bool bson_append_array_begin (bson_t *bson, const char *key, int key_length, bson_t *child);
The bson_append_array_begin() function shall begin appending an array field to bson. This allows for incrementally building a sub-array. Doing so will generally yield better performance as you will serialize to a single buffer. When done building the sub-array, the caller MUST call bson_append_array_end().
true if the operation was applied successfully, otherwise false and bson should be discarded.