ENOTSUP

ENOTSUP is a symbolic error number defined in errno.h on many Unix-like systems. It stands for “Operation not supported” and is used by programs and libraries to indicate that a requested operation cannot be completed by the underlying object, subsystem, or protocol. The value is used by functions that return -1 and set errno to ENOTSUP to signal this condition.

Typical contexts include attempting to perform an operation on a resource that is read-only, using a feature not implemented by a file system or device driver, or issuing a socket or protocol operation that the underlying network protocol does not support. It is distinct from ENOSYS (“Function not implemented”) in that ENOTSUP generally means the operation is recognized as valid but not currently supported by the object, while ENOSYS implies the kernel or interface is not implemented at all.

Relation to other codes: Some platforms provide EOPNOTSUPP (“Operation not supported on socket”) as a separate code; on those systems ENOTSUP may be treated equivalently to EOPNOTSUPP in certain contexts. Because error codes are not standardized identically across all systems, portable software often checks multiple related codes (for example ENOTSUP, EOPNOTSUPP, ENOSYS).

Handling: When ENOTSUP is returned, software can either fall back to a supported alternative, report that the operation is not available, or retry later if capabilities can change. Developers should avoid assuming ENOTSUP is permanent and should consider feature testing and runtime capability discovery to maintain portability.

See also: errno.h, POSIX, EOPNOTSUPP, ENOSYS.