Learning-Bitcoin-from-the-Command-Line/12_2_Accessing_Bitcoind_with_C.md

# 12.2: Accessing 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.

Though command-line `curl` is the easiest way to access the `bitcoind` directly, there are [many other options](https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)) for doing so and most of them support more fully featured programming languages. The best package for doing so in C is currently [libbitcoinrpc](https://github.com/gitmarek/libbitcoinrpc/blob/master/README.md). It uses a `curl` library for accessing the data and it uses the somewhat clunky `jansson` library for decoding the JSON. 

## Set Up libbitcoinrpc

To use `libbitcoinrpc`, you need to install a basic C setup and the dependent packages, `libcurl`, `libjansson`, and `libuuid`. The following will do so on a Ubuntu system:
```
$ sudo apt-get install make gcc libcurl4-openssl-dev libjansson-dev uuid-dev
```
You can then download [libbitcoinrpc from Github](https://github.com/gitmarek/libbitcoinrpc/blob/master/README.md). Clone it or grab a zip file, as you prefer.
```
$ sudo apt-get unzip
$ unzip libbitcoinrpc-master.zip 
$ cd libbitcoinrpc-master/
```

### Compile libbitcoinrpc

Before you can compile and install the package, you'll probably need to adjust your `$PATH`, so that you can access `/sbin/ldconfig`:
```
$ PATH="/sbin:$PATH"
```
For a Ubunto system, you'll also want to adjust the `INSTALL_LIBPATH` in the `libbitcoinrpc` `Makefile` to install to `/usr/lib` instead of `/usr/local/lib`:
```
INSTALL_LIBPATH    := $(INSTALL_PREFIX)/usr/lib
```
(If you prefer not to sully your `/usr/lib`, the alternative is to change your `etc/ld.so.conf` or its dependent files appropriately ... but for a test setup on a test machine, this is probably fine.)

Then you can compile:
```
$ make
```
If that works, you can install the package:
```
$ sudo make install
```

## Write Code in C

`libbitcoinrpc` has well-structured and simple methods for connecting to your `bitcoind`, executing RPC calls, and decoding the response.

### Setup Your Code

To use `libbitcoinrpc`, make sure that your code files include the appropriate headers:
```
#include <jansson.h>
#include <bitcoinrpc.h>
```
You'll also need to link in the appropriate libraries whenever you compile:
```
$ cc mybitcoinclient.c -lbitcoinrpc -ljansson -o rpcclient
```

### Build Your Connection

Building the connection to your `bitcoind` server takes a few simple steps.

First, initialize the library:
```
bitcoinrpc_global_init();
```
Then connect to your `bitcoind`. The four arguments for `bitcoinrpc_cl_init_params` are username, password, IP address, and port. As usual, you should extract the user and password from `~/.bitcoin/bitcoin.conf`, while IP address 127.0.0.1 and port 18332 should be correct for the standard testnet setup described in this documents.
```
bitcoinrpc_cl_t *rpc_client;
rpc_client = bitcoinrpc_cl_init_params ("bitcoinrpc", "d8340efbcd34e312044c8431c59c792c", "127.0.0.1", 18332);
```

> **MAINNET VS TESTNET:** The port would be 8332 for a mainnet setup.

If `rpc_client` is successful, then you can go. 

Later, when you're all done with your `bitcoind` connection, you should close it:
```
bitcoinrpc_global_cleanup();
```

Appendix I shows the complete code for a test of a `bitcoind` connection.

### Make an RPC Call

In order to use an RPC method in `bitcoinrpc`, you must initialize a variable of type `bitcoinrpc_method_t`. You do so with the appropriate value for the method you want to use, all of which are listed in the [bitcoinrpc Reference](https://github.com/gitmarek/libbitcoinrpc/blob/master/doc/reference.md).
```
bitcoinrpc_method_t *getmininginfo  = NULL;
getmininginfo = bitcoinrpc_method_init(BITCOINRPC_METHOD_GETMININGINFO);
```
Usually you would set parameters here, but `getmininginfo` requires no parameters, so you can skip that for now.

