dotnet/runtime · error · RuntimeError

Unknown host os

Error message

Unknown host os: {}

What it means

Raised by determine_coredistools when coreclr_args.host_os is none of osx/linux/windows. Although CoreclrArguments.valid_host_os includes additional platforms (illumos, solaris, haiku, freebsd, browser, android, wasi), the coredistools native library only ships for the three desktop/server OSes, so the loader name cannot be chosen and the function aborts.

Solutions

1. Run SuperPMI asm diffs on a supported host OS (Windows, Linux, or macOS) and use cross-compile target flags for the other platform.
2. Check the detected/override host_os value and pass --host_os windows|linux|osx if you are actually on one of those.
3. If you genuinely need coredistools on a new OS, you must build libcoredistools for that platform and contribute support upstream.

Example fix

// before
if coreclr_args.host_os.lower() == "osx":
    ...
elif coreclr_args.host_os.lower() == "linux":
    ...
elif coreclr_args.host_os.lower() == "windows":
    ...
else:
    raise RuntimeError("Unknown host os: {}".format(coreclr_args.host_os))
// after (no code change; fix invocation)
# Pass an explicit supported host_os:
python superpmi.py asmdiff --host_os linux ...

SUPPORTED_HOST_OS_FOR_COREDISTOOLS = {'windows', 'linux', 'osx'}
def validate_host_os_for_coredistools(host_os):
    if host_os is None:
        raise RuntimeError('host_os is None; pass --host_os')
    if host_os.lower() not in SUPPORTED_HOST_OS_FOR_COREDISTOOLS:
        raise RuntimeError(f"coredistools only supports {sorted(SUPPORTED_HOST_OS_FOR_COREDISTOOLS)}; got '{host_os}'. Run asmdiff on a supported host OS.")

def is_coredistools_supported_os(host_os: str) -> bool:
    return isinstance(host_os, str) and host_os.lower() in {'windows', 'linux', 'osx'}

When it happens

Trigger: Called whenever asm diffs need coredistools on an unsupported host_os. Triggered if the host_os was auto-detected or passed as one of the non-desktop values (e.g. 'freebsd', 'browser', 'wasi', 'android'), or if a custom/misspelled OS string reaches this point.

Common situations: Cross-targeting or building for browser/wasi/android and running superpmi.py asmdiff on that same host_os; an incorrect --host_os argument; running on FreeBSD/Solaris/Haiku where coredistools is not published.

Related errors

• Core_Root not set properly
• Unknown OS.
• Couldn't create git diff
• Couldn't determine baseline git hash
• Couldn't determine current git hash

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/75e63171dac0a8a7. Report an issue: GitHub.

Appendix: source

Thrown at src/coreclr/scripts/superpmi.py:3760

    Args:
        coreclr_args (CoreclrArguments) : parsed args

    Returns:
        coredistools_location (str)     : path of [lib]coredistools.dylib|so|dll
    """

    if not hasattr(coreclr_args, "core_root") or coreclr_args.core_root is None:
        raise RuntimeError("Core_Root not set properly")

    coredistools_dll_name = None
    if coreclr_args.host_os.lower() == "osx":
        coredistools_dll_name = "libcoredistools.dylib"
    elif coreclr_args.host_os.lower() == "linux":
        coredistools_dll_name = "libcoredistools.so"
    elif coreclr_args.host_os.lower() == "windows":
        coredistools_dll_name = "coredistools.dll"
    else:
        raise RuntimeError("Unknown host os: {}".format(coreclr_args.host_os))

    coredistools_location = os.path.join(coreclr_args.core_root, coredistools_dll_name)
    if os.path.isfile(coredistools_location):
        logging.info("Using coredistools found at %s", coredistools_location)
    else:
        coredistools_r2rdump_location = os.path.join(coreclr_args.core_root, "R2RDump", coredistools_dll_name)
        if os.path.isfile(coredistools_r2rdump_location):
            logging.info("Using coredistools found at %s (copying to %s)", coredistools_r2rdump_location, coredistools_location)
            logging.debug("Copying %s -> %s", coredistools_r2rdump_location, coredistools_location)
            shutil.copy2(coredistools_r2rdump_location, coredistools_location)
        else:
            # Often, Core_Root will already exist. However, you can do a product build without
            # creating a Core_Root, and successfully run replay or asm diffs, if we just create Core_Root
            # and copy coredistools there. Note that our replays all depend on Core_Root existing, as we
            # set the current directory to Core_Root before running superpmi.
            if not os.path.isdir(coreclr_args.core_root):
                logging.warning("Warning: Core_Root does not exist at \"%s\"; creating it now", coreclr_args.core_root)
                os.makedirs(coreclr_args.core_root)

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