ℹ️ NOTE: This section has been recently added to the course and is an early draft that may still be awaiting review. Caveat reader.
You've already seen one alternative way to access the Bitcoind's RPC ports: curl, which was covered in a Chapter 4 Interlude. Interacting with bitcoind through an RPC library in C is no different than that, you just need some good libraries to help you out. This section introduces a package called libbitcoinrpc, which allows you to access JSON-RPC bitcoind port. It uses a curl library for accessing the data and it uses the jansson library for encoding and decoding the JSON.
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 your Bitcoin Standup server (or any other Ubuntu server).
$ sudo apt-get install make gcc libcurl4-openssl-dev libjansson-dev uuid-dev
Suggested packages:
libcurl4-doc libidn11-dev libkrb5-dev libldap2-dev librtmp-dev libssh2-1-dev
The following NEW packages will be installed:
libcurl4-openssl-dev libjansson-dev uuid-dev
0 upgraded, 3 newly installed, 0 to remove and 4 not upgraded.
Need to get 358 kB of archives.
After this operation, 1.696 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
You can then download libbitcoinrpc from Github. Clone it or grab a zip file, as you prefer.
$ sudo apt-get install git
$ git clone https://github.com/gitmarek/libbitcoinrpc
⚠️ WARNING A change in the "signrawtransaction" RPC caused signing withlibbitcoinrpcto segfault for Bitcoin 0.17 or higher. A PR has been submitted to resolve the problem, but if it hasn't yet been merged, you can just make the one simple change in the source code tosrc/bitcoinrpc_method.cbefore compiling.
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 an Ubuntu system, you'll also want to adjust the INSTALL_LIBPATH in the libbitcoinrpc Makefile to install to /usr/lib instead of /usr/local/lib:
$ emacs ~/libbitcoinrpc/Makefile
...
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.)
Likewise, you'll also want to adjust the INSTALL_HEADERPATH in the libbitcoinrpc Makefile to install to /usr/include instead of /usr/local/include:
...
INSTALL_HEADERPATH := $(INSTALL_PREFIX)/usr/include
Then you can compile:
$ cd libbitcoinrpc
~/libbitcoinrpc$ make
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -o src/bitcoinrpc_err.o -c src/bitcoinrpc_err.c
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -o src/bitcoinrpc_global.o -c src/bitcoinrpc_global.c
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -o src/bitcoinrpc.o -c src/bitcoinrpc.c
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -o src/bitcoinrpc_resp.o -c src/bitcoinrpc_resp.c
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -o src/bitcoinrpc_cl.o -c src/bitcoinrpc_cl.c
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -o src/bitcoinrpc_method.o -c src/bitcoinrpc_method.c
gcc -fPIC -O3 -g -Wall -Werror -Wextra -std=c99 -D VERSION=\"0.2\" -shared -Wl,-soname,libbitcoinrpc.so.0 \
src/bitcoinrpc_err.o src/bitcoinrpc_global.o src/bitcoinrpc.o src/bitcoinrpc_resp.o src/bitcoinrpc_cl.o src/bitcoinrpc_method.o \
-o .lib/libbitcoinrpc.so.0.2 \
-Wl,--copy-dt-needed-entries -luuid -ljansson -lcurl
ldconfig -v -n .lib
.lib:
libbitcoinrpc.so.0 -> libbitcoinrpc.so.0.2 (changed)
ln -fs libbitcoinrpc.so.0 .lib/libbitcoinrpc.so
If that works, you can install the package:
$ sudo make install
Installing to
install .lib/libbitcoinrpc.so.0.2 /usr/local/lib
ldconfig -n /usr/local/lib
ln -fs libbitcoinrpc.so.0 /usr/local/lib/libbitcoinrpc.so
install -m 644 src/bitcoinrpc.h /usr/local/include
Installing docs to /usr/share/doc/bitcoinrpc
mkdir -p /usr/share/doc/bitcoinrpc
install -m 644 doc/*.md /usr/share/doc/bitcoinrpc
install -m 644 CREDITS /usr/share/doc/bitcoinrpc
install -m 644 LICENSE /usr/share/doc/bitcoinrpc
install -m 644 Changelog.md /usr/share/doc/bitcoinrpc
Installing man pages
install -m 644 doc/man3/bitcoinrpc*.gz /usr/local/man/man3
libbitcoinrpc has well-structured and simple methods for connecting to your bitcoind, executing RPC calls, and decoding the response.
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 yourcode.c -lbitcoinrpc -ljansson -o yourcode
Building the connection to your bitcoind server takes a few simple steps.
First, initialize the library:
bitcoinrpc_global_init();
Then connect to your bitcoind with bitcoinrpc_cl_init_params. The four arguments for bitcoinrpc_cl_init_params are username, password, IP address, and port. You should already know all of this information from your work with Curl. As you'll recall, the IP address 127.0.0.1 and port 18332 should be correct for the standard testnet setup described in these documents, while you can extract the user and password from ~/.bitcoin/bitcoin.conf.
