windows - 调用 FileRead 后的奇怪计数值

Question

我正在尝试将读取的字节数与传递给 FileRead 的计数进行比较这是 WinAPI 的包装器 ReadFile功能。

问题是我根据 ReadFromFile 过程的结构得到了不同的值(添加/减去的行都没有改变计数变量)。

如果你运行下面的代码，你会得到这个输出

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

AV 是因为编译器在范围末尾释放了动态数组(并非总是如此)。正如您在 FileRead 之后看到的 count == 2200 (在此之前我得到的是 0)。如果您注释掉第二个 API 调用或第二行，则计数是正确的

你能告诉我这是什么吗？我该如何解决？

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

您正在使用带有未类型化第二个参数的过程 fileread 的版本:

TmpBuffer: TBytes;
...
ReadRes := FileRead(aFileHandle, TmpBuffer, aCount);

但在这种情况下，您应该取消引用动态数组，如 TmpBuffer[0]

来自您的帮助链接:

 //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;