Protobuf's variable-length lengths are indeed annoying, but can't you encode back-to-front as in ASN.1? That lets you write your lengths just after encoding the

Protobuf's variable-length lengths are indeed annoying, but can't you encode back-to-front as in ASN.1? That lets you write your lengths just after encoding the value, which should work (but may leave your data in e.g. bytes 20+ of a buffer; depending on interfaces, you may need to do one final copy to move the data to the start of the buffer.) (ASN.1 and protobuf are both tag-length-value formats with variable-length integer encodings... h/t to tptacek for mentioning that particular trick.)