Learning-Bitcoin-from-the-Command-Line/12_3_Programming_Bitcoind_with_C.md

12.3: Programming Bitcoind with C

NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.

§12.2 laid out the methodology for creating C programs based on RPC calls to bitcoind. We're now going to show the potential of that C programming by laying out a simplistic, first cut of an actual Bitcoin program.

Plan for Your Code

We're going to program a simplistic first cut version of sendtoaddress, which will allow a user to send money to an address as long as he has a big enough UTXO. Here's what we need to do:

1. Request an address and an amount
2. Set an arbitrary fee
3. Find a UTXO that's large enough for the amount + the fee
4. Create a change address
5. Create a raw transaction that sends from the UTXO to the address and the change address
6. Sign the transaction
7. Send the transaction

Plan for Your Future

Since this is our first functional C program, we're going to try and keep it simple (KISS). If we were producing an actual production program, we'd at least want to do the following:

1. Test and/or sanitize the inputs
2. Calculate a fee automatically
3. Combine multiple UTXOs if necessary

If you want to continue to expand this example, these would be an excellent place to start, especially the latter points, which will approve your understanding and usage of actual RPC commands.

Write Your Transaction Software

We're now going to take that plan step by step

1. Request an Address and an Amount

Inputting the information is easy enough via command line arguments:

if (argc != 3) {

  printf("ERROR: Only %i arguments! Correct usage is '%s [recipient] [amount]'\n",argc-1,argv[0]);
  exit(-1);

}

char *tx_recipient = argv[1];
float tx_amount = atof(argv[2]);

printf("Sending %4.8f BTC to %s\n",tx_amount,tx_recipient);

WARNING: A real program would need much better sanitization of these variables.

2. Set an Arbitrary Fee

We're just setting the 0.0005 BTC fee that we've reguarly used to ensure that our test transactions go through quickly:

float tx_fee = 0.0005;
float tx_total = tx_amount + tx_fee;

WARNING: A real program would calculate a fee that minimized cost while ensuring the speed was sufficient for the sender.

X. Prepare Your RPC

Obviously, you're going to need to get all of your variables ready again, as discussed in §12.2: Accessing Bitcoind with C. You also need to initialize your library, connect your RPC client, and prepare your response object:

bitcoinrpc_global_init();
rpc_client = bitcoinrpc_cl_init_params ("bitcoinrpc", "73bd45ba60ab8f9ff9846b6404769487", "127.0.0.1", 18332);
btcresponse = bitcoinrpc_resp_init();

3. Find a UTXO

To find a UTXO you must call the listunspent RPC:

rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_LISTUNSPENT);
bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);

However, the real work comes in decoding the response. We said that the jansson library was "somewhat clunky" and this is why: you have to create (and clear) a very large set of json_t objects in order to dig down to what you want.

First, you must retrieve the result field from JSON:

json_t *lu_response = NULL;
json_t *lu_result = NULL;

lu_response = bitcoinrpc_resp_get (btcresponse);
lu_result = json_object_get(lu_response,"result");

WARNING: You only get a result if there wasn't an error. Here's another place for better error checking for production code.

Then, you go into a loop, examining each unspent transaction, which appears as an element in your JSON result array:

int i;

const char *tx_id = 0;
int tx_vout = 0;
double tx_value = 0.0;

for (i = 0 ; i < json_array_size(lu_result) ; i++) {

  json_t *lu_data = NULL;
  lu_data = json_array_get(lu_result, i);
  
  json_t *lu_value = NULL;
  lu_value = json_object_get(lu_data,"amount");
  tx_value = json_real_value(lu_value);

Is the UTXO large enough to pay out your transaction? If so, grab it!

  if (tx_value > tx_total) {

    json_t *lu_txid = NULL;
    lu_txid = json_object_get(lu_data,"txid");
    tx_id = strdup(json_string_value(lu_txid));

    json_t *lu_vout = NULL;
    lu_vout = json_object_get(lu_data,"vout");
    tx_vout = json_integer_value(lu_vout);

    json_decref(lu_value);
    json_decref(lu_txid);
    json_decref(lu_vout);
    json_decref(lu_data);
    break;

  } 

Make sure to clean things up as you go through failed loops and then when you finish the loops:

  json_decref(lu_value);
  json_decref(lu_data);

}

json_decref(lu_result);
json_decref(lu_response);

If you didn't find any large-enough UTXOs, you'll have to report that sad fact to the user ... and perhaps suggest that they should use a better program that will correctly merge UTXOs.

if (!tx_id) {

  printf("Very Sad: You don't have any UTXOs larger than %f",tx_total);
  exit(-1);
}

WARNING: A real program would use subroutines for this sort of lookup, so that you could confidentally call various RPCs from a library of C functions. We're just going to blob it all into main as part of our KISS philosophy of simple examples.

