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DependentExtensions/jansson-2.4/doc/tutorial.rst
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DependentExtensions/jansson-2.4/doc/tutorial.rst
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.. _tutorial:
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********
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Tutorial
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********
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.. highlight:: c
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In this tutorial, we create a program that fetches the latest commits
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of a repository in GitHub_ over the web. `GitHub API`_ uses JSON, so
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the result can be parsed using Jansson.
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To stick to the the scope of this tutorial, we will only cover the the
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parts of the program related to handling JSON data. For the best user
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experience, the full source code is available:
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:download:`github_commits.c`. To compile it (on Unix-like systems with
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gcc), use the following command::
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gcc -o github_commits github_commits.c -ljansson -lcurl
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libcurl_ is used to communicate over the web, so it is required to
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compile the program.
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The command line syntax is::
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github_commits USER REPOSITORY
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``USER`` is a GitHub user ID and ``REPOSITORY`` is the repository
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name. Please note that the GitHub API is rate limited, so if you run
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the program too many times within a short period of time, the sever
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starts to respond with an error.
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.. _GitHub: https://github.com/
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.. _GitHub API: http://developer.github.com/
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.. _libcurl: http://curl.haxx.se/
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.. _tutorial-github-commits-api:
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The GitHub Repo Commits API
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===========================
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The `GitHub Repo Commits API`_ is used by sending HTTP requests to
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URLs like ``https://api.github.com/repos/USER/REPOSITORY/commits``,
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where ``USER`` and ``REPOSITORY`` are the GitHub user ID and the name
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of the repository whose commits are to be listed, respectively.
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GitHub responds with a JSON array of the following form:
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.. code-block:: none
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[
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{
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"sha": "<the commit ID>",
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"commit": {
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"message": "<the commit message>",
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<more fields, not important to this tutorial...>
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},
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<more fields...>
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},
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{
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"sha": "<the commit ID>",
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"commit": {
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"message": "<the commit message>",
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<more fields...>
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},
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<more fields...>
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},
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<more commits...>
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]
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In our program, the HTTP request is sent using the following
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function::
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static char *request(const char *url);
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It takes the URL as a parameter, preforms a HTTP GET request, and
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returns a newly allocated string that contains the response body. If
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the request fails, an error message is printed to stderr and the
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return value is *NULL*. For full details, refer to :download:`the code
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<github_commits.c>`, as the actual implementation is not important
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here.
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.. _GitHub Repo Commits API: http://developer.github.com/v3/repos/commits/
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.. _tutorial-the-program:
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The Program
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===========
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First the includes::
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#include <string.h>
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#include <jansson.h>
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Like all the programs using Jansson, we need to include
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:file:`jansson.h`.
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The following definitions are used to build the GitHub API request
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URL::
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#define URL_FORMAT "https://api.github.com/repos/%s/%s/commits"
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#define URL_SIZE 256
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The following function is used when formatting the result to find the
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first newline in the commit message::
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/* Return the offset of the first newline in text or the length of
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text if there's no newline */
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static int newline_offset(const char *text)
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{
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const char *newline = strchr(text, '\n');
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if(!newline)
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return strlen(text);
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else
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return (int)(newline - text);
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}
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The main function follows. In the beginning, we first declare a bunch
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of variables and check the command line parameters::
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int main(int argc, char *argv[])
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{
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size_t i;
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char *text;
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char url[URL_SIZE];
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json_t *root;
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json_error_t error;
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if(argc != 3)
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{
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fprintf(stderr, "usage: %s USER REPOSITORY\n\n", argv[0]);
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fprintf(stderr, "List commits at USER's REPOSITORY.\n\n");
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return 2;
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}
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Then we build the request URL using the user and repository names
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given as command line parameters::
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snprintf(url, URL_SIZE, URL_FORMAT, argv[1], argv[2]);
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This uses the ``URL_SIZE`` and ``URL_FORMAT`` constants defined above.
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Now we're ready to actually request the JSON data over the web::
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text = request(url);
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if(!text)
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return 1;
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If an error occurs, our function ``request`` prints the error and
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returns *NULL*, so it's enough to just return 1 from the main
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function.
