Creating a BSON Document

The bson_t Structure

BSON documents are created using the bson_t structure. This structure encapsulates the necessary logic for encoding using the BSON Specification. At the core, bson_t is a buffer manager and set of encoding routines.

Let's start by creating a new BSON document on the stack. Whenever using libbson, make sure you #include <bson/bson.h>.

bson_t b;
bson_init (&b);

This creates an empty document. In JSON, this would be the same as {}.

We can now proceed to adding items to the BSON document. A variety of functions prefixed with bson_append_ can be used based on the type of field you want to append. Let's append a UTF-8 encoded string.

bson_append_utf8 (&b, "key", -1, "value", -1);

Notice the two -1 parameters. The first indicates that the length of key in bytes should be determined with strlen(). Alternatively, we could have passed the number 3. The same goes for the second -1, but for value.

Libbson provides macros to make this less tedious when using string literals. The following two appends are identical.

bson_append_utf8 (&b, "key", -1, "value", -1);
BSON_APPEND_UTF8 (&b, "key", "value");

Now let's take a look at an example that adds a few different field types to a BSON document.

bson_t b = BSON_INITIALIZER;
BSON_APPEND_INT32 (&b, "a", 1);
BSON_APPEND_UTF8 (&b, "hello", "world");
BSON_APPEND_BOOL (&b, "bool", true);

Notice that we omitted the call to bson_init. By specifying BSON_INITIALIZER we can remove the need to initialize the structure to a base state.

Sub-Documents and Sub-Arrays

To simplify the creation of sub-documents bson_append_document_begin can be used to build a sub-document using the parent's memory region as the destination buffer.

bson_t parent = BSON_INITIALIZER;
bson_t child;
bson_append_document_begin (&parent, "foo", 3, &child);
bson_append_int32 (&child, "baz", 3, 1);
bson_append_document_end (&parent, &child);
char *str = bson_as_relaxed_extended_json (&parent, NULL);
printf ("%s\n", str); // Prints: { "foo" : { "baz" : 1 } }
bson_free (str);
bson_destroy (&parent);

To simplify the creation of sub-arrays bson_array_builder_t can be used to build a sub-array using the parent's memory region as the destination buffer.

bson_t parent = BSON_INITIALIZER;
bson_array_builder_t *bab;
bson_append_array_builder_begin (&parent, "foo", 3, &bab);
bson_array_builder_append_int32 (bab, 9);
bson_array_builder_append_int32 (bab, 8);
bson_array_builder_append_int32 (bab, 7);
bson_append_array_builder_end (&parent, bab);
char *str = bson_as_relaxed_extended_json (&parent, NULL);
printf ("%s\n", str); // Prints: { "foo" : [ 9, 8, 7 ] }
bson_free (str);
bson_destroy (&parent);

Simplified BSON C Object Notation

Creating BSON documents by hand can be tedious and time consuming. BCON, or BSON C Object Notation, was added to allow for the creation of BSON documents in a format that looks closer to the destination format.

The following example shows the use of BCON. Notice that values for fields are wrapped in the BCON_* macros. These are required for the variadic processor to determine the parameter type.

bson_t *doc;
doc = BCON_NEW ("foo",
                "{",
                "int",
                BCON_INT32 (1),
                "array",
                "[",
                BCON_INT32 (100),
                "{",
                "sub",
                BCON_UTF8 ("value"),
                "}",
                "]",
                "}");

Creates the following document

{ "foo" : { "int" : 1, "array" : [ 100, { "sub" : "value" } ] } }