CODEWITHSUNDEEP
c++serialization
katang
katang

Reputation: 2774

Serializing/deserializing a bitfield structure in c++

I have a 

typedef struct {
    uint32_t  Thread: HTHREAD_BUS_WIDTH; 
    uint32_t Member: 3;                  
    uint32_t Proxy:3;  
// Other members, fill out 32 bits                 
} MyStruct;

that I must transfer from one system to another as an item of a buffer comprising 32-bit words.

What is the best way to serialize the struct, and on the other side, to deserialize it? "best" means here safe casting, and no unneeded copying. For one direction of casting, I have found (as member function)

int &ToInt() {
    return *reinterpret_cast<int *>(this);}

Is there similar valid casting in the other way round, i.e. from integer to MyStruct; the best would be as a member function?

How can I define which bit means which field? (It may even the case, that the deserialization happens in another program, in another language, in little/big endian systems?

Upvotes: 0

Views: 1354

Answers (2)

eerorika
eerorika

Reputation: 238381

How can I define which bit means which field?

You cannot. You have no control over the layout of bitfields.

"best" means here safe casting, and no unneeded copying.

There is no portable safe cast that could avoid copying.

A portable way to serialise bitfields is to manually shift into an integer, in the desired order. For example:

MyStruct value = something;
uint32_t out = 0;
out |= value.Thread;
out << HTHREAD_BUS_WIDTH;
out |= value.Member;
out << 3;
out |= value.Proxy;

In the shown example, the least significant bits contain the field Proxy while the other fields are adjacent in more significant bits.

Of course, in order to serialise this generated integer correctly, just like serialising any integer, you must take endianness into consideration. Serialisation of an integer can be portably implemented by repeatedly shifting the integer, and copying the bytes in order of significance into an array.

Upvotes: 1

Jean-Marc Volle
Jean-Marc Volle

Reputation: 3333

If you need to read from other system which might have different endianess you cannot rely on a portable bitfield. A solution is to "expanse" your structure so that each field is serialyzed as a 32 bit value in the "transport" buffer. A safe implementation could be something like:

typedef struct {
    uint32_t  Thread: HTHREAD_BUS_WIDTH; 
    uint32_t Member: 3;                  
    uint32_t Proxy:3;  
// Other members, fill out 32 bits                 
   std::vector<uint32_t > to_buffer() const;
} MyStruct;

Implementation of to_buffer():

std::vector<uint32_t > MyStruct::to_buffer() const
{
   std::vector<uint32_t> buffer;
   buffer.push_back((uint32_t )(Thread);
   buffer.push_back((uint32_t )(Member);
   buffer.push_back((uint32_t )(Proxy);
   // push other members

  return buffer;
}

then on the reception side you can do the "buffer" to struct.

If you do not want to expanse the fields that do not use 32 bits you can always implement you own packing function by shifting and masking bits eg:

uint32_t menber_and_procy = (Member << 3) | proxy; // and so one for other members.

It is much more error prone.

From my own experience, if communication bandwith is not an issue, relying on "text like" content is a better choice (no endianess issues and very easy to debug).

Upvotes: 0

Related Questions