[M3devel] I know, I know...

[Dragiša Durić]

You mean, for system programming I have to use C?

--
Divided by a common language

Dragiša Durić
dragisha at m3w.org




On Aug 23, 2012, at 9:26 PM, Jay K wrote:

> How about you just use "bytes" or integers and helper functions like this:
>  
>  
> typedef struct _BIT_FIELD {
>     UINT8 ByteOffset;
>     UINT8 BitOffset;
>     UINT8 BitCount;
> } BIT_FIELD;
> 
>  
> HRESULT
> ExtractBitField(__in const BIT_FIELD* BitField,
>                 __in const UINT8* Bytes,
>                 __in size_t Size,
>                 __out_opt UINT8* Value)
> /*++
> Extract a bitfield from an array of bytes, as specified
>     by a byte offset, a bit offset within the first byte, and a bit count.
> The bitfield is presumed to be no more than 8 bits -- however that could be easily fixed.
> The bitfield can definitely cross byte boundaries.
> --*/
> {
>     UINT8 const bitCount = BitField->BitCount;
>     UINT8 const bitOffset = BitField->BitOffset;
>     UINT8 const byteOffset = BitField->ByteOffset;
>     UINT8 const FF = (UINT8)~(UINT8)0;
>     BOOL8 const twoBytes = ((bitOffset + bitCount) > 8);
>     UINT8 const secondBitOffset = (8 - bitOffset);
>     UINT8 localValue = { 0 };
>     HRESULT error = { 0 };
>     if (Value)
>         *Value = 0;
>     if ((bitCount > 8) || (bitOffset > 7) || (bitCount == 0))
>     {
>         error = E_INVALIDARG;
>         goto Exit;
>     }
>     if ((byteOffset + twoBytes) >= Size)
>     {
>         error = HRESULT_FROM_WIN32(ERROR_BUFFER_OVERFLOW);
>         goto Exit;
>     }
>     localValue = (Bytes[byteOffset] >> bitOffset);
>     if (twoBytes)
>     {
>         localValue &= ~(FF << secondBitOffset);
>         localValue |= (Bytes[byteOffset + 1] << secondBitOffset);
>     }
>     localValue &= ~(FF << bitCount);
>     error = 0;
>     if (Value)
>         *Value = localValue;
> Exit:
>     return error;
> }
> 
>  
> arrays of bytes are really the only way to control the layout.
> Look at how GNU binutils works, for example...
> Bitfields in C don't yield predictable/portable layout either.
>  
>  
>  - Jay
>  
> > From: hosking at cs.purdue.edu
> > Date: Thu, 23 Aug 2012 09:30:57 -0400
> > To: dragisha at m3w.org
> > CC: m3devel at elegosoft.com
> > Subject: Re: [M3devel] I know, I know...
> > 
> > PACKED?
> > 
> > Sent from my iPad
> > 
> > On Aug 23, 2012, at 3:24 AM, Dragiša Durić <dragisha at m3w.org> wrote:
> > 
> > > I know this will probably be very dense subject, but. What about:
> > > 
> > > TSPacketHeader = <* ENDIAN = BIG*>RECORD
> > > sync: BITS 8 FOR [16_0..16_ff]; (* Always 0x47 *)
> > > tErrInd, (* Transport Error Indicator *)
> > > pusi: BITS 1 FOR BOOLEAN; (* Payload Unit Start Indicator *)
> > > transPrio: BITS 1 FOR [0..1];
> > > pid: BITS 13 FOR PID;
> > > transScramControl: BITS 2 FOR [0..3]; (* 00 means no scrambling *)
> > > afc: BITS 2 FOR [0..3]; (* 01 - no adaptation field, payload only *) 
> > > cc: BITS 4 FOR Nibble;
> > > END;
> > > 
> > > Meaning: bit data is packed from left to right, and all multi-byte data is packed MSB first.
> > > 
> > > Right now I am doing this, based on knowledge of my platform (little endian, x86_64), like this:
> > > 
> > > TSPacketHeader = RECORD
> > > sync: BITS 8 FOR [16_0..16_ff]; (* Always 0x47 *)
> > > 
> > > pidHi: BITS 5 FOR [16_00..16_1f];
> > > transPrio: BITS 1 FOR [0..1];
> > > pusi,
> > > tErrInd: BITS 1 FOR BOOLEAN;
> > > 
> > > pidLo: BITS 8 FOR [16_00..16_ff];
> > > 
> > > cc: BITS 4 FOR Nibble;
> > > afc: BITS 2 FOR [0..3]; (* 01 - no adaptation field, payload only *) 
> > > transScramControl: BITS 2 FOR [0..3]; (* 00 means no scrambling *)
> > > END;
> > > 
> > > And please don't tell me "write in C", because then I will just "offload" this problem to C preprocessor and still only hope for the best. Modula-3 is provably very adept to systems programming and I hope it can be more so.
> > > 
> > > Thanks in advance..
> > > --
> > > Divided by a common language
> > > 
> > > Dragiša Durić
> > > dragisha at m3w.org
> > > 
> > > 
> > > 
> > >

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