efg's Delphi File I/O

Topic

Description

Unit

general info

File I/O, Delphi in a Nutshell, pp. 26-27

{$I+}, {$I-}

See IOResults and Critical Errors Page

Absolute Sectors

Read/Write absolution positions on disk

Absolute Read/Write HardDisk on Win32 Platform. Delphi 3 full source included. http://delphi.icm.edu.pl/ftp/d30free/abs_rw.zip

Append

procedure Append(var f: Text);
Opens an existing file so that new text can be added

The procedure Append is intended for text files. If you use Append on a file of another type the compiler does not catch the error and appends anyway. D5 Bug.

Example: (Append vs. Rewrite)

// Write Log file.
PROCEDURE WriteToLog(CONST key: STRING; CONST level: INTEGER; CONST description: STRING);
VAR
    DirectoryName: STRING;
    FileName: STRING;
    LogFile: TextFile;
BEGIN
DirectoryName := ExtractFilePath(ParamStr(0)) + 'LOG';

// Create directory, if necessary.
IF       NOT FileCtrl.DirectoryExists(DirectoryName)
THEN FileCtrl.ForceDirectories(DirectoryName);

FileName := FormatDateTime('yyyymm', Now) + '.LOG';
AssignFile(LogFile, DirectoryName + '\' + FileName);

TRY
    IF        SysUtils.FileExists(DirectoryName + '\' + FileName)
    THEN Append(LogFile)    // If existing file
    ELSE Rewrite(Logfile);     // Create if new

    IF       Level = 0
    THEN WRITELN(LogFile) // blank line
    ELSE WRITELN (LogFile, FormatDateTime('dd hh:nn:ss', Now), ' ', Format('%-16s', [key]), ' ', description);

    CloseFile(LogFile)
EXCEPT
    // Ignore exceptions while writing log files
END;

END {WriteToLog};

System

Assign

Use AssignFile below. Exists only for backward compatibility.

System

AssignFile

procedure AssignFile(var F);
Associates the name of an external file with a file variable.

See:
- efg's UseNet Reply about AssignFile and UNC names
- ExclusiveIO Example
- example as part of Append information (above)
- College Student Guide to Reading and Writing Files, Borland's TI 641D
- How to copy files in Delphi, Borland's TI 910D

System

AssignPrn

procedure AssignPrn(var F: Text);
Assigns a text-file variable to the printer.

Printers

BlockRead

D1:
procedure BlockRead(var F: File; var Buf; Count: Word; [; var Result: Word]);

D2-D4:
procedure BlockRead(var F: File; var Buf; Count: Integer; [; var AmtTransferred: Integer]);

See:
- Erwin's Ahlers' UseNet Post explaining Count parameter
- Read and BlockRead file function failure in 32 bit, FAQ 2798D
- How to copy files in Delphi, Borland's TI 910D
- Fragment of Patching EXE with CRC-32 between Sentinels
- ExclusiveIO Example.

- Bob Villiers' UseNet Post with VarArrayLoadFile, which loads a bitmap into a variant using BlockRead
- Example from CRC Lab Report:

PROCEDURE CalcFileCRC32 (FromName: STRING; VAR CRCvalue: INTEGER;
VAR IOBuffer: pointer; BufferSize: INTEGER; VAR TotalBytes: INTEGER;
VAR error: INTEGER);
VAR
BytesRead: INTEGER;
FromFile : FILE;
BEGIN
FileMode := 0; {Turbo default is 2 for R/W; 0 is for R/O}
CRCValue := $FFFFFFFF;
ASSIGN (FromFile,FromName);
{$I-} RESET (FromFile,1); {$I+}
error := IOResult;
IF       error = 0
THEN BEGIN
    TotalBytes := 0;

    REPEAT
      BlockRead (FromFile,IOBuffer^,BufferSize,BytesRead);
      CalcCRC32 (IOBuffer,BytesRead,CRCvalue);
      INC (TotalBytes, BytesRead)
    UNTIL BytesRead = 0;
    CLOSE (FromFile)
END;
CRCvalue := NOT CRCvalue
END {CalcFileCRC32};

System

BlockWrite

D1:
procedure BlockWrite(var F: File; var Buf; Count: Word; [; var Result: Word]);

D2-D4:
procedure BlockWrite(var F: File; var Buf; Count: Integer; [; var AmtTransferred: Integer]);

See:
- fragment of Patching EXE with CRC-32 between Sentinels
- How to copy files in Delphi, Borland's TI 910D

System

Use CloseFile below. Exists only for backward compatibility.

System

Close a File

How do I close a file that was opened in a DLL (Delphi made) and called from VB?
Borland's TI 561D

CloseFile

procedure CloseFile(var F);
Associates the name of an external file with a file variable.

Can be abbreviated as Close for backward compatibility.

See
- ExclusiveIO Example
- College Student Guide to Reading and Writing Files, Borland's TI 641D

System

DeleteFile

function DeleteFile(const FileName: string): Boolean;
Erase the specified file.

