sending C structures to a future version of your code?

Question

I'm working on a hot-upgrade feature and need to package up an array of structs to be stashed away for the new version to find them. I really want to avoid adding a conversion function for every possible version transition. Is this reasonable?
The most likely change to the struct is for more fields to be added to the structure in the future and if this happens then a default value for the new field will be available. I will also soon face the task of saving the array of structs into a configuration file, so extra credit for answers that can be applied to both hot-upgrade and configuration saving.
I don't have to worry about the hot-update mechanism I just give it a pointer and a size and it does fantastic magic :)

Answer 1

The most likely change to the struct is for more fields to be added to the structure in the future and if this happens then a default value for the new field will be available.

From version 1, always include sizeof(myStruct) as a field in the beginning of each struct. Then, when you need to add new fields, always do so in the end of each struct, never in the middle. Now when receiving (or reading from a file), first read the size field only, so that you know how many bytes will be coming after it. If the size is less than sizeof(myStruct) as determined by the receiver/reader, then you know that something is missing, and default values are needed.

Answer 2

I'd recommend using something like Google's protocol buffers, which automatically handle versioning. If you add new fields to your messages, it's very easy to handle.