Hi Marcio,
I'm well versed in the issues you are hitting upon.
Are you using C or C++?
The solution is similar in C and C++, but C++ allows encapsulation so the implementation (from an outside perspective) may seem a little cleaner.
In either case, I recommend saving your "unsigned int a : 1" into it's very own byte. Save "unsigned int b : 26" into 4 bytes (in network order in your serialized persistent store canonical file). And save your "unsigned int c : 3" in its very own byte.
What you'll be writing are two routines: a serializing "write" (memory to store) and marshalling "read" (store to memory). In C they'll be two standalone routine named "writeStructA(FILE* out, struct A* x);" and "readStructA(FILE* in, struct A* x);". For C++, you'll add two member methods, "write(std::ostream&)" and "read(std::istream&)".
Once you let me know the C or C++ question, I can provide a more complete short example using your struct A.
--Eljay