Two more objects are required, a "response object" and an "error object". They're created via standard `bitcoinrpc` function calls:
```
bitcoinrpc_resp_t *btcresponse  = NULL;
btcresponse = bitcoinrpc_resp_init();

bitcoinrpc_err_t btcerror;
```
And now you can put it all together to make a `getmininginfo` RPC call:
```
bitcoinrpc_call (rpc_client, getmininginfo, btcresponse, &btcerror);
```
### Output Your Response

Retrieve the output of your call as a JSON object with `bitcoinrpc_resp_get`.
```
json_t *jsonresponse = NULL;
jsonresponse = bitcoinrpc_resp_get (btcresponse);
```
If you want to output the complete JSON results of the RPC call, you can do so with a simple invocation of `json_dumps`, from the `jansson` library:
```
printf ("%s\n", json_dumps (j, JSON_INDENT(2)));
```
However since you're now writing complete programs, you're probably going to want to do more subtle work, such as pulling out individual JSON values for specific usage. The [jansson Reference](https://jansson.readthedocs.io/en/2.10/apiref.html) details how to do so.

You can drill down to the `result` JSON object:
```
json_t *jsonresult = NULL;
jsonresult = json_object_get(jsonresponse,"result");
printf ("%s\n", json_dumps (jsonresult, JSON_INDENT(2)));
```
Alternatively, you can drill down to an individual item like `blocks`:
```
json_t *jsonblocks = NULL;
jsonblocks = json_object_get(jsonresult,"blocks");

int blocks;
blocks = json_integer_value(jsonblocks);
printf("Block Count: %d\n",blocks);
```

> **WARNING:** It's extremely easy to segfault your C code when working with jansson objects if you get confused with what type of object you're retrieving. Make careful use of `bitcoin-cli help` to know what you should expect, and if you experience a segmentation fault, first look at your JSON retrieval functions.

Appendix II has an example of this complete code.

### Make an RPC Call with Arguments

But what if your RPC call _did_ have arguments? 

#### Create a JSON Array

To send parameters to your RPC call using `libbitcoinrpc` you have to wrap them in a JSON array. An array is just a simple listing of values, so all you have to do is encode the parameters as elements of the array. 

Create the array using the `json_array` function from `jansson`:
```
json_t *params = NULL;
params = json_array();
```
You can then fill it by converting C-typed objects into JSON-typed objects and appending them to the array:
```
json_array_append_new(params,json_string(tx_rawhex));
```
Note that there are two variants to the append command: `json_array_append_new`, which appends a newly created variable, and `json_array_append`, which appends an existing variable.

This methodology will serve for the majority of RPC commands with parameters, but there are some that are much more complex. In these cases you may need to create subsidiary JSON objects or JSON arrays, which you then append to the parameters array as usual. The next section contains an example of doing so using `createrawtransaction`, which contains an array of objects for the inputs, an object for the outputs and the `locktime` parameter. 

#### Assign the Parameters

When you've created your parameters JSON array, you simply assign it after you've initialized your RPC method, as follows:
```
bitcoinrpc_method_set_params(rpc_method, params)
```
This section doesn't include a full example of this more complex methodology, but we'll see it in action multiple times in our first comprehensive RPC-based C program, in the next section.

## Summary: Accessing Bitcoind with C

By linking to the `bitcoinrpc` and `jansson` libraries, you can easily access `bitcoind` via RPC calls from a C library. To do so, you create an RPC connection, then make individual RPC calls, some of them with parameters. `jansson` then allows you to decode the JSON responses.

_What is the power of C?_ C allows you to take the next step beyond shell-scripting, permitting the creation of more comprehensive and robust programs. A more comprehensive example will appear in the next chapter.

