C++ struct alignment question

Question

I have a predefined struct (actually several) where variables span across 32-bit word boundary. In Linux (and Windows using GCC) I am able to get my structs to pack to the correct size using 'attribute((packed))'. However I cannot get it to work the same way using VC++ and #pragma pack.

Using GCC this returns a correct size of 6 bytes:

struct
{
    unsigned int   a    : 3;
    unsigned int   b    : 1;
    unsigned int   c    : 15;
    unsigned int   troubleMaker  : 16;
    unsigned short padding   : 13;
} __attribute__((packed)) s;

Using VC++ this returns an incorrect size of 8 bytes

#pragma pack(push)
#pragma pack(1)

struct
{
    unsigned int   a    : 3;
    unsigned int   b    : 1;
    unsigned int   c    : 15;
    unsigned int   troubleMaker  : 16;
    unsigned short padding   : 13;
} s;

#pragma pack(pop)

I can get things to work by splitting 'troubleMaker' across the boundary manually but I'd prefer not to. Any ideas?

Answer 1

I believe VC++ doesn't support this, and I have grave doubts whether GCC's behaviour in this respect is actually standard.

Answer 2

I don't believe this behavior is supported in Visual Studio. In addiction to the pack macro I tried using __declspec(align(1)) and got the same behavior. I think you are stuck with 12 bytes or re-ordering your structure a bit.

Answer 3

Crazy idea: just write a C99 or C++03 -conforming program in the first place

---

I would suggest not using vendor-specific C language extensions to match device or network bit formats. Even if you get the fields to line up using a series of one-per-vendor language extensions, you still have byte order to worry about, and you still have a struct layout that requires extra instructions to access.

You can write a C99 conforming program that will work on any architecture or host and at maximum speed and cache efficiency by using the standardized C API string and memory copy functions and the Posix hton and ntoh functions.

A good practice is to use the following functions for which there exist published standards:

C99: memcpy(), Posix: htonl(), htons(), ntohl(), ntohs()

Update: here is some code that should work the same everywhere. You may need to get <stdint.h> from this project if Microsoft still hasn't implemented it for C99, or just make the usual assumptions about int sizes.

#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <arpa/inet.h>

struct packed_with_bit_fields {  // ONLY FOR COMPARISON
    unsigned int   a     : 3;
    unsigned int   b     : 1;
    unsigned int   c     : 15;
    unsigned int   troubleMaker : 16;
    unsigned short padding  : 13;
} __attribute__((packed));       // USED ONLY TO COMPARE IMPLEMENTATIONS

struct unpacked { // THIS IS THE EXAMPLE STRUCT
    uint32_t a;
    uint32_t b;
    uint32_t c;
    uint32_t troubleMaker;
}; // NOTE NOT PACKED

struct unpacked su;
struct packed_with_bit_fields sp;
char *bits = "Lorem ipsum dolor";

int main(int ac, char **av) {
  uint32_t x;   // byte order issues ignored in both cases

  // This should work with any environment and compiler
  memcpy(&x, bits, 4);
  su.a = x & 7;
  su.b = x >> 3 & 1;
  su.c = x >> 4 & 0x7fff;
  memcpy(&x, bits + 2, 4);
  su.troubleMaker = x >> 3 & 0xffff;

  // This section works only with gcc
  memcpy(&sp, bits, 6);
  printf( sp.a == su.a
      &&  sp.b == su.b
      &&  sp.c == su.c
      &&  sp.troubleMaker == su.troubleMaker
      ? "conforming and gcc implementations match\n" : "huh?\n");
  return 0;
}

Answer 4

If it absoloutely defnitely needs to be 6 bytes then define it as 3 shorts and get the data out yourself ... it won't slow things down ... the compiler is just doing this anyway ...

Answer 5

Alignment and ordering of bitfields are notoriously implementation-specific. It is much safer to declare a normal integer field and manipulate the "bitfields" within using masks and bitwise (| & ^) operators .

Answer 6

Shorter example with only conforming code

---

struct unpacked {  // apparently my other example was too long and confusing
    uint32_t a;    // ...here is a much shorter example with only the conforming
    uint32_t b;    // ...code. (The other program had the gcc-specific declaration,
    uint32_t c;    // but only for test code. Still, it was a bit long.)
    uint32_t troubleMaker;
};

struct unpacked su;
char *bits = "Lorem ipsum dolor";

void f(void) {
  uint32_t x;

  memcpy(&x, bits, 4);
  su.a = x & 7;
  su.b = x >> 3 & 1;
  su.c = x >> 4 & 0x7fff;
  memcpy(&x, bits + 2, 4);
  su.troubleMaker = x >> 3 & 0xffff;
  return 0;
}