When using ossEncode() with a pre-allocated buffer, it seems that which way the buffer fills vary depending on the encoding rules (PER or BER) that I use. Is that true, and what determines where the buffer fills from?

By default PER and BER indefinite-length encodings fill from the top. DER and BER definite-length encodings by default fill from the bottom.

You can force the filling to always be from the top by specifying the ossEncode() flag FRONT_ALIGN, and you can force filling to always be from the bottom by specifying BACK_ALIGN. These flags will force a movement of the encoded data if the position that it is in is not the one you stipulated.

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