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Next we'll call :func:`json_loads()` to decode the JSON text we got
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as a response::
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root = json_loads(text, 0, &error);
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free(text);
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if(!root)
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{
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fprintf(stderr, "error: on line %d: %s\n", error.line, error.text);
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return 1;
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}
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We don't need the JSON text anymore, so we can free the ``text``
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variable right after decoding it. If :func:`json_loads()` fails, it
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returns *NULL* and sets error information to the :type:`json_error_t`
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structure given as the second parameter. In this case, our program
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prints the error information out and returns 1 from the main function.
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Now we're ready to extract the data out of the decoded JSON response.
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The structure of the response JSON was explained in section
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:ref:`tutorial-github-commits-api`.
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We check that the returned value really is an array::
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if(!json_is_array(root))
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{
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fprintf(stderr, "error: root is not an array\n");
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return 1;
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}
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Then we proceed to loop over all the commits in the array::
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for(i = 0; i < json_array_size(root); i++)
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{
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json_t *data, *sha, *commit, *message;
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const char *message_text;
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data = json_array_get(root, i);
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if(!json_is_object(data))
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{
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fprintf(stderr, "error: commit data %d is not an object\n", i + 1);
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return 1;
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}
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...
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The function :func:`json_array_size()` returns the size of a JSON
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array. First, we again declare some variables and then extract the
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i'th element of the ``root`` array using :func:`json_array_get()`.
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We also check that the resulting value is a JSON object.
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Next we'll extract the commit ID (a hexadecimal SHA-1 sum),
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intermediate commit info object, and the commit message from that
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object. We also do proper type checks::
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sha = json_object_get(data, "sha");
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if(!json_is_string(sha))
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{
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fprintf(stderr, "error: commit %d: sha is not a string\n", i + 1);
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return 1;
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}
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commit = json_object_get(data, "commit");
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if(!json_is_object(commit))
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{
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fprintf(stderr, "error: commit %d: commit is not an object\n", i + 1);
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return 1;
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}
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message = json_object_get(commit, "message");
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if(!json_is_string(message))
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{
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fprintf(stderr, "error: commit %d: message is not a string\n", i + 1);
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return 1;
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}
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...
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And finally, we'll print the first 8 characters of the commit ID and
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the first line of the commit message. A C-style string is extracted
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from a JSON string using :func:`json_string_value()`::
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message_text = json_string_value(message);
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printf("%.8s %.*s\n",
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json_string_value(id),
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newline_offset(message_text),
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message_text);
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}
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After sending the HTTP request, we decoded the JSON text using
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:func:`json_loads()`, remember? It returns a *new reference* to the
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JSON value it decodes. When we're finished with the value, we'll need
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to decrease the reference count using :func:`json_decref()`. This way
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Jansson can release the resources::
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json_decref(root);
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return 0;
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For a detailed explanation of reference counting in Jansson, see
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:ref:`apiref-reference-count` in :ref:`apiref`.
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The program's ready, let's test it and view the latest commits in
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Jansson's repository::
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$ ./github_commits akheron jansson
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1581f26a Merge branch '2.3'
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aabfd493 load: Change buffer_pos to be a size_t
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bd72efbd load: Avoid unexpected behaviour in macro expansion
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e8fd3e30 Document and tweak json_load_callback()
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873eddaf Merge pull request #60 from rogerz/contrib
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bd2c0c73 Ignore the binary test_load_callback
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17a51a4b Merge branch '2.3'
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09c39adc Add json_load_callback to the list of exported symbols
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cbb80baf Merge pull request #57 from rogerz/contrib
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040bd7b0 Add json_load_callback()
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2637faa4 Make test stripping locale independent
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<...>
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Conclusion
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==========
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In this tutorial, we implemented a program that fetches the latest
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commits of a GitHub repository using the GitHub Repo Commits API.
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Jansson was used to decode the JSON response and to extract the commit
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data.
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This tutorial only covered a small part of Jansson. For example, we
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did not create or manipulate JSON values at all. Proceed to
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:ref:`apiref` to explore all features of Jansson.
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