Pad a C++ structure to a power of two

问题:

I'm working on some C++ code for an embedded system. The I/O interface the code uses requires that the size of each message (in bytes) is a power of two. Right now, the code does something like this (in several places):

#pragma pack(1)
struct Message
{
   struct internal_
   {
      unsigned long member1;
      unsigned long member2;
      unsigned long member3;
      /* more members */
   } internal;
   char pad[64-sizeof(internal_)];
};
#pragma pack()

I'm trying to compile the code on a 64-bit Fedora for the first time, where long is 64-bits. In this case, sizeof(internal_) is greater than 64, the array size expression underflows, and the compiler complains that the array is too large.

Ideally, I'd like to be able to write a macro that will take the size of the structure and evaluate at compile time the required size of the padding array in order to round the size of the structure out to a power of two.

I've looked at the Bit Twiddling Hacks page, but I don't know if any of the techniques there can really be implemented in a macro to be evaluated at compile time.

Any other solutions to this problem? Or should I perpetuate the problem and just change the magical 64 to a magical 128?

回答1:

Use a template metaprogram. (Edited in response to comment).

#include <iostream>
#include <ostream>
using namespace std;

template <int N>
struct P
{
    enum { val = P<N/2>::val * 2 };
};

template <>
struct P<0>
{
    enum { val = 1 };
};

template <class T>
struct PadSize
{
    enum { val = P<sizeof (T) - 1>::val - sizeof (T) }; 
};

template <class T, int N>
struct PossiblyPadded
{
    T       payload;
    char    pad[N]; 
};

template <class T>
struct PossiblyPadded<T, 0>
{
    T       payload;
};

template <class T>
struct Holder : public PossiblyPadded<T, PadSize<T>::val>
{
};


int main()
{
    typedef char Arr[6];

    Holder<Arr> holder;
    cout << sizeof holder.payload << endl;

    // Next line fails to compile if sizeof (Arr) is a power of 2
    // but holder.payload always exists
    cout << sizeof holder.pad << endl;
}

回答2:

Probably the most obvious way would be to just use the ternary operator:

#define LOG2_CONST(n) ((n) <= 1 ? 0 :
                      ((n) <= 2 ? 1 :
                      ((n) <= 4 ? 2 :
                      /* ... */
                      ))))))))))))))))))))))))))))))
#define PADDED_STRUCT(ResultName, BaseName) \
  typedef union { BaseName data; char pad[1 << LOG2_CONST(sizeof(BaseName))]; } ResultName

回答3:

Why not use a union?

union Message
{
    struct internal_
    {
        unsigned long member1;
        /* more members */
    };
    char[64];
};

or better yet use anonymous structs

union Message
{
    struct
    {
        unsigned long member1;
        /* more members */
    };
    char[64];
};

So you can access members like this: Message.member1;

Edit: obviously this doesn't solve your greater than 64 problem, but provides a cleaner way of padding.

回答4:

One way of working around the problem would be to replace the hardcoded 64 with a multiple of size(long), turning the padding into something like this:

char pad[4*sizeof(unsigned long) - sizeof(internal_)];

It's ugly but it should be portable to 64-bit.

That said, an API that requires the message size to be a power of 2 sounds a bit odd and like a design issue. Requiring the size being an even number makes sense as on some processors to pay quite a penalty for accessing data on odd addresses but your #pragma pack almost makes that inevitable.

回答5:

How about just writing a small wrapper around the send and receive message function that handle any size message and they just allocate a larger buffer ( next power of 2 ) and memclear it, copy the struct to the beginning and send it along.

回答6:

You're already using #pragma pack, I don't know what compiler(s) your using specifically, but you should see if they support arguments for pack that control the alignment/padding and then you can just get rid of the padding field altogether. I know MSVC's version of pragma pack supports this, as does GCC's.

回答7:

You can macroize this as follows (for 32-bit architecture):

#define align_step(N, shift) ((N) | ((N) >> shift))
#define align_up(N) (align_step(align_step(align_step(align_step(align_step((N)-1, 1), 2), 4), 8), 16) + 1)
#define alignment_padding(N) (align_up((N)) - (N))

Then you can apply that using the union trick or some other means. In your example:

#pragma pack(1)
struct Message
{
   struct internal_
   {
      unsigned long member1;
      unsigned long member2;
      unsigned long member3;
      /* more members */
   } internal;
   char pad[alignment_padding(sizeof(internal_))];
};
#pragma pack()

回答8:

You can get a compile-time constant for the size of the structure rounded up to a power of two using templates :

template<int N, int C = 1>
struct Recurse
{
    enum {result = Recurse<N/2, C*2>::result};
};

template<int C>
struct Recurse<0, C>
{
    enum {result = C};
};

template<typename T>
struct Calc
{
    enum {size = Recurse<sizeof(Test)-1>::result};
};

struct Test
{
    int a;
    double b;
    double c;
};

int main()
{
    std::cout << sizeof(Test) << " -> " << Calc<Test>::size << std::endl;
    return 0;
}

The padding value should then be easy.

回答9:

union Message
{
    struct
    {
        unsigned long member1;
        unsigned long member2; //...
    };
    char pad[1 << 5]; //change 5 to whatever size you need...
};

Would be a little cleaner.

回答10:

I like Niki's answer, especially the part with anonymous structs.

One thing that answer didn't solve was the greater-than-64-bytes problem, but that can be solved by conditionally declaring a char[128] struct member if sizeof(long)==8 and declaring char[64] otherwise.

回答11:

And yet another template solution (robbing hugely from fizzer):

#include <iostream>
#include <ostream>
using namespace std;

template <int TSize, int PSize = 1, bool = false>
struct PadSize
{
  static const int val =
    ( PadSize <    TSize, (PSize*2), (TSize <= (PSize*2)  )    > :: val );
};

template < int TSize, int PSize>
struct PadSize <TSize, PSize, true>  // As soon as TSize is <= to PSize
{
  static const int val = PSize;
};

int main()
{
    typedef char Arr[8];
    char pad[ PadSize <sizeof(Arr)>::val  ];

    cout << sizeof pad << endl;
}

My approach is simply to keep doubling the padding size until it's at least as big as the type size.