## Appendix I: Testing a Bitcoind Connection

Here's the complete code for a test of the connection to `bitcoind`.
```
file: testbitcoin.c

#include <jansson.h>
#include <bitcoinrpc.h>

int main(void) {
  
  bitcoinrpc_global_init();

  bitcoinrpc_cl_t *rpc_client;
  rpc_client = bitcoinrpc_cl_init_params ("bitcoinrpc", "d8340efbcd34e312044c8431c59c792c", "127.0.0.1", 18332);

  if (rpc_client) {

    printf("Successfully connected to server!\n");
  
  } else {

    printf("Failed to connect to server!\n");

  }

  bitcoinrpc_global_cleanup();

}
```
You can compile and run this as follows:
```
$ cc testbitcoin.c -lbitcoinrpc -ljansson -o testbitcoin
$ ./testbitcoin 
Successfully connected to server!
```
## Appendix II: Getting Mining Info

Here's the complete code for the `getmininginfo` command, with organized variable initiatialization, error checking, and variable cleanup.
```
file: getmininginfo.c

#include <jansson.h>
#include <bitcoinrpc.h>

int main(void) {

  bitcoinrpc_cl_t *rpc_client;
  bitcoinrpc_method_t *getmininginfo  = NULL;
  bitcoinrpc_resp_t *btcresponse  = NULL;
  bitcoinrpc_err_t btcerror;

  json_t *jsonresponse = NULL;
  json_t *jsonresult = NULL;
  json_t *jsonblocks = NULL;
  int blocks;

  bitcoinrpc_global_init();

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

  if (rpc_client) {
    getmininginfo = bitcoinrpc_method_init(BITCOINRPC_METHOD_GETMININGINFO);

    if (!getmininginfo) {

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

    }

    btcresponse = bitcoinrpc_resp_init();
    if (!btcresponse) {

      printf("Error: Cannot initialize response object!\n");
      exit(-1);

    }

    bitcoinrpc_call(rpc_client, getmininginfo, btcresponse, &btcerror);
    
    if (btcerror.code != BITCOINRPCE_OK) {


      printf("Error: getmininginfo error code %d [%s]\n", btcerror.code,btcerror.msg);
      exit(-1);

    }

    printf("Full Response: ");
    jsonresponse = bitcoinrpc_resp_get (btcresponse);
    printf ("%s\n", json_dumps (jsonresponse, JSON_INDENT(2)));

    printf("\nJust the Result: ");
    jsonresult = json_object_get(jsonresponse,"result");
    printf ("%s\n", json_dumps (jsonresult, JSON_INDENT(2)));

    jsonblocks = json_object_get(jsonresult,"blocks");
    blocks = json_integer_value(jsonblocks);
    printf("\nBlock Count: %d\n",blocks);

    json_decref(jsonblocks);
    json_decref(jsonresult);
    json_decref(jsonresponse);

  } else {

    printf("ERROR: Failed to connect to server!\n");

  }

  bitcoinrpc_cl_free(rpc_client);
  bitcoinrpc_global_cleanup();

}
```
As usual, you can compile and run as follows:
```
$ cc getmininginfo.c -lbitcoinrpc -ljansson -o getmininginfo
$ ./getmininginfo 
Full Response: {
  "id": "03406237-cd8f-466d-ac31-86711ea9d1db",
  "result": {
    "blocks": 1147154,
    "errors": "Warning: unknown new rules activated (versionbit 28)",
    "pooledtx": 0,
    "currentblocksize": 0,
    "currentblockweight": 0,
    "currentblocktx": 0,
    "difficulty": 313525.08513550513,
    "networkhashps": 3958339463617.417,
    "chain": "test"
  },
  "error": null
}

Just the Result: {
  "blocks": 1147154,
  "errors": "Warning: unknown new rules activated (versionbit 28)",
  "pooledtx": 0,
  "currentblocksize": 0,
  "currentblockweight": 0,
  "currentblocktx": 0,
  "difficulty": 313525.08513550513,
  "networkhashps": 3958339463617.417,
  "chain": "test"
}

Block Count: 1147154
```