* Bitfield packing
@ 2002-08-17 2:33 eddy
2002-08-17 11:58 ` Der Herr Hofrat
0 siblings, 1 reply; 2+ messages in thread
From: eddy @ 2002-08-17 2:33 UTC (permalink / raw)
To: gcc-help
Hi,
I am using gcc for a microcontroller application. I want to use
bitfields to describe the processor registers,
similar to this:
typedef union {
IO_BYTE byte;
struct {
IO_BYTE P30 :1;
IO_BYTE P31 :1;
IO_BYTE P32 :1;
IO_BYTE P33 :1;
IO_BYTE P34 :1;
IO_BYTE P35 :1;
IO_BYTE P36 :1;
IO_BYTE P37 :1;
} bit;
} PDR2;
How does gcc handle bitfields like thisone? Above I have defined 8 bits,
are they distributed on several bytes with some optimization levels?
How do I setup gcc, so that it will always assign the bits 1:1 (no
packing/optimization)?
Thanks
Eddy Ilg
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Bitfield packing
2002-08-17 2:33 Bitfield packing eddy
@ 2002-08-17 11:58 ` Der Herr Hofrat
0 siblings, 0 replies; 2+ messages in thread
From: Der Herr Hofrat @ 2002-08-17 11:58 UTC (permalink / raw)
To: eddy; +Cc: gcc-help
> Hi,
>
> I am using gcc for a microcontroller application. I want to use
> bitfields to describe the processor registers,
> similar to this:
>
> typedef union {
> IO_BYTE byte;
> struct {
> IO_BYTE P30 :1;
> IO_BYTE P31 :1;
> IO_BYTE P32 :1;
> IO_BYTE P33 :1;
> IO_BYTE P34 :1;
> IO_BYTE P35 :1;
> IO_BYTE P36 :1;
> IO_BYTE P37 :1;
> } bit;
> } PDR2;
>
> How does gcc handle bitfields like thisone? Above I have defined 8 bits,
> are they distributed on several bytes with some optimization levels?
if you don't use any __attribute__((packed)) on the bitfields the size of
this object will be alligned machin dependant (probably 32bit on a 32bit box)
you can see that by using the __alignof__() (just like sizeof) function to
report allignement - internal ordering I guess is nothing you should ever
relie on. See the section "storage layout" in the gcc manual for infos on
what may influence your setup (i.e. BITS_PER_UNIT).
If you want to be shure of the bit-positions I guess you must use
bit-operators and allocate an unsigned char / u8 object.
hofrat
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