dotnet/runtime · error · RuntimeError

Unknown host OS.

Error message

Unknown host OS.

What it means

Thrown by determine_jit_name() in jitutil.py when host_os is not 'osx', 'linux', or 'windows'. The host OS determines the file extension of the JIT binary (.dylib for macOS, .so for Linux, .dll for Windows). An unrecognized host OS means the function cannot produce a valid JIT filename.

Source

Thrown at src/coreclr/scripts/jitutil.py:515

        elif target_arch.startswith("wasm"):
            jit_os_name = "universal"
        elif target_os == "windows":
            jit_os_name = "win"
        elif target_os == "osx" or target_os == "linux":
            jit_os_name = "unix"
        else:
            raise RuntimeError("Unknown target OS.")

        jit_base_name = 'clrjit_{}_{}_{}'.format(jit_os_name, target_arch, host_arch)

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


def get_deepest_existing_directory(path):
    """ Given a path, find the deepest existing directory containing it. This
        might be the path itself, or a parent directory. If no such directory
        is found, None is returned.

    Args:
        path (str) : path to check

    Returns:
        As described above
    """
    path = os.path.abspath(path)
    lastPath = ""

    # When os.path.dirname() is called on the root directory ("C:\\" on Windows or "/" on Linux),
    # if returns itself.

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Check -host_os and ensure it is exactly 'windows', 'linux', or 'osx'.
2. Verify platform auto-detection: run 'python -c "import platform; print(platform.system().lower())"' and compare.
3. If running on a genuinely new OS, add the appropriate file extension mapping in determine_jit_name().

Example fix

# before: incorrect host OS string
python superpmi.py collect -host_os mac

# after: correct OS name
python superpmi.py collect -host_os osx

# Validate host_os before calling determine_jit_name
valid_host_oses = {'osx', 'linux', 'windows'}
if host_os not in valid_host_oses:
    raise ValueError(f'Unknown host OS: {host_os}. Must be one of: {valid_host_oses}')

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

try:
    jit_name = determine_jit_name(host_os)
except RuntimeError as e:
    if 'Unknown host OS' in str(e):
        logging.error('host_os must be windows, linux, or osx')
        sys.exit(1)
    raise

When it happens

Trigger: At lines 508-515: after resolving jit_base_name (either default 'clrjit' or a cross-compile name), the function checks host_os to append the correct extension. If host_os is not 'osx', 'linux', or 'windows', line 515 raises.

Common situations: Running on an unsupported operating system (e.g., FreeBSD, Solaris, AIX) where the .NET JIT infrastructure scripts haven't been adapted. The host_os auto-detection returned an unusual platform string. A typo in a manually-specified -host_os argument.

Related errors

• Unknown target OS.
• Unknown OS.
• Unsupported OS.
• event.eventName is a required parameter, in event: ${JSON.st
• "arguments" should be an array, but was ${request.arguments}

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/ab459cc7ac8ccd82. Report an issue: GitHub.