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Update 12_3_Programming_Bitcoind_with_C.md
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Shannon Appelcline
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@ -324,3 +324,340 @@ The entire code, with a _little_ more error-checking appears in the Appendix.
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## Summary: Programming Bitcoind with C
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Using the techniques outlined in [§12.2](12_2_Accessing_Bitcoind_with_C.md) you can write a much more complex program using C calls. This section offers an example, with the first cut of a program that will send money to an address, without your users worrying about where it's coming from, how much they're paying as a fee, or how they get their change back. Obviously, a real-world program would need much better user-input control and error handling, but by outlining how the RPC code works, this section opens up the programming doorways to allow you to take the next step.
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## Appendix: Sending to an Address
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Here's the complete code for sending funds to someone using C.
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WARNING: This code has not been robustly checked. Beyond that, it's to be functionally minimalistic, not containing necessary error-checking. If you use it as a foundation of your own work, be sure to check it carefully and to make it a lot more robust.
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```
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file: sendtoaddress.c
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#include <jansson.h>
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#include <bitcoinrpc.h>
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#include <string.h>
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int main(int argc, char *argv[]) {
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/* 1. Request an Address and an Amount */
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if (argc != 3) {
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printf("ERROR: Only %i arguments! Correct usage is '%s [recipient] [amount]'\n",argc-1,argv[0]);
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exit(-1);
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}
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char *tx_recipient = argv[1];
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double tx_amount = atof(argv[2]);
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/* 2. Set an Arbitrary Fee */
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double tx_fee = 0.0005;
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double tx_total = tx_amount + tx_fee;
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/* X. Prepare Your RPC */
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bitcoinrpc_cl_t *rpc_client;
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bitcoinrpc_method_t *rpc_method = NULL;
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bitcoinrpc_resp_t *btcresponse = NULL;
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bitcoinrpc_err_t btcerror;
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json_t *lu_response = NULL;
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json_t *lu_result = NULL;
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bitcoinrpc_global_init();
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rpc_client = bitcoinrpc_cl_init_params ("bitcoinrpc", "73bd45ba60ab8f9ff9846b6404769487", "127.0.0.1", 18332);
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if (rpc_client) {
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/* 3. Find a UTXO */
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rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_LISTUNSPENT);
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if (!rpc_method) {
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printf("ERROR: Unable to initialize listunspent method!\n");
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exit(-1);
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}
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btcresponse = bitcoinrpc_resp_init();
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if (!btcresponse) {
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printf("Error: Cannot initialize response object!");
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exit(-1);
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}
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bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);
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if (btcerror.code != BITCOINRPCE_OK) {
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printf("Error: listunspent error code %d [%s]\n", btcerror.code,btcerror.msg);
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exit(-1);
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}
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lu_response = bitcoinrpc_resp_get (btcresponse);
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lu_result = json_object_get(lu_response,"result");
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int i;
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const char *tx_id = 0;
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int tx_vout = 0;
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double tx_value = 0.0;
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for (i = 0 ; i < json_array_size(lu_result) ; i++) {
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json_t *lu_data = NULL;
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lu_data = json_array_get(lu_result, i);
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json_t *lu_value = NULL;
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lu_value = json_object_get(lu_data,"amount");
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tx_value = json_real_value(lu_value);
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if (tx_value > tx_total) {
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json_t *lu_txid = NULL;
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lu_txid = json_object_get(lu_data,"txid");
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tx_id = strdup(json_string_value(lu_txid));
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json_t *lu_vout = NULL;
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lu_vout = json_object_get(lu_data,"vout");
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tx_vout = json_integer_value(lu_vout);
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json_decref(lu_value);
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json_decref(lu_txid);
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json_decref(lu_vout);
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json_decref(lu_data);
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break;
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}
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}
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json_decref(lu_result);
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json_decref(lu_response);
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if (!tx_id) {
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printf("Very Sad: You don't have any UTXOs larger than %f",tx_total);
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exit(-1);
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}
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bitcoinrpc_method_free(rpc_method);
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/* 4. Create a Change Address */
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rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_GETRAWCHANGEADDRESS);
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if (!rpc_method) {
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printf("ERROR: Unable to initialize createrawchangeaddress method!\n");
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exit(-1);
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}
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bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);
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if (btcerror.code != BITCOINRPCE_OK) {
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printf("Error: createrawchangeaddress error code %d [%s]\n", btcerror.code, btcerror.msg);
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exit(-1);
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}
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lu_response = bitcoinrpc_resp_get (btcresponse);
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lu_result = json_object_get(lu_response,"result");
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char *changeaddress = strdup(json_string_value(lu_result));
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json_decref(lu_result);
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json_decref(lu_response);
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bitcoinrpc_method_free(rpc_method);
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/* 5. Create a Raw Transaction */
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/* 5.1. Create the Input Parameters */
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json_t *inputtxid = NULL;
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inputtxid = json_object();
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json_object_set_new(inputtxid,"txid",json_string(tx_id));
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json_object_set_new(inputtxid,"vout",json_integer(tx_vout));
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json_t *inputparams = NULL;
