bson_writer_begin man page
bson_writer_begin — bson_writer_begin()
bool bson_writer_begin (bson_writer_t *writer, bson_t **bson);
writer: A bson_writer_t.
bson: A bson_t.
Begins writing a new document. The caller may use the bson structure to write out a new BSON document. When completed, the caller must call either bson_writer_end() or bson_writer_rollback().
true if there was space in the underlying buffers to begin the document.
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