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E_BLUETOOTH_ATT_PREPARE_QUEUE_FULL
Reliable writes consume server-side queue entries before commit
E_BLUETOOTH_ATT_PREPARE_QUEUE_FULL is ATT error 0x09. Prepare Write stores fragments on the server until an Execute Write commits or cancels the transaction. The queue can fill because the value is large, fragments are unnecessarily small, multiple attributes are included, or a previous transaction was not completed.
This condition differs from a generic memory failure: it identifies the reliable-write staging queue. Retrying the same additional fragment before cancelling or executing the existing queue does not release capacity and can leave the connection stuck in the same state.
Recover without committing incomplete data
- Send Execute Write with the cancel flag when the transaction cannot be completed safely.
- Use the largest fragment permitted by the negotiated MTU to reduce queue entries.
- Ensure each connection serializes reliable-write transactions.
- Check whether disconnect paths and timeouts clear the server queue as required by the implementation.
References
- Bluetooth Core: queued writes and Prepare Queue Full
- BlueZ GATT API: prepare-authorize writes
- Zephyr: ATT queued-write error constants
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