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.
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: