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EOPNOTSUPP
The request exceeds this socket's supported semantics
EOPNOTSUPP is a socket-operation compatibility error. The descriptor may be valid and the network may be reachable, but the requested operation, flag, or behavior is not supported for that socket type or protocol context.
Its meaning depends on the API. A send-family call can reject inappropriate flags; a bind-style operation can reject a socket type that does not support address binding. That is why an error record without the syscall and argument values is rarely actionable.
How to diagnose the unsupported request
- Log the exact syscall, socket family/type/protocol, symbolic and numeric flags, option level if applicable, and the current connection state.
- Compare with
ENOPROTOOPT, which points to an unavailable protocol option rather than the broader operation or flag being unsupported. - Do not silently drop a requested flag unless the caller's protocol contract permits changing the behavior; a fallback can alter message ordering, acknowledgements, or security properties.
What it does not show
The error is local capability negotiation within the socket API. It is not evidence that the remote endpoint rejected an application request or that a route is unavailable.
References
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