errno 25 (ENOTTY): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: An ioctl-style request does not apply to the descriptor’s object type, driver, or supported operation set.

Could be also:
Constant	Type	OS
ERROR_SEEK	Win32 error	Windows
KERN_MEMORY_RESTART_COPY	Kern return	Mac
ippStsInsufficientEntropy	Intel Ipp Status	Any
BAD_POOL_HEADER	BugCheck Code	Windows

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EMFILE	ETXTBSY

ENOTTY

Why the name is historical

ENOTTY historically referred to an operation that required a terminal device. In modern UNIX-like systems it is commonly returned when an ioctl() request does not apply to the object referenced by the file descriptor. The descriptor may be valid, yet point to a regular file, socket, pipe, namespace, or device driver that does not implement that command.

It is therefore usually an API-contract mismatch, not an indication that a physical terminal is missing. A feature can also be unavailable because the active kernel, filesystem, driver, or library wrapper does not support the requested ioctl.

How to investigate

Record the ioctl request constant, descriptor origin, and object type.
Verify that the descriptor is for the device or filesystem the operation expects, rather than merely checking that it is open.
Check documented kernel, driver, and filesystem support for the exact command and version.
Provide a capability check or explicit unsupported-feature path instead of treating every ENOTTY as a transient error.

References

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