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ENOCSI
ENOCSI: Linux legacy errno
ENOCSI (errno 3) reports that the legacy communications interface cannot obtain the expected CSI control/status structure. 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, CSI identifier, module state, and request that requires the structure. 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
compare against a known-good stream with an initialized CSI structure on the same driver. Keep the process, device/endpoint and deployed driver version fixed so the comparison changes only the suspected boundary.
Correction
initialize/attach the required control structure or correct its lifetime. 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 ENOCSI 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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