$ cat bitcoin.conf
server=1
dbcache=1536
par=1
maxuploadtarget=137
maxconnections=16
rpcuser=StandUp
rpcpassword=6305f1b2dbb3bc5a16cd0f4aac7e1eba
rpcallowip=127.0.0.1
debug=tor
prune=550
testnet=1
[test]
rpcbind=127.0.0.1
rpcport=18332
[main]
rpcbind=127.0.0.1
rpcport=8332
[regtest]
rpcbind=127.0.0.1
rpcport=18443
Which you then place in the bitcoinrpc_cl_init_params:
bitcoinrpc_cl_t *rpc_client;
rpc_client = bitcoinrpc_cl_init_params("StandUp", "6305f1b2dbb3bc5a16cd0f4aac7e1eba", "127.0.0.1", 18332);
MAINNET VS TESTNET: The port would be 8332 for a mainnet setup.
If rpc_client is successfully initialized, you'll be able to send off RPC commands.
Later, when you're all done with your bitcoind connection, you should close it:
bitcoinrpc_global_cleanup();
Test code can be found at 16_1_testbitcoin.c in the src directory. Download it to your testnet machine, then insert the correct RPC password (and change the RPC user if you didn't create your server with StandUp).
You can compile and run this as follows:
$ cc testbitcoin.c -lbitcoinrpc -ljansson -o testbitcoin
$ ./testbitcoin
Successfully connected to server!
⚠️ WARNING: If you forget to enter your RPC password in this or any other code samples that depend on RPC, you will receive a mysteriousERROR CODE 5.
In order to use an RPC method using libbitcoinrpc, 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.
bitcoinrpc_method_t *getmininginfo = NULL;
getmininginfo = bitcoinrpc_method_init(BITCOINRPC_METHOD_GETMININGINFO);
Usually you would set parameters next, but getmininginfo requires no parameters, so you can skip that for now.
You must also create two other objects, a "response object" and an "error object". They can be initialized as follows:
bitcoinrpc_resp_t *btcresponse = NULL;
btcresponse = bitcoinrpc_resp_init();
bitcoinrpc_err_t btcerror;
You use the rpc_client variable that you already learned about in the previous test, and add on your getmininginfo method and the two other objects:
bitcoinrpc_call(rpc_client, getmininginfo, btcresponse, &btcerror);
You'll want to know what the RPC call returned. To do so, retrieve the output of your call as a JSON object with bitcoinrpc_resp_get and save it into a standard jansson object, of type json_t:
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, also from the jansson library:
printf ("%s\n", json_dumps(j, JSON_INDENT(2)));
However, since you're now writing complete programs, you probably want to do more subtle work, such as pulling out individual JSON values for specific usage. The jansson Reference details how to do so.
Just as when you were using Curl, you'll find that RPC returns a JSON object containing an id, an error, and most importantly a JSON object of the result.
The json_object_get function will let you retrieve a value (such as the result) from a JSON object by key:
json_t *jsonresult = NULL;
jsonresult = json_object_get(jsonresponse,"result");
printf ("%s\n", json_dumps (jsonresult, JSON_INDENT(2)));
However, you probably want to drill down further, to get a specific variable. Once you've retrieved the appropriate value, you will need to convert it to a standard C object by using the appropriate json_*_value function. For example, accessing an integer uses json_integer_value:
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 withjanssonobjects if you get confused with what type of object you're retrieving. Make careful use ofbitcoin-cli helpto know what you should expect, and if you experience a segmentation fault, first look at your JSON retrieval functions.
Retrieve the test code from the src directory.
$ cc getmininginfo.c -lbitcoinrpc -ljansson -o getmininginfo
$ ./getmininginfo
Full Response: {
"result": {
"blocks": 1804406,
"difficulty": 4194304,
"networkhashps": 54842097951591.781,
"pooledtx": 127,
"chain": "test",
"warnings": "Warning: unknown new rules activated (versionbit 28)"
},
"error": null,
"id": "474ccddd-ef8c-4e3f-93f7-fde72fc08154"
}
Just the Result: {
"blocks": 1804406,
"difficulty": 4194304,
"networkhashps": 54842097951591.781,
"pooledtx": 127,
"chain": "test",
"warnings": "Warning: unknown new rules activated (versionbit 28)"
}
Block Count: 1804406
But what if your RPC call did have arguments?
To send parameters to your RPC call using libbitcoinrpc you have to wrap them in a JSON array. Since an array is just a simple listing of values, all you have to do is encode the parameters as ordered elements in the array.
Create the JSON array using the json_array function from jansson:
json_t *params = NULL;
params = json_array();
You'll then reverse the procedure that you followed to access JSON values: you'll convert C-typed objects to JSON-typed objects using the json_* functions. Afterward, you'll append 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 simple json_array_append_new methodology will serve for the majority of RPC commands with parameters, but some RPC commands require more complex inputs. In these cases you may need to create subsidiary JSON objects or JSON arrays, which you will then append to the parameters array as usual. The next section contains an example of doing so using createrawtransaction, which contains a JSON array of JSON objects for the inputs, a JSON object for the outputs, and the locktime parameter.
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.
By linking to the bitcoinrpc RPC and jansson JSON 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. The next section will demonstrate how this can be used for a pragmatic, real-world program.
- 🔥 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.
Learn more about "Talking to Bitcoind with C" in 16.2: Programming Bitcoind in C with RPC Libraries.