What characterizes a process that is in uninterruptible sleep?

A process in uninterruptible sleep is characterized by its need to wait for an external event to occur before it can continue execution. This state is often associated with I/O operations, such as waiting for disk reads or network packets. The uninterruptible nature means that the process cannot be interrupted or woken up by signals or other requests from the system until the specific event it is waiting for has occurred.

This contrasts with a normal sleep state, where a process might be waiting for other internal conditions and can be interrupted by signals for immediate attention. By being in uninterruptible sleep, the process effectively relinquishes its ability to respond to any signaling, ensuring that it remains in this state until completion of the I/O task at hand.

While this state does not consume CPU resources, that characteristic alone does not define uninterruptible sleep. Instead, it is the dependency on an external event that most accurately encompasses the nature of this sleep state. As a result, the state embodies the combination of waiting for an external event and being unresponsive to interruptions.