lightning-fundpsbt – Command to populate PSBT inputs from the wallet¶
fundpsbt satoshi feerate startweight [minconf] [reserve] [locktime]
fundpsbt is a low-level RPC command which creates a PSBT using unreserved
inputs in the wallet, optionally reserving them as well.
satoshi is the minimum satoshi value of the output(s) needed (or the string “all” meaning use all unreserved inputs). If a value, it can be a whole number, a whole number ending in sat, a whole number ending in 000msat, or a number with 1 to 8 decimal places ending in btc.
feerate can be one of the feerates listed in lightning-feerates(7), or one of the strings urgent (aim for next block), normal (next 4 blocks or so) or slow (next 100 blocks or so) to use lightningd’s internal estimates. It can also be a feerate is a number, with an optional suffix: perkw means the number is interpreted as satoshi-per-kilosipa (weight), and perkb means it is interpreted bitcoind-style as satoshi-per-kilobyte. Omitting the suffix is equivalent to perkb.
startweight is the weight of the transaction before fundpsbt has added any inputs.
minconf specifies the minimum number of confirmations that used outputs should have. Default is 1.
reserve is a boolean: if true (the default), then reserveinputs is called (successfully, with exclusive true) on the returned PSBT.
locktime is an optional locktime: if not set, it is set to a recent block height.
Let’s assume the caller is trying to produce a 100,000 satoshi output.
First, the caller estimates the weight of the core (typically 42) and known outputs of the transaction (typically (9 + scriptlen) * 4). For a simple P2WPKH it’s a 22 byte scriptpubkey, so that’s 124 weight.
It calls “fundpsbt 100000sat slow 166”, which succeeds, and returns the psbt and feerate_per_kw it used, the estimated_final_weight and any excess_msat.
If excess_msat is greater than the cost of adding a change output, the caller adds a change output randomly to position 0 or 1 in the PSBT. Say feerate_per_kw is 253, and the change output is a P2WPKH (weight 124), the cost is around 31 sats. With the dust limit disallowing payments below 546 satoshis, we would only create a change output if excess_msat was greater or equal to 31 + 546.
On success, returns the psbt containing the inputs, feerate_per_kw showing the exact numeric feerate it used, estimated_final_weight for the estimated weight of the transaction once fully signed, and excess_msat containing the amount above satoshi which is available. This could be zero, or dust. If satoshi was “all”, then excess_msat is the entire amount once fees are subtracted for the weights of the inputs and startweight.
If reserve was true, then a reservations array is returned, exactly like reserveinputs.
On error the returned object will contain
code being one of the following:
- -32602: If the given parameters are wrong.
- -1: Catchall nonspecific error.
- 301: Insufficient UTXOs to meet satoshi value.
lightning-utxopsbt(7), lightning-reserveinputs(7), lightning-unreserveinputs(7).