The padding issue is simple to avoid. Simply do not write/read structs. Instead, write/read each member, recursively when a nested struct is encountered.

Since you're using fixed-width numeric types, implementation-defined size variance is not an issue.

However, you still have to worry about endianness if you're reading/writing multibyte types across different hardware platforms (even when they're running the same operating system). In this case, all members of your struct are simple numeric types (flattening the nested struct), so you may be able to write/read them using the standard ntoh*/hton* libc functions if the c libraries of all relevant platforms provide a 64-bit extension. If no 64-bit version of those functions is available, you could play games with casting, pointers, bitmasks, bitshifting, etc ... to massage the 64-bit types into network byte order. Or, perhaps you could avoid all of the above and use an implementation of XDR (or something similar).

In case you you haven't used them, the keywords 'serialization' and 'marshalling' should be helpful in searching/googling for further information on this topic.

Regards,
Alister