c - How to enforce the struct bit order with the GCC compiler?

Question

I was wondering if there is a GCC C Compiler directive that allows me to determine the bit order for packing of a structure? Something to the likes of:

#pragma bit_order left

The rationale for such a need is that I have the following structure:

struct {
       union {
             unsigned char BYTE;
             struct {
                 unsigned char B0: 1;
                 unsigned char B1: 1;
                 unsigned char B2: 1;
                 unsigned char B3: 1;
                 unsigned char B4: 4;
             }BIT;
       }ITEM;
} myStruct;

With this structure, I would like the compiler to pack it this way:

Bit order: | 7  6  5  4  3  2  1  0 |
Label:     |B0 B1 B2 B3 B4 B5 B6 B7 |

Rather than how GCC does it:

Bit order: | 7  6  5  4  3  2  1  0 |
Label:     |B7 B6 B5 B4 B3 B2 B1 B0 |

I am dealing with MCUs that have huge header files that have structures that compute bit offsets according to stipulated hardware addresses. I am hoping that there is a compiler directive in GCC C Compiler that does the bit order swap for me before I attempt the flip all the fields in the manufacturer supplied file.

Answer 1

Which version of GCC are you using and which platform? A pragma exists that may do the trick, but it doesn't work on x86 starting with GCC 4.

#pragma reverse_bitfields on

More details at:

http://groups.google.com/group/gnu.gcc.help/browse_thread/thread/747918655affa5c0?pli=1

If you don't mind rebuilding GCC, all the relevant build settings are here (search for bitfield):

http://gcc.gnu.org/onlinedocs/gccint/Storage-Layout.html

Some details about bitfields being bad:

C/C++: Force Bit Field Order and Alignment