Strange Count value after calling FileRead - windows

Question

I'm trying to compare the number of bytes read with the count passed to FileRead which is a wrapper around the WinAPi ReadFile function.
The problem is that I get different values based on the structure of my ReadFromFile procedure (none of the added/subtracted lines change the count variable).
If you run the below code you get this output
FileHandle: 400
SizeOfFile: 8672
Current position: 8655
aCount before SetLength: 17
aCount before FileRead: 17
Number of bytes read: 17
aCount after FileRead: 2200
EAccessViolation: Access violation at address 0040C5BC in module 'Project7.exe'. Read of address C23C30BA
the AV is because of freeing the dynamic array by the compiler at the end of scope (this is not always).
as you can see the count == 2200 here (I got 0 before this) after the FileRead. If you comment out the second API Call or Line two the count is right
Can you tell me what is this and how can I solve it?
program Project7;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils, WinAPI.Windows, System.Classes;
procedure ReadFromFile(aFileHandle: THandle; aCount: Longint);
var
aPosition, ReadRes: Int64;
TmpBuffer: TBytes;
begin
writeln('aCount before SetLength: ',aCount);
SetLength(TmpBuffer, aCount);
writeln('aCount before FileRead: ',aCount);
ReadRes := FileRead(aFileHandle, TmpBuffer, aCount);
Writeln('Number of bytes read: ', ReadRes);
//aPosition := FileSeek(aFileHandle, 0, Ord(soCurrent)); // second API call
//Writeln('Current position after read: ', aPosition); // line two
writeln('aCount after FileRead: ',aCount);
if ReadRes <> aCount then
//Raise Exception.Create('hi there');
// DoWrite(TmpBuffer[0], aCount);
end;
var
FFileHandle: THandle;
aFileName: string;
I1: Integer;
aFilePhysicalSize: Int64;
FPosition: Int64;
begin
try
aFileName := 'C:\Users\nacereddine\Desktop\ascii-table.gif';
{ TODO -oUser -cConsole Main : Insert code here }
FFileHandle := CreateFile(PChar(aFileName), GENERIC_READ,
FILE_SHARE_READ, nil, OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL, 0);
Writeln('FileHandle: ', FFileHandle);
aFilePhysicalSize := FileSeek(FFileHandle, 0 , Ord(soEnd));
Writeln('SizeOfFile: ', aFilePhysicalSize);
FPosition := FileSeek(FFileHandle, aFilePhysicalSize - 17 , Ord(soBeginning));
Writeln('Current position: ', FPosition);
I1 := 17;
ReadFromFile(FFileHandle, I1);
readln;
except
on E: Exception do
begin
Writeln(E.ClassName, ': ', E.Message);
readln;
end;
end;
end.

Answer 1

You are using version of procedure fileread with untyped second parameter here:
TmpBuffer: TBytes;
...
ReadRes := FileRead(aFileHandle, TmpBuffer, aCount);
but in this case you should dereference dynamic array like TmpBuffer[0]
From your help link:
//this version is used
function FileRead(Handle: THandle; var Buffer; Count: LongWord): Integer;
//perhaps you wanted that one:
function FileRead(Handle: THandle; var Buffer: TBytes; Offset, Count: LongWord): Integer;