- The C compiler identification is GNU 15.1.0
-- The ASM compiler identification is GNU
-- Found assembler: /usr/local/bin/gcc-15.1
-- Detecting C compiler ABI info
gcc-15 toolchain CMAKE_C_COMPILER = /usr/local/bin/gcc-15.1
gcc-15 toolchain CMAKE_CXX_COMPILER = /usr/local/bin/g++-15.1
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/local/bin/gcc-15.1 - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- TARGET: <native> (OpenBLAS getarch/getarch_2nd)
CMake Warning at CMakeLists.txt:104 (message):
  CMake support is experimental.  It does not yet support all build options
  and may not produce the same Makefiles that OpenBLAS ships with.


CMake Error at cmake/system_check.cmake:88 (if):
  if given arguments:

    "STREQUAL" "CORE2"

  Unknown arguments specified

Any help would be appreciated!

[Osyotr]

Likely a bug in openblas port, but you can try to skip crosscompiling by setting host triplet = target triplet.

[tuero]

It turns out setting both VCPKG_HOST_TRIPLET and VCPKG_TARGET_TRIPLET to both the same will enable cross-compiling, setting it true in the OpenBLAS configuration and OpenBLAS isn't properly setting the target platform and I'm getting garbage defaults.

The fix was to copy the default toolchain for Linux where vcpkg hard codes CMAKE_CROSSCOMPILING to OFF.

[Osyotr]

It turns out setting both VCPKG_HOST_TRIPLET and VCPKG_TARGET_TRIPLET to both the same will enable cross-compiling

Correction: it will disable (vcpkg) cross-compiling (i.e. you don't have to build x64-linux).

The fix was to copy the default toolchain for Linux where vcpkg hard codes CMAKE_CROSSCOMPILING to OFF.

Hmm you probably should have set(CMAKE_SYSTEM_NAME "Linux") instead and let CMake figure out that this is not a crosscompiling toolchain.

[tuero]

Could you please expand on this so I can try out of curiosity? Where should set(CMAKE_SYSTEM_NAME "Linux") be set (triplet, toolchain, CMakePresets cache var? And would I still set both the host/target for vcpkg in my presets cache vars?

[tuero]

I've been playing around with this, and from what I can tell, cross compiling is being set by cmake because the system name is user specified. In the toolchain file provided for linux, it checks whether to turn it off:

  if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux" AND CMAKE_SYSTEM_PROCESSOR STREQUAL CMAKE_HOST_SYSTEM_PROCESSOR)
      set(CMAKE_CROSSCOMPILING OFF CACHE BOOL "")
  endif()

The variables CMAKE_SYSTEM_PROCESSOR and CMAKE_HOST_SYSTEM_PROCESSOR are not defined it seems, so this is evaluating to false (and thus cross compiling). What works is if I just turn off CMAKE_CROSSCOMPILING unconditionally in the toolchain file. What is the preferred way of doing so? I started all of this to simply specify the compiler version I want to compile both my project and dependencies.

[dg0yt]

Where should set(CMAKE_SYSTEM_NAME "Linux") be set (triplet, toolchain, CMakePresets cache var? And would I still set both the host/target for vcpkg in my presets cache vars?

The triplet file doesn't set any toolchain properties directly. It is read in script mode. https://learn.microsoft.com/en-us/vcpkg/contributing/maintainer-guide#portfiles-are-run-in-script-mode

The toolchain file seeds/overrides toolchain setup in CMake project mode. In vcpkg, the toolchain file will use information from the triplet. Toolchains are expected to set CMAKE_SYSTEM_NAME.

CMake presets are a per-project or per-user convenience.

The toolchain has some quirks in vcpkg. But ATM the accepted answer makes a false statement, as pointed out in #48436 (reply in thread).

[dg0yt]

Its building both a x64-linux and x64-linux-gcc-15 openblas, which I'm not sure means something is not setup correctly?

It means you are doing a vcpkg cross-build.

[dg0yt]

Maybe your custom toolchain needs to set CMAKE_SYSTEM_PROCESSOR.

[dg0yt]

The fix was to copy the default toolchain for Linux where vcpkg hard codes CMAKE_CROSSCOMPILING to OFF.

The default toolchain also sets CMAKE_SYSTEM_PROCESSOR.
The lack of the CMAKE_SYSTEM_PROCESSOR value is a reasonable explanation for

CMake Error at cmake/system_check.cmake:88 (if):
  if given arguments:

    "STREQUAL" "CORE2"

[tuero]

Thanks. I think at least for now what works for me is to just take the default vcpkg toolchain file, and just hardcode in cross compiling to be off as that's not a feature I need to utilize/deal with right now.

[dg0yt]

• The custom toolchain file lacks an important value, CMAKE_SYSTEM_PROCESSOR. This breaks port openblas.
• The custom toolchain sets CMAKE_SYSTEM_NAME. This causes CMAKE_CROSSCOMPILING (unless overridden, as in the vcpkg toolchain).

To be fair, getting CMake toolchains right and complete can be painful.