BSON (i.e., binary structured object notation) is the binary format used by MongoDB to store data and express queries and commands. To work with MongoDB is to trade in BSON objects. This document describes how to create, read, and destroy BSON objects using the MongoDB C Driver.


A brief note on libraries.

When you compile the driver, the BSON library is included in the driver. This means that when you include mongo.h, you have access to all the functions declared in bson.h.

If you want to use BSON independently, you don’t need libmongoc: when you compile the driver, you’ll also get shared and static libraries for libbson. You can link to this library and simple require bson.h.

Using BSON objects

The pattern of BSON object usage is pretty simple. Here are the steps:

  1. Initiate a new BSON object.
  2. Construct the object using the bson_append_* methods.
  3. Pass the object to bson_finish() to finalize it. The object is now ready to use.
  4. When you’re done with it, pass the object to bson_destroy() to free up any allocated memory.

To demonstrate, let’s create a BSON object corresponding to the simple JSON object {count: 1001}.

bson b[1];

bson_init( b );
bson_append_int( b, "count", 1001 );
bson_finish( b );

/* BSON object now ready for use */

bson_destroy( b );

That’s all there is to creating a basic object.

Creating complex BSON objects

BSON objects can contain arrays as well as sub-objects. Here we’ll see how to create these by building the bson object corresponding to the following JSON object:

  name: "Kyle",

  colors: [ "red", "blue", "green" ],

  address: {
    city: "New York",
    zip: "10011-4567"
bson b[1];

bson_init( b );
bson_append_string( b, "name", "Kyle" );

bson_append_start_array( b, "colors" );
  bson_append_string( b, "0", "red" );
  bson_append_string( b, "1", "blue" );
  bson_append_string( b, "2", "green" );
bson_append_finish_array( b );

bson_append_start_object( b, "address" );
  bson_append_string( b, "city", "New York" );
  bson_append_string( b, "zip", "10011-4567" );
bson_append_finish_object( b );

if( bson_finish( b ) != BSON_OK )
    printf(" Error. ");

Notice that for the array, we have to manually set the index values from “0” to n, where n is the number of elements in the array.

You’ll notice that some knowledge of the BSON specification and of the available types is necessary. For that, take a few minutes to consult the official BSON specification.

Error handling

The names of BSON object values, as well as all strings, must be encoded as valid UTF-8. The BSON library will automatically check the encoding of strings as you create BSON objects, and if the objects are invalid, you’ll be able to check for this condition. All of the bson_append_* methods will return either BSON_OK for BSON_ERROR. You can check in your code for the BSON_ERROR condition and then see the exact nature of the error by examining the bson->err field. This bitfield can contain any of the following values:


The most important of these is BSON_NOT_UTF8 because the BSON objects cannot be used with MongoDB if they’re not valid UTF8.

To keep your code clean, you may want to check for BSON_OK only when calling bson_finish(). If the object is not valid, it will not be finished, so it’s quite important to check the return code here.

Reading BSON objects

You can read through a BSON object using a bson_iterator. For a complete example, you may want to read through the implementation of bson_print_raw() (in bson.h). But the basic idea is to initialize a bson_iterator object and then iterate over each successive element using bson_iterator_next(). Let’s take an example. Suppose we have a finished object of type bson* called b:

bson_iterator i[1];
bson_type type;
const char * key;

bson_iterator_init( i, b );

type = bson_iterator_next( i );
key = bson_iterator_key( i );

printf( "Type: %d, Key: %s\n", type, key );

We’ve advanced to the first element in the object, and we can print both it’s BSON numeric type and its key name. To print the value, we need to use the type to find the correct method for reading the value. For instance, if the element is a string, then we use bson_iterator_string to return the result:

printf( "Value: %s\n", bson_iterator_string( i ) );

In addition to iterating over each successive BSON element, we can use the bson_find() function to jump directly to an element by name. Again, suppose that b is a pointer to a bson object. If we want to jump to the element named “address”, we use bson_find() like so:

bson_iterator i[1], sub[1];
bson_type type;

type = bson_find( i, b, "address" );

This will initialize the iterator, i, and position it at the element named “address”. The return value will be the “address” element’s type.

Reading sub-objects and arrays

Since “address” is a sub-object, we need to specially iterate it. To do that, we get the raw value and initialize a new BSON iterator like so:

type = bson_find( i, b, "address" );

bson_iterator_subiterator( i, sub );

The function bson_iterator_subiterator initializes the iterator sub and points it to the beginning of the sub-object. From there, we can iterate over sub until we reach BSON_EOO, indicating the end of the sub-object.

If you want to work with a sub-object by itself, there’s a function, bson_iterator_subobject, for initializing a new bson object with the value of the sub-object. Note that this does not copy the object. If you want a copy of the object, use bson_copy().

bson copy[1];

bson_copy( copy, sub );

Getting a Raw BSON Pointer

Sometimes you’ll want to access the char * that points to the buffer storing the raw BSON object. For that, use the bson_data() function. You can use this in concert with the bson_iterator_from_buffer() function to initialize an iterator:

bson_iterator i[1];

bson_iterator_from_buffer( i, bson_data( b ) );