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ENOSYS
Missing implementation, not an unsupported object feature
ENOSYS says that the called function is not implemented in the current system context. This can arise from a compatibility layer, an older OS version, a sandboxed or emulated environment, or a syscall/API path that the platform does not provide.
Do not confuse it with ENOTSUP. ENOTSUP means an operation is unsupported for a particular object; ENOSYS means the requested function itself is absent. The appropriate fallback is therefore an explicitly supported alternative code path, selected by capability detection rather than an unbounded retry.
Evidence that helps
- Record the exact interface and ABI used, OS version, architecture, and whether a compatibility or virtualization layer is involved.
- Verify that the process is invoking the intended symbol or syscall number for its target SDK and runtime.
- Gate optional features before use and make the fallback visible in logs or telemetry.
References
- Apple XNU: Darwin errno definitions
- Apple system-call reference: ENOSYS
- POSIX: open() and implementation-defined support
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