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EPROTO
EPROTO points at a protocol contract failure rather than a missing route or a local permission problem. Linux lists it as “Protocol error”; POSIX describes it as a protocol error reported by a STREAMS or protocol-oriented interface.
The code is deliberately more specific than a generic I/O failure but less specific than transport results such as ECONNRESET or ETIMEDOUT. Use it as a signal to inspect the protocol state machine, negotiated options, framing, peer behavior, and device or kernel logs.
What usually matters
- Which protocol layer produced the errno: socket family, filesystem client, driver, or parser.
- The last valid state transition before the failure.
- Whether the peer sent an unsupported frame, invalid sequence, or corrupted record.
- Whether the implementation is mixing protocol versions or incompatible feature flags.
POSIX errno.h · Linux errno(3) · Linux UAPI errno definitions
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