What does errno 40 (ENOSYS) mean?

ENOSYS

ENOSYS denotes an unavailable operation, not an invalid argument. The caller asked for a defined kind of action, but the active implementation does not provide it. In portable code this may be a platform capability gap, a feature omitted by a particular runtime, or an API surface compiled without the required backend.

Do not use this value as a generic description of every “not supported” Windows failure. Native Win32 APIs report their own status through their documented mechanism, and an error-mapping layer may intentionally translate or collapse those values. Preserve both the original platform status and the mapped errno value whenever a boundary performs that conversion.

Useful follow-up checks

• Identify the exact library, version, target architecture, and feature flag that produced the result.
• Verify whether the operation is optional and whether the API offers a documented capability probe or alternate code path.
• Do not treat an unavailable operation as a transient error; retries without a configuration or implementation change do not add support.
• Keep fallback behavior explicit so that a missing capability does not silently weaken security, durability, or protocol semantics.

References

• Microsoft: errno constants
• Microsoft: Win32 system error codes
• POSIX: <errno.h>

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