What does errno 134 (EPROTO) mean?

EPROTO

EPROTO is a UCRT POSIX compatibility constant for a protocol contract failure. It indicates that the caller or peer reached a point where the received state, order, field set, or operation is invalid for the protocol being implemented.

Unlike ECONNRESET or a timeout, this status does not by itself say that the network path failed. It can arise after transport succeeds, for example when a peer sends an unexpected reply, negotiates an unsupported state transition, or violates a version-specific constraint.

Evidence that makes the error actionable

• The protocol name and version, connection or session identifier, and current state-machine phase.
• The expected message or operation, the received or attempted one, and any negotiated capabilities.
• The parser, validation rule, or transition check that classified the condition as a protocol error.

For a Windows build, first determine whether the code was emitted by UCRT, a POSIX compatibility layer, or an application library. Native Windows and Winsock APIs retain their own documented error contracts.

References

• POSIX: <errno.h>
• Microsoft: errno constants
• Microsoft: errno and _doserrno

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