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

Answer 1

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

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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 from this project if Microsoft still hasn't implemented it for C99, or just make the usual assumptions about int sizes.

#include 
#include 
#include 
#include 
#include 

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;
}