etxn_fee_base

Summary. Compute the minimum fee (in drops) that a to-be-emitted transaction must pay, given its fully-serialized form.

Signature.

int64_t etxn_fee_base(uint32_t read_ptr, uint32_t read_len);

Parameters.

Name	Type	Description
read_ptr	uint32_t	Pointer to the serialized transaction.
read_len	uint32_t	Length of the serialized transaction.

Return value. Returns the required fee in drops on success. Errors: OUT_OF_BOUNDS (-1) for a bad buffer; PREREQUISITE_NOT_MET (-9) if etxn_reserve was not called; INVALID_TXN (-37) if the buffer cannot be parsed as a transaction.

Common failure patterns.

• Calling it before etxn_reserve → PREREQUISITE_NOT_MET.
• Passing a malformed or incomplete transaction blob → INVALID_TXN.

Caveats / notes.

• Internally this runs the transaction through the same base-fee calculation the network uses (calculateBaseFee / invoke_calculateBaseFee under fixHookAPI20251128), so it doubles as a sanity check that the blob is a valid transaction.
• Compute the fee after the transaction is otherwise complete (including sfEmitDetails), then write the returned value into sfFee. emit rejects a transaction whose sfFee is below this minimum with EMISSION_FAILURE.

Minimal example.

int64_t fee = etxn_fee_base((uint32_t)tx, tx_len);

Practical example (the pattern generated by the Transaction Builder).

// After the template is otherwise complete (including sfEmitDetails),
// compute the fee and patch it into sfFee via the generated FEE_OUT pointer.
int64_t fee = etxn_fee_base(SBUF(txn));
if (fee < 0)
    rollback(SBUF("fee calc failed"), fee);
SET_NATIVE_AMOUNT(FEE_OUT, fee);

This is exactly what the generated PREPARE_TXN() macro does for you — see Emitted Transaction.

Related APIs. emit, fee_base, prepare.