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Asked: 2026-05-23T18:15:34+00:00

I was wondering if there is a GCC C Compiler directive that allows me

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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.

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1 Answer

  1. Editorial Team

    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

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