From bec47102e80d8bd20a078d2556a059e8c71beb18 Mon Sep 17 00:00:00 2001
From: Shannon Appelcline <shannona@skotos.net>
Date: Wed, 21 Jun 2017 10:52:28 -0700
Subject: [PATCH] Update 12_3_Programming_Bitcoind_with_C.md

---
 12_3_Programming_Bitcoind_with_C.md | 85 ++++++++++++++++++++++++++++-
 1 file changed, 84 insertions(+), 1 deletion(-)

diff --git a/12_3_Programming_Bitcoind_with_C.md b/12_3_Programming_Bitcoind_with_C.md
index 4517358..a0fbe07 100644
--- a/12_3_Programming_Bitcoind_with_C.md
+++ b/12_3_Programming_Bitcoind_with_C.md
@@ -59,7 +59,90 @@ 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
+### 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](12_2_Accessing_Bitcoind_with_C.md). 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");
+```
+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_amount = 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_amount = NULL;
+  lu_amount = json_object_get(lu_data,"amount");
+  tx_amount = json_real_value(lu_amount);
+```
+Is the UTXO large enough to pay out your transaction? If so, grab it!
+```
+  if (tx_amount > 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_amount);
+	  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:
+```
+  json_decref(lu_amount);
+  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 sending 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);
+}
+
+printf("We are using txid %s with vout %i\n",tx_id,tx_vout);
+```
+
+> **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