@yperess would you be so kind as to clarify a bit more what you would like to do?
Are you using gcc, or clang or something else alltogether? are you trying to cross-compile? and/or just trying to use a different C library than glibc or picolibc or minimal? (or a different runtime)
At this point all I can say is that the current approach in this PR parsing for the sysroot command line option in the top level cmake file does not seem right. But I'm not able to say what would be a good way without knowing more about what you'd like to achieve.

@aescolar happy to and thank you for being so responsive on this issue. This is what I'm trying to get passing https://pigweed-review.git.corp.google.com/c/pigweed/pigweed/+/293592. This chage calls twister for the Pigweed project and tests all the Zephyr backends. The important details for this call are:

The environment variables are set here

def _env_with_zephyr_vars(ctx: PresubmitContext) -> dict:
    """Returns the environment variables with ... set for Zephyr."""
    env = os.environ.copy()
    # Set some variables here.
    env['ZEPHYR_BASE'] = str(ctx.package_root / 'zephyr')
    env['ZEPHYR_MODULES'] = str(ctx.root)
    env['ZEPHYR_TOOLCHAIN_VARIANT'] = 'llvm'
    return env

Then the call:

def zephyr_build(ctx: PresubmitContext) -> None:
    """Run Zephyr compatible tests"""
    # Install the Zephyr package
    build.install_package(ctx, 'zephyr')
    # Configure the environment
    env = _env_with_zephyr_vars(ctx)
    sysroot_dir = (
        ctx.pw_root
        / 'environment'
        / 'cipd'
        / 'packages'
        / 'pigweed'
        / 'clang_sysroot'
    )
    platform_filters = (
        ['-P', 'native_sim']
        if platform.system() in ['Windows', 'Darwin']
        else []
    )
    # Run twister
    call(
        'pw',
        'twister-runner',
        '-vvv',
        '--ninja',
        '--integration',
        '--clobber-output',
        '--inline-logs',
        '--verbose',
        '--coverage',
        '--coverage-basedir',
        str(ctx.pw_root),
        *platform_filters,
        f'-x=TOOLCHAIN_C_FLAGS=--sysroot={sysroot_dir}',
        f'-x=TOOLCHAIN_LD_FLAGS=--sysroot={sysroot_dir}',
        '--testsuite-root',
        str(ctx.pw_root),
        env=env,
    )

Pigweed sets up a hermetic environment so things don't get installed in the "normal" places. Where ever you download pigweed into is your ${PW_ROOT} from there, all the dependencies are installed:

PW_ROOT
├── environment/cipd/packages/pigweed/bin/clang
├── environment/cipd/packages/pigweed/clang_sysroot/
├── environment/packages/zephyr/

So you can see everything is under the environment/ subdirectory. The twister runner is a wrapper script that sanitizes the call because Zephyr is also a subdirectory, just running twister on ${PW_ROOT} would lead to running all the upstream tests too.

Some links for reference:

• https://cs.opensource.google/pigweed/pigweed/+/main:pw_presubmit/py/pw_presubmit/pigweed_presubmit.py;l=544
• https://cs.opensource.google/pigweed/pigweed/+/main:pw_build/py/pw_build/zephyr_twister_runner.py

@yperess thanks. Now I understand. I'm guessing that clang installation does not have as default sysroot that ${PW_ROOT}/environment/cipd/packages/pigweed/clang_sysroot/?
Are you passing it to the compiler thru TOOLCHAIN_C_FLAGS because Zephyr's SYSROOT_DIR did not work?

Maybe we should fix setting SYSROOT_DIR for clang (and maybe as a bonus also for the host gcc) and when it is set have it passed to the native sim runner build(?)

@aescolar I think I need more context, I know my way around the Zephyr build but I'm not an expert by any means. Let's chat on discord to avoid polluting this PR comments.

@aescolar this is much better, thank you for helping clean this PR up. It passed my CI downstream using https://pigweed-review.git.corp.google.com/c/pigweed/pigweed/+/293592 (Check https://ci.chromium.org/ui/p/pigweed/builders/pigweed.try/pigweed-linux-zephyr/b8713553788173861361/overview).

Once this PR merges I will clean up my downstream change and we'll be good to go.

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Looking good to me on a very quick look. Will re-review a bit later.
@tejlmand added as assignee for the cmake changes.

Hi @pdgendt & @tejlmand , friendly ping on the review here. Thanks.

Hi @pdgendt & @tejlmand , friendly ping on the review here. Thanks.

I could only really comment on the styling, not experienced with the content of the PR.