JSON to BSON converter

Now that we have a basic grasp of the library, we'll write a solution to a real life problem: converting JSON to BSON.

Our program will expect correctly formatted JSON, in condensed one-line format, and will output a BSON document for each line of JSON received.

#define __STRICT_ANSI__ 1

#include <bson.h>
#include <json.h>

#include <stdio.h>
#include <unistd.h>
#include <glib.h>

First, we forward declare the json_to_bson() function, because we'll recursively use it later on:

static bson *json_to_bson (struct json_object *json);

Next, we create the heart of the program, a function that takes a BSON object, a value and a key, and appends the key-value pair to the bson object, with the correct type.

static void
json_key_to_bson_key (bson *b, void *val,
                      const gchar *key)
{

We do this by checking the JSON object's type, and acting up on it:

  switch (json_object_get_type (val))
    {

The boolean, double, integer and string types are easy: we just use the appropriate bson_append_*() function:

    case json_type_boolean:
      bson_append_boolean (b, key, json_object_get_boolean (val));
      break;
    case json_type_double:
      bson_append_double (b, key, json_object_get_double (val));
      break;
    case json_type_int:
      bson_append_int32 (b, key, json_object_get_int (val));
      break;
    case json_type_string:
      bson_append_string (b, key, json_object_get_string (val), -1);
      break;

Converting a JSON object to BSON is a bit more complicated, yet, straightforward nevertheless:

    case json_type_object:
      {
        bson *sub;

        sub = json_to_bson (val);
        bson_append_document (b, key, sub);
        bson_free (sub);
        break;
      }

This is one of the reasons we needed to forward-declare json_to_bson(): we're using it to turn the JSON value into BSON, and append it as a subdocument.

Next up: arrays! This is even trickier than sub-documents, as we need to iterate over the elements, and append each individually. But, trickier as it may be, it's still straightforward;

    case json_type_array:
      {
        gint pos;
        bson *sub;

        sub = bson_new ();

        for (pos = 0; pos < json_object_array_length (val); pos++)
          {
            gchar *nk = g_strdup_printf ("%d", pos);

            json_key_to_bson_key (sub, json_object_array_get_idx (val, pos),
                                  nk);
            g_free (nk);
          }
        bson_finish (sub);

        bson_append_array (b, key, sub);
        bson_free (sub);
        break;
      }

Anything else, we ignore:

    default:
      break;

    }

}

And to bind this together with JSON-C's API, we need two more functions. The first one will simply iterate over a JSON object, and call the function we wrote above:

static void
json_to_bson_foreach (bson *b, struct json_object *json)
{
  json_object_object_foreach (json, key, val)
    {
      json_key_to_bson_key (b, val, key);
    }
}

The next one is another wrapper around this former: it creates a BSON document, calls the foreach method, then finishes the BSON object and we're done:

static bson *
json_to_bson (struct json_object *json)
{
  bson *b;

  b = bson_new ();
  json_to_bson_foreach (b, json);
  bson_finish (b);

  return b;
}

We're almost done! All that is left is writing our program's entry point: something that will read the input, turn it into BSON, and write it out:

int
main (int argc, char **argv)
{
  GIOChannel *input;
  GString *json_str;
  GError *error = NULL;
  struct json_tokener *tokener;

We do some setting up, creating a new IO channel, and a JSON tokenizer:

  input = g_io_channel_unix_new (0);

  json_str = g_string_new (NULL);
  tokener = json_tokener_new ();

Then, until we have something to read...

  while (g_io_channel_read_line_string (input, json_str,
                                        NULL, &error) == G_IO_STATUS_NORMAL)
    {
      struct json_object *json;
      bson *bson;

We reset the tokenizer before parsing another line, then parse the JSON we received:

      json_tokener_reset (tokener);

      json = json_tokener_parse_ex (tokener, json_str->str, json_str->len);
      if (!json)
        {
          fprintf (stderr, "Error parsing json: %s\n", json_str->str);
          break;
        }

If we received something other than a JSON object, we can't turn that into BSON, so we write an error to STDERR, and skip this line:

      if (json_object_get_type (json) != json_type_object)
        {
          fprintf (stderr,
                   "Error: json's top-level object is not object: %s\n",
                   json_str->str);
          json_object_put (json);
          break;
        }

Otherwise, we turn it into BSON, and write it to STDOUT:

      bson = json_to_bson (json);
      json_object_put (json);

      write (1, bson_data (bson), bson_size (bson));

      bson_free (bson);

    }

And that was our program, a very simple application that turns each line of JSON into BSON.

  return 0;
}