xpop_slot

Summary. Load the inner transaction and metadata of an XPOP (cross-chain proof) into two slots. Only valid while processing an Import (ttIMPORT) transaction.

Requires the featureHooksUpdate1 amendment.

Signature.

int64_t xpop_slot(uint32_t slot_no_tx, uint32_t slot_no_meta);

Parameters.

Name	Type	Description
slot_no_tx	uint32_t	Destination slot for the inner transaction, or 0 to allocate.
slot_no_meta	uint32_t	Destination slot for the inner metadata, or 0 to allocate.

Return value. Returns (slot_no_tx << 16) | slot_no_meta — both chosen slot numbers packed into one int64_t — on success. Errors: PREREQUISITE_NOT_MET (-9) if the originating transaction is not an Import; INVALID_ARGUMENT (-7) if either slot exceeds max_slots, or if both are the same non-zero slot (a collision); NO_FREE_SLOTS (-6) if not enough slots are free for the auto-allocation requested; INVALID_TXN (-37) if Xahau cannot extract either the XPOP inner transaction or its metadata.

Common failure patterns.

• Calling it outside Import processing → PREREQUISITE_NOT_MET.
• Passing the same non-zero slot for both txn and meta → INVALID_ARGUMENT.

Caveats / notes.

• Unpack the result: tx_slot = ret >> 16, meta_slot = ret & 0xFFFF.
• Requires featureHooksUpdate1; before that amendment, a hook that references xpop_slot cannot be installed successfully.

Minimal example.

int64_t r = xpop_slot(0, 0);       // auto-allocate both
uint32_t tx_slot   = (r >> 16) & 0xFFFF;
uint32_t meta_slot = r & 0xFFFF;

Related APIs. meta_slot, slot_subfield.