[DUG] RFID - EAccess violation

[Karl Reynolds]

*Incidentally, "buffer" is used in the original code - it receives the
keydata string via the call to StrPCopy.  "key" ends up as effectively
@buffer[0]*

Oh right, I missed that! Then what I wrote was wrong, especially if the
function is expecting to write back over buffer.. Yes, we do really need to
know more about the function being called; in the absence of that
information though, it wouldn't hurt to initialise the arrays.

var
  key, bufferr: PChar;
  keydata, bufferrdata: array[0..255] of char;
  ...
begin
  for i := 0 to 255 do
  begin
    if i <= 6 then
    begin
      keydata[i] := #255;
    end
    else keydata[i] := #0;
    bufferrdata[i] := #0;
  end;
  key := PChar(@keydata);
  bufferr := PChar(@buffdata);
  receive := API_PCDRead(0,0,$00,10,1,key,bufferr);
  ...

(I called it keydata rather than buffer to save myself confusion :))

Cheers,
Carl

On Thu, May 28, 2015 at 3:41 PM, Jolyon Smith <jsmith at deltics.co.nz> wrote:

> Fair enough.  That wasn't entirely clear so thought it worth clarifying in
> case the OP was confused into thinking this was somehow involved in the AV.
>
> (Incidentally, "buffer" is used in the original code - it receives the
> keydata string via the call to StrPCopy.  "key" ends up as effectively
> @buffer[0]).
>
> On 28 May 2015 at 15:31, Karl Reynolds <kreynolds at chocfishsoft.co.nz>
> wrote:
>
>> Jolyon, I was pointing out a separate error that had nothing to do with
>> the AV he was getting.
>>
>> *> there looks to be an error in your code even if it works*
>>
>> Cheers,
>> Carl
>>
>> On Thu, May 28, 2015 at 2:43 PM, Jolyon Smith <jsmith at deltics.co.nz>
>> wrote:
>>
>>> @Carl,
>>>
>>> Although the code probably is incorrect in the way that it references
>>> buffer after the call, rather than bufferr, this in itself will not cause
>>> an AV since both buffer and bufferr are both statically allocated arrays
>>> and thus references to items in these arrays will be valid, even if not
>>> correct, as long as those references are within the bounds of the array,
>>> which they are in this case.
>>>
>>> On 28 May 2015 at 12:33, Karl Reynolds <kreynolds at chocfishsoft.co.nz>
>>> wrote:
>>>
>>>> I don't know the API_PCDRead function. But there looks to be an error
>>>> in your code even if it works. You are passing bufferr (two 'r's) to the
>>>> function but referring to buffer (one 'r') afterwards, which wasn't used
>>>> (unless there's other code you didn't show). So you need to look at that.
>>>>
>>>> That aside, for the bit leading up to the call, try something like:
>>>>
>>>> var
>>>>   key, bufferr: PChar;
>>>>   keydata: string;
>>>>   buffdata: array[0..255] of char;
>>>>   ...
>>>> begin
>>>>  // Allocate a string of six consecutive $FFs
>>>>   keydata := #255#255#255#255#255#255;
>>>>
>>>>  // Convert to a PChar, an easy way to avoid having to allocate memory
>>>> explicitly
>>>>   key := PChar(keydata);
>>>>
>>>> // Once again avoid having to allocate memory explicitly
>>>> // Point bufferr at the address of an array of 256 characters
>>>>   bufferr := PChar(@buffdata);
>>>>
>>>>   receive:=API_PCDRead(0,0,$00,10,1,key,bufferr);
>>>>
>>>>   ...
>>>>
>>>> Cheers,
>>>> Carl
>>>>
>>>> On Thu, May 28, 2015 at 11:33 AM, Marshland Engineering <
>>>> marshland at marshland.co.nz> wrote:
>>>>
>>>>> Thanks for the replies.
>>>>>
>>>>> this is the line
>>>>>
>>>>> >   receive:=API_PCDRead(0,0,$00,10,1,key,bufferr);
>>>>>
>>>>> I'm way out my depth with the workings of this function. I just do
>>>>> simple
>>>>> coding - A ex dBase3 programmer.
>>>>>
>>>>> I took the sample code that the RFID supplier gave, stripped off
>>>>> everything I
>>>>> didn't need other than the read line for RFID card and then copied the
>>>>> bits
>>>>> into my program which now crashes.
>>>>>
>>>>> I was wondering if the XPMENU call or something like that is
>>>>> interfering with
>>>>> other modules ?
>>>>>
>>>>> My debugging is limited to finding the line with the error.
>>>>>
>>>>> Both programs are run in Delphi 6, same machine and same environment.
>>>>>
>>>>> I can't find how to code this, where ever I put it, it crashes. -
>>>>> Expected
>>>>> expression but ARRAY found.
>>>>>
>>>>>    const
>>>>>      KEY = array[0..6] of Byte = ($FF,$FF,$FF,$FF,$FF,$FF,$00);
>>>>>
>>>>> Thanks Wallace.
>>>>>
>>>>>
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