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inputparams = json_array();
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json_array_append(inputparams,inputtxid);
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/* 5.2 Create the Output Parameters */
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json_t *outputparams = NULL;
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outputparams = json_object();
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char tx_amount_string[32];
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sprintf(tx_amount_string,"%.8f",tx_amount);
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char tx_change_string[32];
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sprintf(tx_change_string,"%.8f",tx_value - tx_total);
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json_object_set(outputparams,
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tx_recipient,
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json_string(tx_amount_string));
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json_object_set(outputparams,
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changeaddress,
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json_string(tx_change_string));
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/* 5.3 Create the Parameter Array */
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json_t *params = NULL;
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params = json_array();
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json_array_append(params,inputparams);
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json_array_append(params,outputparams);
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/* 5.4 Make the RPC Call */
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rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_CREATERAWTRANSACTION);
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if (!rpc_method) {
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printf("ERROR: Unable to initialize createrawtransaction method!\n");
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exit(-1);
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}
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if (bitcoinrpc_method_set_params(rpc_method, params) != BITCOINRPCE_OK) {
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fprintf (stderr, "Error: Could not set params for createrawtransaction");
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}
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json_decref(inputtxid);
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json_decref(inputparams);
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json_decref(outputparams);
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json_decref(params);
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bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);
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if (btcerror.code != BITCOINRPCE_OK) {
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printf("Error: createrawtransaction error code %d [%s]\n", btcerror.code,btcerror.msg);
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exit(-1);
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}
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lu_response = bitcoinrpc_resp_get(btcresponse);
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lu_result = json_object_get(lu_response,"result");
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char *tx_rawhex = strdup(json_string_value(lu_result));
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json_decref(lu_result);
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json_decref(lu_response);
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bitcoinrpc_method_free(rpc_method);
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/* 6. Sign the Transaction */
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params = json_array();
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json_array_append_new(params,json_string(tx_rawhex));
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rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_SIGNRAWTRANSACTION);
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if (!rpc_method) {
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printf("ERROR: Unable to initialize signrawtransaction method!\n");
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exit(-1);
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}
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if (bitcoinrpc_method_set_params(rpc_method, params) != BITCOINRPCE_OK) {
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fprintf (stderr, "Error: Could not set params for signrawtransaction");
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}
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json_decref(params);
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bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);
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if (btcerror.code != BITCOINRPCE_OK) {
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printf("Error: signrawtransaction error code %d [%s]\n", btcerror.code,btcerror.msg);
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exit(-1);
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}
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lu_response = bitcoinrpc_resp_get(btcresponse);
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lu_result = json_object_get(lu_response,"result");
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json_t *lu_signature = json_object_get(lu_result,"hex");
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char *tx_signrawhex = strdup(json_string_value(lu_signature));
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json_decref(lu_signature);
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json_decref(lu_result);
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json_decref(lu_response);
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bitcoinrpc_method_free(rpc_method);
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/* 7. Sign the Transaction */
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params = json_array();
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json_array_append_new(params,json_string(tx_signrawhex));
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rpc_method = bitcoinrpc_method_init(BITCOINRPC_METHOD_SENDRAWTRANSACTION);
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if (!rpc_method) {
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printf("ERROR: Unable to initialize sendrawtransaction method!\n");
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exit(-1);
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}
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if (bitcoinrpc_method_set_params(rpc_method, params) != BITCOINRPCE_OK) {
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fprintf (stderr, "Error: Could not set params for sendrawtransaction");
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}
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json_decref(params);
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bitcoinrpc_call(rpc_client, rpc_method, btcresponse, &btcerror);
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if (btcerror.code != BITCOINRPCE_OK) {
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printf("Error: endrawtransaction error code %d [%s]\n", btcerror.code,btcerror.msg);
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exit(-1);
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}
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lu_response = bitcoinrpc_resp_get(btcresponse);
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lu_result = json_object_get(lu_response,"result");
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char *tx_newid = strdup(json_string_value(lu_result));
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printf("Txid: %s\n",tx_newid);
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json_decref(lu_result);
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json_decref(lu_response);
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bitcoinrpc_method_free(rpc_method);
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} else {
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printf("ERROR: Failed to connect to server!\n");
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}
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bitcoinrpc_cl_free(rpc_client);
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bitcoinrpc_global_cleanup();
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}
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```
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Compile this as usual:
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```
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$ cc sendtoaddress.c -lbitcoinrpc -ljansson -o sendtoaddress
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```
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You can then use it to send funds to an address:
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```
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$ ./sendtoaddress mwCwTceJvYV27KXBc3NJZys6CjsgsoeHmf .10
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Txid: 23f3faea4e1e933e981fe7ed66008f33c00983a865f6ccce9af0005e9670e916
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```
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You can see information on this transaction that we sent [here](https://live.blockcypher.com/btc-testnet/tx/23f3faea4e1e933e981fe7ed66008f33c00983a865f6ccce9af0005e9670e916/).
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