Mongo C Driver accessing a document by its _id

How can I access a document in a Mongo database using only its _id, with mongo-c-driver? I want to define a function

void *get_doc_by_id(int obj_id) {
    // return document if exists
} 

First off, keep in mind that an int is not guaranteed to be large enough to hold a MongoDB object id (and in all cases I know of, is not). MongoDB object ids are 96-bit in size and int is typically 32-bits in size.

You might try something like:

/**
 * get_doc_by_id:
 * @conn: A mongo connection.
 * @db_and_collection: The "db.collection" namespace.
 * @oid: A bson_oid_t containing the document id.
 *
 * Fetches a document by the _id field.
 *
 * Returns: A new bson* that should be freed with bson_destroy() if
 *    successful; otherwise NULL on failure.
 */
bson *
get_doc_by_id (mongo            *conn,
               const char       *db_and_collection,
               const bson_oid_t *oid)
{
    bson query;
    bson *ret = NULL;
    mongo_cursor *cursor;

    assert(conn);
    assert(db_and_collection);
    assert(oid);

    bson_init(&query);
    bson_append_oid(&query, "_id", oid);

    cursor = mongo_find(conn, db_and_collection, &query, NULL, 1, 0, 0);
    if (!cursor) {
        goto cleanup;
    }

    if (mongoc_cursor_next(cursor) != MONGO_OK) {
        goto cleanup;
    }

    ret = calloc(1, sizeof *ret);
    if (!ret) {
        goto cleanup;
    }

    bson_copy(ret, mongoc_cursor_bson(cursor));

cleanup:
    if (cursor) {
        mongo_cursor_destroy(cursor);
    }
    bson_destroy(&query);

    return ret;
}