[fpc-pascal] Setting bit in array of byte

[Michalis Kamburelis]

Koenraad Lelong wrote:
> Hi,
> I have following code (almost literally translated from C) :
> //-------------------------------------------------------------------------- 
> 
> // Bit utility to read and write a bit in the buffer 'buf'.
> //
> // 'op'    - operation (1) to set and (0) to read
> // 'state' - set (1) or clear (0) if operation is write (1)
> // 'loc'   - bit number location to read or write
> // 'buf'   - pointer to array of bytes that contains the bit
> //           to read or write
> //
> // Returns: 1   if operation is set (1)
> //          0/1 state of bit number 'loc' if operation is reading
> //
> type
>  PByteArray = ^array of byte;
> 

"array of byte" is a dynamic array. It's a pointer, to data and to some 
internal fields. "^array of byte" is a pointer to a pointer to data. You 
don't want this.

What you probably want is something like

type
    TByteArray = array[0..MaxInt - 1] of Byte;
    PByteArray = ^TByteArray;

Actually this should be already defined somewhere in the RTL (SysUtils 
unit, or maybe even in System) so you should just omit declaring 
PByteArray type yourself and all should be OK.

> function bitacc(op, state, loc : byte; buf : PByteArray) : byte;
> {C-code : static SMALLINT bitacc(SMALLINT op, SMALLINT state, SMALLINT 
> loc, uchar *buf)}
> 
> var
>  nbyt,
>  nbit : byte;
> 
> begin
> 
> nbyt := loc div 8;
> nbit := loc mod 8;
> if (op = WRITE_FUNCTION) then
>  begin
>   if state=1 then
>    buf^[nbyt] := buf^[nbyt] or ($01 << nbit)
>   else
>    buf^[nbyt] := buf^[nbyt] and not ($01 << nbit);
>   result:=1;
>  end
> else
>  result:=(buf^[nbyt] >> nbit) and $01;
> end;
> 
> It's called with this code :
> 
> bitacc(WRITE_FUNCTION, bitacc(READ_FUNCTION,0,i, at SerialNum[portnum][0]), 
> (i * 2 + 1), @sendpacket[pos])
> {C-code :
> bitacc(WRITE_FUNCTION, bitacc(READ_FUNCTION,0,i,&SerialNum[portnum][0]),
>  (short)(i * 2 + 1), &sendpacket[pos]);}
> 
> Executing the code gives an an access violation exception. What am I 
> doing wrong ?
> I included some debugging code and it seems accessing buf^[nbyt] is the 
> problem.
> 
> TIA
> Koenraad Lelong.
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