SysUtils.FileRead Function

Reads a specified number of bytes from a file.

Pascal

function FileRead(Handle: Integer; var Buffer; Count: LongWord): Integer;

C++

int FileRead(int Handle,  Buffer, LongWord Count);

File

SysUtils

Description

FileRead reads Count bytes from the file specified by Handle into the buffer. The Count parameter indicates the size, in bytes, of the buffer. The function result is the actual number of bytes read, which may be less than Count. The Handle that is passed to FileRead must be opened with FileOpen or FileCreate.

Note: :In Delphi, do not mix routines that take or return file handles with those that use Delphi language file variables (typically seen as var F). To read from a file specified by a Delphi file variable, use the BlockRead procedure or the Read procedure instead.

C++ Examples:

Copy Code

/*
The following example uses a button, a string grid, and an
Open dialog box on a form. When the button is clicked, the
user is prompted for a filename. When the user clicks OK,
the specified file is opened, read into a buffer, and closed.
Then the buffer is displayed in two columns of the string
grid. The first time you use this app, you should do a "Fill
Grid" and then "Write Grid to File".   Always use files
generated in this way to do a "Read File into Grid".
*/
// Read File to Grid

#include <memory>       //for STL auto_ptr class

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  int iFileHandle;
  int iFileLength;
  int iBytesRead;
  int iLength;
  if (OpenDialog1->Execute())
  {
    try
    {
      iFileHandle = FileOpen(OpenDialog1->FileName, fmOpenRead);
      iFileLength = FileSeek(iFileHandle,0,2);
      FileSeek(iFileHandle,0,0);
      iBytesRead = FileRead(iFileHandle, reinterpret_cast<char *>(&(StringGrid1->ColCount)), 4);
      iBytesRead = FileRead(iFileHandle, reinterpret_cast<char *>(&(StringGrid1->RowCount)), 4);
      for (int x = 0; x < StringGrid1->ColCount; x++)
      {
        for (int y = 0; y < StringGrid1->RowCount; y++)
        {
          // Write out the length of each string, followed by the string itself.
          FileRead(iFileHandle, reinterpret_cast<char *>(&iLength), 4);
          std::auto_ptr<AnsiChar> pszBuffer(new AnsiChar[iLength + 1]);
          FileRead(iFileHandle, pszBuffer.get(), iLength);
          pszBuffer.get()[iLength] = 0;
          StringGrid1->Cells[x][y] = pszBuffer.get();
        }
      }
      FileClose(iFileHandle);
    }
    catch(...)
    {
#if defined(_DELPHI_STRING_UNICODE)
      Application->MessageBox(
        L"Can't perform one of the following file operations: Open, Seek, Read, Close.",
        L"File Error",
        IDOK);
#else
      Application->MessageBox(
        "Can't perform one of the following file operations: Open, Seek, Read, Close.",
        "File Error",
        IDOK);
#endif
    }
  }
}

// Write Grid into File

#include <dir.h>

void __fastcall TForm1::Button2Click(TObject *Sender)
{
  char szFileName[MAXFILE+4];
  int iFileHandle;
  int iLength;
  if (SaveDialog1->Execute())
  {
    if (FileExists(SaveDialog1->FileName))
    {
      fnsplit(AnsiString(SaveDialog1->FileName).c_str(), 0, 0, szFileName, 0);
      strcat(szFileName, ".BAK");
      RenameFile(SaveDialog1->FileName, szFileName);
    }
    iFileHandle = FileCreate(SaveDialog1->FileName);
    // Write out the number of rows and columns in the grid.
    FileWrite(iFileHandle, reinterpret_cast<char *>(&(StringGrid1->ColCount)), sizeof(StringGrid1->ColCount));
    FileWrite(iFileHandle, reinterpret_cast<char *>(&(StringGrid1->RowCount)), sizeof(StringGrid1->RowCount));
    for (int x=0;x<StringGrid1->ColCount;x++)
    {
      for (int y=0;y<StringGrid1->RowCount;y++)
      {
        // Write out the length of each string, followed by the string itself.
        iLength = StringGrid1->Cells[x][y].Length();
        FileWrite(iFileHandle, (AnsiChar *)&iLength, sizeof(iLength));
        FileWrite(iFileHandle, StringGrid1->Cells[x][y].t_str(), StringGrid1->Cells[x][y].Length());
      }
    }
    FileClose(iFileHandle);
  }
}

