otxn_id

Summary. Write the 32-byte transaction id (hash) of the originating transaction into hook memory.

Signature.

int64_t otxn_id(uint32_t write_ptr, uint32_t write_len, uint32_t flags);

Parameters.

Name	Type	Description
write_ptr	uint32_t	Destination buffer in WASM memory.
write_len	uint32_t	Buffer length; must be at least 32.
flags	uint32_t	0 = during a failed-emit callback, return the emitted txn's stored hash (sfTransactionHash); otherwise return the computed transaction id.

Return value. Returns the number of bytes written (32) on success. Errors: TOO_SMALL (-4) if write_len < 32; OUT_OF_BOUNDS (-1) if the buffer is outside memory.

Common failure patterns.

• Buffer shorter than 32 bytes → TOO_SMALL.
• Out-of-range pointer or length → OUT_OF_BOUNDS.

Caveats / notes.

• The flags parameter only matters inside a failed-emit callback. There, flags == 0 yields the hash the emitted transaction was stored under (sfTransactionHash), while a non-zero flags recomputes the transaction id from the object (getTransactionID()). Outside a callback the two are equivalent and you can pass 0.
• Writes 32 raw bytes; there is no serialization prefix to strip.

Minimal example.

uint8_t id[32];
otxn_id((uint32_t)id, 32, 0);   // hash of the originating transaction

Practical example.

// Persist the originating transaction id as a state key so the hook can
// detect a replay/callback for the same transaction later.
uint8_t id[32];
if (otxn_id((uint32_t)id, 32, 0) != 32)
    rollback(SBUF("could not read otxn id"), __LINE__);

uint8_t one[1] = {1};
state_set(SBUF(one), (uint32_t)id, 32);   // mark this txn as seen
accept(SBUF("recorded"), 0);

Related APIs. otxn_field, state_set, hook_again.