From: Der Herr Hofrat <der.herr@mail.hofr.at>
To: eddy@fericom.net
Cc: gcc-help@gcc.gnu.org
Subject: Re: Bitfield packing
Date: Sat, 17 Aug 2002 11:58:00 -0000 [thread overview]
Message-ID: <200208171802.g7HI2wN24657@hofr.at> (raw)
In-Reply-To: <20020817073337.GA816@fericom.net> from "eddy@fericom.net" at "Aug 17, 2002 09:33:37 am"
> 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
prev parent reply other threads:[~2002-08-17 18:58 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-08-17 2:33 eddy
2002-08-17 11:58 ` Der Herr Hofrat [this message]
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