Copy Code

/*
The following example uses a button, a string grid, and an
Open dialog box on a form. When the button is clicked, the
user is prompted for a filename. When the user clicks OK,
the specified file is opened, read into a buffer, and closed.
Then the buffer is displayed in two columns of the string
grid. The first column contains the character values in the
buffer. The second column contains the numeric values of the
characters in the buffer.
*/

#include <memory>       //for STL auto_ptr class

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  int iFileHandle;
  int iFileLength;
  int iBytesRead;
  char *pszBuffer;
  if (OpenDialog1->Execute())
  {
    try
    {
      iFileHandle = FileOpen(OpenDialog1->FileName, fmOpenRead);
      iFileLength = FileSeek(iFileHandle,0,2);
      FileSeek(iFileHandle,0,0);
//    pszBuffer = PChar(AllocMem(iFileLength+1));
      std::auto_ptr<char> pszBuffer(new char[iFileLength+1]);
      iBytesRead = FileRead(iFileHandle, pszBuffer.get(), iFileLength);
      FileClose(iFileHandle);
      for (int i=0;i<iBytesRead;i++)
      {
        StringGrid1->RowCount += 1;
        StringGrid1->Cells[1][i+1] = pszBuffer.get()[i];
        StringGrid1->Cells[2][i+1] = IntToStr((int)pszBuffer.get()[i]);
      }
    }
    catch(...)
    {
      Application->MessageBox(
        L"Can't perform one of the following file operations: Open, Seek, Read, Close.",
        L"File Error", IDOK);
    }
  }
}

Delphi Examples:

Copy Code

{
The following example uses a button, a string grid, and an
Open dialog box on a form. When the button is clicked, the
user is prompted for a filename. When the user clicks OK,
the specified file is opened, read into a buffer, and closed.
Then the buffer is displayed in two columns of the string
grid. The first column contains the character values in the
buffer. The second column contains the numeric values of the
characters in the buffer.
} 
procedure TForm1.Button1Click(Sender: TObject);
var
  iFileHandle: Integer;
  iFileLength: Integer;
  iBytesRead: Integer;
  Buffer: PAnsiChar;
  i: Integer;
begin
  if OpenDialog1.Execute then
  begin
    try
      iFileHandle := SysUtils.FileOpen(OpenDialog1.FileName, fmOpenRead);
      iFileLength := SysUtils.FileSeek(iFileHandle,0,2);
      FileSeek(iFileHandle,0,0);
      Buffer := System.AllocMem(iFileLength + 1);
      iBytesRead := SysUtils.FileRead(iFileHandle, Buffer^, iFileLength);
      FileClose(iFileHandle);
      for i := 0 to iBytesRead-1 do
      begin
        StringGrid1.RowCount := StringGrid1.RowCount + 1;
        StringGrid1.Cells[1,i+1] := Buffer[i];
        StringGrid1.Cells[2,i+1] := IntToStr(Integer(Buffer[i]));
      end;
    finally
      FreeMem(Buffer);
    end;
  end;
end;

