What is marshalling? What is happening when something is "marshalled?"
I know this question has been asked, at least here.
But there wasn't a satisfactory answer, at least not to me. There is a lot of talk about marshalling as regards interoperating with unmanaged code, but what about marshalling from one thread to another, as we have to do in .NET sometimes.
This makes me ask, what is marshalling, really? When you give a definition of marshalling, how would you define it so that it is explaining the case of interoperability, as well as the cases where you are "marshalling" between threads?
Answer
Computations often need to move data from one site to another, and don't have any shared memory. So one computation sends a message containing the data to the other.
How should that data, if it is arbitrarily complicated, be sent in a message?
Marshalling is the process of converting a data field, or an entire set of related structures, into a serialized string that can be sent in a message. To marshall a binary number, one might convert it to hexadecimal digit string, if the message format must be text. If the message will carry binary data, the binary number might be converted into 4 little-endian normalized binary bytes and sent that way. Pointers are harder; one often has to convert them into an abstract reference (e.g., a "node number") that is independent of the actual memory locations.
Of course, if you "marshall" data, you must eventually "unmarshall", which is the process of reading the serial stream and reconstructing the transmitted data (structure).
Often there are (un)marshalling routines in a library that are used to accomplish this purpose, and sometimes there are even tools that will manufacture all the calls needed on the (un)marshalling routines to send/recieve the data.