EOPNOTSUPP

EOPNOTSUPP is an errno value used in Unix-like systems to indicate that an operation is not supported on a transport endpoint, typically a socket. It originates from the POSIX error set and is returned by system calls or library routines when the requested operation cannot be performed because the underlying transport protocol or endpoint does not implement it.

Common usage and meaning

- EOPNOTSUPP signaling occurs mainly in socket-related contexts, where certain operations or options are not supported by the current transport (for example, a protocol feature that the socket’s endpoint does not implement).

- It communicates a more specific situation than a general “operation not allowed”; the limitation lies with the transport layer rather than with permissions or input parameters.

Scope and interpretation

- The error is most often seen with socket operations or protocol-specific control commands. It can also arise in other transport endpoints where the interface does not support the requested action.

- The exact meaning can vary by platform, and some systems may provide a different but equivalent code (for example ENOTSUP) for similar situations.

Relation to other codes

- EOPNOTSUPP is distinct from ENOTSUP, which is a broader indication that an operation is not supported by a component in general, not necessarily tied to a transport endpoint.

- Other related transport-related codes include ENOPROTOOPT (protocol not available) and ENOTSOCK (not a socket) used in complementary contexts.

Practical note

- If a program encounters EOPNOTSUPP, it should either fall back to an alternative implementation compatible with the transport, or report that the requested transport-specific feature is not available for the current endpoint.