Copy Code

{
The following example uses a button, a string grid, and an
Open dialog box on a form. When the button is clicked, the
user is prompted for a filename. When the user clicks OK,
the specified file is opened, read into a buffer, and closed.
Then the buffer is displayed in two columns of the string
grid. The first column contains the character values in the
buffer. The second column contains the numeric values of the
characters in the buffer.
}
// Read File to Grid
procedure TForm1.Button1Click(Sender: TObject);
var
  iFileHandle: Integer;
  iFileLength: Integer;
  iBytesRead: Integer;
  Buffer: PAnsiChar;
  X, Y, I: Integer;
  StringLen, colcnt, rowcnt: Integer;
  cellString: String;
begin
  if OpenDialog1.Execute then
  begin
    try
      iFileHandle := SysUtils.FileOpen(OpenDialog1.FileName, fmOpenRead);
      iFileLength := SysUtils.FileSeek(iFileHandle,0,2);
      FileSeek(iFileHandle,0,0);
//      Buffer := PChar(System.AllocMem(iFileLength + 1));
//      iBytesRead := SysUtils.FileRead(iFileHandle, Buffer^, iFileLength);
//      FileClose(iFileHandle);
//      iFileHandle := SysUtils.FileOpen(OpenDialog1.FileName, fmOpenRead);
//      iFileLength := SysUtils.FileSeek(iFileHandle,0,2);
//      FileSeek(iFileHandle,0,0);
      colcnt := StringGrid1.ColCount;
      iBytesRead := SysUtils.FileRead(
        iFileHandle,
        colcnt,
        4);  // 4 because it's an integer
      rowcnt := StringGrid1.RowCount;
      iBytesRead := SysUtils.FileRead(
        iFileHandle,
        rowcnt,
        4);  // 4 because it's an integer
    for X := 0 to StringGrid1.ColCount - 1 do
    begin
      for Y := 0 to StringGrid1.RowCount - 1 do
      begin
        try
          iBytesRead := SysUtils.FileRead(iFileHandle, StringLen, 4);  // 4 because it's an integer
          Buffer := System.AllocMem(StringLen + 1);
//          GetMem(Buffer, StringLen); { allocate the buffer }
          iBytesRead := SysUtils.FileRead(iFileHandle, Buffer^, StringLen);
//          for I := 1 to StringLen do
//            cellString[I] := Buffer[I - 1];
          StringGrid1.Cells[X,Y] := AnsiString(Buffer);
        finally
          FreeMem(Buffer);
        end;
      end;
    end;
    finally
      FileClose(iFileHandle);
    end;
  end;
end;

// Write Grid into File
procedure TForm1.Button2Click(Sender: TObject);
var
  BackupName: string;
  FileHandle: Integer;
  StringLen: Integer;
  X, Y, I: Integer;
  colCountLength, rowCountLength: Integer;
  Buffer: PAnsiChar;
  cellString: AnsiString;
begin
  if SaveDialog1.Execute then
  begin
    if FileExists(SaveDialog1.FileName) then
    begin
      BackupName := SysUtils.ExtractFileName(SaveDialog1.FileName);
      BackupName := ChangeFileExt(BackupName, '.BAK');
      if not RenameFile(SaveDialog1.FileName, BackupName) then
        raise Exception.Create('Unable to create backup file.');
    end;
    FileHandle := FileCreate(SaveDialog1.FileName);
    { Write out the number of rows and columns in the grid. }
    colCountLength := SizeOf(StringGrid1.ColCount);
    FileWrite(FileHandle, 
      PAnsiChar(StringGrid1.ColCount), colCountLength);
    rowCountLength := SizeOf(StringGrid1.RowCount);
    FileWrite(FileHandle,
      PAnsiChar(StringGrid1.RowCount), rowCountLength);
    for X := 0 to StringGrid1.ColCount - 1 do
    begin
      for Y := 0 to StringGrid1.RowCount - 1 do
      begin
        try
          { Write out the length of each string, followed by the string itself. }
          StringLen := Length(StringGrid1.Cells[X,Y]);
          FileWrite(FileHandle, PChar(StringLen), SizeOf(StringLen));
          GetMem(Buffer, StringLen); { allocate the buffer }
          cellString := StringGrid1.Cells[X,Y];
          for I := 1 to StringLen do
             Buffer[I - 1] := cellString[I];
          FileWrite(FileHandle, Buffer^, StringLen);
        finally
          FreeMem(Buffer, StringLen);
        end;
      end;
    end;
    FileClose(FileHandle);
  end;
end;