dotnet/runtime · error · RuntimeError

Unknown target OS.

Error message

Unknown target OS.

What it means

Raised by determine_jit_name() in jitutil.py in the cross-compilation JIT branch. When use_cross_compile_jit is set or host/target arch differ or target OS differs from host, the function builds a cross-JIT name; if target_os is not 'windows', 'osx', or 'linux' (and target_arch is not arm*/wasm*) it raises 'Unknown target OS.'

Solutions

1. Pass target_os as one of: windows, osx, linux (lowercase).
2. Omit -target_os if you do not need cross-OS compilation.
3. Verify target_arch/host_arch are consistent and that you actually need the cross-compile path.

Example fix

// before
jit_name = determine_jit_name(host_os='linux', target_os='unix', host_arch='x64', target_arch='arm64')  # 'Unknown target OS.'
// after
jit_name = determine_jit_name(host_os='linux', target_os='linux', host_arch='x64', target_arch='arm64')

VALID_TARGET_OS = {"windows", "osx", "linux"}
if use_cross_compile_jit or (host_arch != target_arch) or (target_os is not None and host_os != target_os):
    if not (target_arch.startswith("arm") or target_arch.startswith("wasm")) and target_os not in VALID_TARGET_OS:
        raise SystemExit(f"target_os must be one of {sorted(VALID_TARGET_OS)} for cross-compile, got {target_os!r}")

def is_supported_target_os(s: str) -> bool:
    return s in {"windows", "osx", "linux"}

When it happens

Trigger: Calling determine_jit_name with a target_os outside {windows, osx, linux} while requesting a cross-compile JIT (e.g. target_arch differs from host_arch, or target_os passed and differs from host_os).

Common situations: Passing a target_os typo or an unsupported platform string; calling superpmi collect/replay with -target_os set incorrectly; mixing capitalized forms.

Related errors

• Unknown host OS.
• Unknown OS.
• Unsupported OS.
• Azure failed to download
• {coreclr_args.mode}

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/69d5bd962d9836a5. Report an issue: GitHub.

Appendix: source

Thrown at src/coreclr/scripts/jitutil.py:504

        If you pass one of target_os, host_arch, or target_arch, you must pass them all.

    Return:
        (str) : name of the jit for this OS
    """

    jit_base_name = 'clrjit'

    if use_cross_compile_jit or (host_arch != target_arch) or ((target_os is not None) and (host_os != target_os)):
        if target_arch.startswith("arm"):
            jit_os_name = "universal"
        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.

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