4. Create a Change Address

Repeat the standard RPC-lookup methodology to get a change address:

rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_GETRAWCHANGEADDRESS);

if (!rpc_method) {

  printf("ERROR: Unable to initialize listunspent method!\n");
  exit(-1);

}

bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);

if (btcerror.code != BITCOINRPCE_OK) {

printf("Error: listunspent error code %d [%s]\n", btcerror.code,btcerror.msg);

  exit(-1);

}

lu_response = bitcoinrpc_resp_get (btcresponse);
lu_result = json_object_get(lu_response,"result");
char *changeaddress = strdup(json_string_value(lu_result));

The only difference is in what particular information we extract from our JSON object.

WARNING: Here's another place that a subroutine would be really nice: to abstract out the whole RPC method initialization and call.

5. Create a Raw Transaction

Creating the actual raw transaction is the other tricky part of programming your sendtoaddress replacement. That's because it requires the creation of a complex JSON object as a paramter.

To correctly create these parameters, you'll need to review what the createrawtransaction RPC expects. Fortunately, this is easy to determine using the bitcoin-cli help functionality:

$ bitcoin-cli help createrawtransaction
createrawtransaction [{"txid":"id","vout":n},...] {"address":amount,"data":"hex",...} ( locktime )

To review, your inputs will be a JSON array containing one JSON object for each UTXO. Then the ouputs will all be in one JSON object. It's easiest to create these JSON elements from the inside out, using jansson commands.

5.1. Create the Input Parameters

To create the input object for your UTXO, use json_object, then fill it with key-values using either json_object_set_new (for newly created references) or json_object_set (for existing references):

json_t *inputtxid = NULL;
inputtxid = json_object();

json_object_set_new(inputtxid,"txid",json_string(tx_id));
json_object_set_new(inputtxid,"vout",json_integer(tx_vout));

You'll note that you have to translate each C variable type into a JSON variable type using the appropriate function, such as json_string or json_integer.

To create the overall input array for all your UTXOs, use json_array, then fill it up with objects using json_array_append:

json_t *inputparams = NULL;
inputparams = json_array();
json_array_append(inputparams,inputtxid);

5.2 Create the Output Parameters

To create the output array for your transaction, follow the same format, creating a JSON object with json_object, then filling it with json_object_set:

json_t *outputparams = NULL;
outputparams = json_object();

char tx_amount_string[32];
sprintf(tx_amount_string,"%.8f",tx_amount);
char tx_change_string[32];
sprintf(tx_change_string,"%.8f",tx_value - tx_total);

json_object_set(outputparams, tx_recipient, json_string(tx_amount_string));
json_object_set(outputparams, changeaddress, json_string(tx_change_string));

WARNING: You might expect to input your Bitcoin values as numbers, using json_real. Unfortunately, this exposes one of the major problems with integrating the jansson library and Bitcoin. Bitcoin is only valid to eight significant digits past the decimal point. You might recall that .00000001 BTC is a satoshi, and that's the smallest possible division of a Bitcoin. Doubles in C offer more significant digits than that, though they're often imprecise out past eight decimals. If you try to convert straight from your double value in C (or a float value, for that matter) to a Bitcoin value, the imprecision will often create a Bitcoin value with more than eight significant digits. Before Bitcoin Core 0.12 this appears to work, and you could use json_real. But as of Bitcoin Core 0.12, if you try to give createrawtransaction a Bitcoin value with too many significant digits, you'll instead get an error and the transaction will not be created. As a result, if the Bitcoin value has ever become a double or float, you must reformat it to eight significant digits past the digit before feeding it in as a string. This is obviously a kludge, so you should make sure it continues to work in future versions of Bitcoin Core.

5.3 Create the Parameter Array

To finish creating your parameters, simply to bundle them all up in a JSON array:

json_t *params = NULL;
params = json_array();
json_array_append(params,inputparams);
json_array_append(params,outputparams);

5.4 Make the RPC Call

Use the normal method to create your RPC call:

rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_CREATERAWTRANSACTION);

However, now you must feed it your parameters. This simply done with bitcoinrpc_method_set_params:

if (bitcoinrpc_method_set_params(rpc_method, params) != BITCOINRPCE_OK) {

  fprintf (stderr, "Error: Could not set params for createrawtransaction");

}

Afterward, run the RPC and get the results as usual:

bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);

lu_response = bitcoinrpc_resp_get(btcresponse);
lu_result = json_object_get(lu_response,"result");

char *tx_rawhex = strdup(json_string_value(lu_result));