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EUNATCH
EUNATCH: Linux legacy errno
EUNATCH (errno 3) reports that the required protocol driver/module is not attached to the legacy stream. These names come from historical System V/XENIX compatibility interfaces; on modern Linux they usually point to a driver, STREAMS-compatibility layer, or legacy IPC contract rather than a general filesystem error.
Evidence to capture
Record stream/device, expected protocol module, current module stack, and failing request. Save errno immediately after the failing call, together with its return value, because later library or logging calls can overwrite thread-local error state.
Focused test
enumerate the stream modules and reproduce after attaching only the required protocol module. Keep the process, device/endpoint and deployed driver version fixed so the comparison changes only the suspected boundary.
Correction
attach/configure the required protocol module through the supported stream lifecycle. If the interface is provided by a compatibility driver, verify its documented ABI and object lifetime instead of assuming that a numerically similar errno from another UNIX has the same contract.
Verification
Repeat the original system call and confirm it passes the former EUNATCH boundary with the same object identity. If another errno appears, preserve it as a separate result from the next failing operation.
Technical references
- Linux man-pages: errno(3)
- Linux kernel: asm-generic errno definitions
- GNU C Library: error codes
- illumos: STREAMS programming documentation
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