What does errno 51 (EL2HLT) in Linux mean?

EL2HLT

EL2HLT: the level-2 subsystem is halted

EL2HLT is a historical System V/XENIX-style errno retained by Linux headers. It describes a stopped level-2 communications component; it is not a generic “device failed” or filesystem error.

What to preserve

Capture the system call that returned the error, the device or stream object, the loaded compatibility/driver modules, and the immediately returned errno. If a legacy stack exposes separate protocol levels, record which level-2 object was expected to be active and the last control operation that changed its state.

Verification

Restore or reattach the level-2 component through the owning driver or compatibility interface, then repeat the original call against the same endpoint. A different errno after that transition belongs to a later boundary and should be diagnosed separately.

Technical references

• Linux asm-generic errno definitions.
• Linux errno(3).