What does errno 130 (EOPNOTSUPP) mean?

EOPNOTSUPP

EOPNOTSUPP is a UCRT POSIX compatibility constant for an operation that the selected endpoint, object, or protocol does not support. Its value does not say whether the rejected operation was a socket option, an IPC operation, a file operation, or an application-defined portability call.

The useful distinction is operational: the object may be valid and reachable, yet the requested action is not part of its contract. Changing network reachability, restarting a process, or retrying the same unsupported action does not add the missing capability.

What to compare

• The exact operation and option that were requested.
• The object, protocol family, endpoint type, backend, and negotiated version.
• The documented feature set of the concrete implementation, rather than a similarly named API on another platform.

UCRT defines EOPNOTSUPP separately from ENOTSUP. Portable code should preserve the producer's meaning, but Windows code must still inspect the documented error channel of the API that actually failed.

References

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

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