“Fast Copy” is a special run-time flag that can be set for the decoder that can substantially reduce the number of copy operations that need to be done to decode a message. The copy operations are reduced by taking advantage of the fact that the data contents of some ASN.1 types already exist in decoded form in the message buffer. Therefore, there is no need to allocate memory for the data and then copy the data from the buffer into the allocated memory structure.

As an example of what fast copy does, consider a simple ASN.1 SEQUENCE consisting of an element a, an INTEGER and b, an OCTET STRING:

   Simple ::= SEQUENCE {
      a INTEGER,
      b OCTET STRING
   }

Assume an encoded value of this type contains a value of a = 123 (hex 7B) and b contains the hex octets 0x01 0x02 0x03. The generated variable for the OCTET STRING will contain a data pointer. So rather than allocate memory for this string and copy the data to it, fast copy will simply store a pointer directly to the data in the buffer:

The pointer stored in the data structure points directly at data in the message buffer. No memory allocation or copy is done.

The user must keep in mind that if this technique is used, the message buffer containing the decoded message must be available as long as the type variable containing the decoded data is in use. This will not work in a producer-consumer threading model where one thread is decoding messages and the next thread is processing the contents. The producer thread will overwrite the buffer contents and therefore data referenced in the decoded message type variable that the consumer is processing.

This will also not work if the message buffer is an automatic variable in a function and the decoded result type is being passed out. The result type variable will point at data in the buffer variable that has gone out of scope.

To set fast copy, the rtSetFastCopy function must be invoked with the initialized context variable that will be used to decode a message. This should be done once prior to entering the loop that will be used to decode a stream of messages.