dotnet/runtime · error · RuntimeError

Unknown OS.

Error message

Unknown OS.

What it means

Raised by get_native_library_name in superpmi_aspnet.py when target_os is not one of 'osx','linux','windows'. The helper maps an OS to its native library suffix and has no default.

Source

Thrown at src/coreclr/scripts/superpmi_aspnet.py:84

def determine_native_name(coreclr_args, base_lib_name, target_os):
    """ Determine the name of the native lib based on the OS.

    Args:
        coreclr_args (CoreclrArguments): parsed args
        base_lib_name (str) : root name of the lib
        target_os (str) : os to run tests on
    Return:
        (str) : name of the native lib for this OS
    """

    if target_os == "osx":
        return "lib" + base_lib_name + ".dylib"
    elif target_os == "linux":
        return "lib" + base_lib_name + ".so"
    elif target_os == "windows":
        return base_lib_name + ".dll"
    else:
        raise RuntimeError("Unknown OS.")

# Where there is an option, we generally target the less performant machines
# See https://github.com/aspnet/Benchmarks/tree/master/scenarios
#
def determine_benchmark_machine(coreclr_args):
    """ Determine the name of the benchmark machine to use

    Args:
        coreclr_args (CoreclrArguments): parsed args

    Return:
        (str) : name of the benchmnark machine
    """

    if coreclr_args.arch == "x64":
        if coreclr_args.host_os == "windows":
            return "aspnet-perf-win"
#            return "aspnet-citrine-win"

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Print coreclr_args.host_os / target_os at the call site to see the actual value.
2. Normalize casing: `.lower()` the os before calling, or extend the if-chain for the new OS.
3. Fix the pipeline variable feeding --host_os.
4. For wasm/browser flows use superpmi_aspnet2.py or the dedicated wasm path instead.

Example fix

// before
# host_os='Linux' -> falls through to raise
// after
target_os = target_os.lower()  # or extend helper to handle the new OS

SUPPORTED = {'osx','linux','windows'}
if target_os not in SUPPORTED:
    raise SystemExit(f'Unsupported OS {target_os!r}; expected one of {SUPPORTED}')

def is_supported_native_os(os: str) -> bool:
    return os in {'osx','linux','windows'}

try:
    get_native_library_name(coreclr_args, base_lib_name, target_os)
except RuntimeError as e:
    if 'Unknown OS' in str(e):
        raise SystemExit(f'configure pipeline TargetOS; got {target_os}')

When it happens

Trigger: coreclr_args.host_os (or a passed target_os) is e.g. 'freebsd','browser','wasm', an empty string, or a case variant the comparison doesn't accept.

Common situations: New/experimental target_os added to the build matrix without updating this helper; mismatched casing ('Linux'); typo in pipeline variable; passing target_os from a future wasm flow.

Related errors

• Invalid OS for x64.
• Invalid OS for arm64.
• Invalid arch.
• Couldn't determine current git hash
• No JIT changes found starting with baseline hash

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/5ab356f79514e121. Report an issue: GitHub.