If I provide the output buffer for the encoder, the value field of the ossbuf returned has a different address, why?

If you provide the output buffer for the encoder, then whether the encoder changes the value field of the instance of OssBuf depends on which encoding rules you use.

In PER, the value field is never changed.

In BER and in DER, it changes it by default (BACK_ALIGN & DEFINITE length case) to point to the beginning of the encoded PDU. So in general the provided buffer start address will not be the same as that returned in the value field. Only if you pass FRONT_ALIGN or INDEFINITE, will the value field be the same as the start address of the buffer provided.

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