SysUtils

EOF

function Eof(var F): Boolean;

Text files:
function Eof [ (Var F: Text) ]: Boolean;

Tests whether the current file position is the end-of-file.

System

EOLN

function Eoln [ (Var F: Text) ]: Boolean;

Tests whether the current file position is the end-of-line of a text file.

System

Erase

procedure Erase(var F);
Deletes the external file associate with F.

System

Exception Handling

See IOResults and Critical Errors Page

Exclusive I/O

Delphi only directly supports exclusive "reads" from typed or untyped files.
This UNIT provides a TExclusiveIO class to read a TextFile in an exclusive mode.
Use in a network when files are shared. Also, handles either DOS or UNIX format of
ASCII files.

ExclusiveIO

File I/O

"Dealing with File I/O," Delphi 5 Developer's Guide (pp. 338-360) by Teixeria and Pacheco

File Input and Output

Delphi in a Nutshell, pp. 494-497

File mode constants

Use the file mode constants when opening and closing disk files. The Mode field of TFileRec and TTextRec will contain one of these values.

fmClosed		$D7B0
fmInput		$D7B1
fmOutput		$D7B2
fmInOut		$D7B3

SysUtils

File Open Mode constants

Use the file open mode constants to control the shareability
of a file or stream when it's opened.

fmOpenRead	$0000	Open for read access only
fmOpenWrite	$0001	Open for write access only
fmOpenReadWrite	$0002	Open for read and write access
fmShareCompat	$0000	Compatible with the way FCBs are opened
fmShareExclusive	$0010	Open the file, disallowing other applications to open it for reading or writing.
fmShareDenyWrite	$0020	Open the file, disallowing other applications to open it for writing. Philippe Ranger's UseNet Post with File Locking Example
fmShareDenyRead	$0030	Open the file, disallowing other applications to open it for reading
fmShareDenyNone	$0040	Open the file, disallowing other applications to open it for exclusive use.

SysUtils

FileClose

procedure FileClose(Handle: Integer);
Closes the specified file.

How to Match File Date/Time, Borland's TI 719D
AlterFileTimeStamp.TXT example.

SysUtils

FileCreate

function FileCreate(const Filename: string): Integer;
Closes the specified file.

SysUtils

FileMode

The FileMode variable (a byte) determines the access mode to use when typed and untyped files (not text files) are opened using the Reset procedure. The default FileMode is 2. Assigning another value to FileMode causes all subequent Resets to use that mode.

0 = Read Only, 1 = Write Only, 2= Read/Write

See:
- ExclusiveIO Example using fmShareExclusive.
- fragment of Patching EXE.

System

FileOpen

How to Match File Date/Time, Borland's TI 719D
AlterFileTimeStamp.TXT example.

SysUtils

FilePos

System

FileRead

SysUtils

FileSeek

Read a text file backwards
www.preview.org/q/q1038.shtml

SysUtils

FileSetDate

See AlterFileTimeStamp.TXT example.

FileSize

FileWrite

SysUtils

FlushFileBuffers

Flush binary file from disk cache to disk, FAQ1926D

IOResult

See IOResults and Critical Errors Page

System

input variable

System

LoadFromFile

output variable

System

Read, ReadLn

- College Student Guide to Reading and Writing Files, Borland's TI 641D
- How to readln longer than 255 chars (D1), Borland's TI 571D
- Read and BlockRead file function failure in 32 bit, Borland's FAQ 2798D

System

Reading & Writing Files

College Student Guide to Reading and Writing Files, Borland's TI 641D

Rename

System

Reset

See:
- FileMode variable (above)
- fragment of Patching EXE.
- How to copy files in Delphi, Borland's TI 910D

System

Rewrite

See:
- fragment of Patching EXE.
- College Student Guide to Reading and Writing Files, Borland's TI 641D
- How to copy files in Delphi, Borland's TI 910D

System

SaveToFile

Seek

Ken Aston's UseNet Post explaining "seek" on a text file (TextRead - a simple Seek access to text files in read mode.)

System

SeekEof

System

SeekEoln

System

SetTextBuf

VAR
FromBuffer: ARRAY[1..10240] OF CHAR; {speed up I/O}
...
SetTextBuf (FromFile, FromBuffer);
ASSIGN (FromFile, FromName);

System

Streams

See Stream I/O page.

System.FileMode

See"File Open Modes" (above)

Test Files

The GenText program can be used to create up to 99,999,999 records in a file of up to about 5.9 GB. Gen.ZIP

TextFile

System

TFileRec

TFileRec is the internal format for typed and untyped files. TFileRec enables you to typecast a file variable to access its internal fields. Note: You would normally never declare a variable of this type.

type TFileRec =
packed record
Handle: Integer;
Mode: Integer;
RecSize: Cardinal;
Private: array[1..28] of Byte;
UserData: array[1..32] of Byte;
Name: array[0..259] of Char;
end;

SysUtils

TTextRec

TTextRec is the internal format of a variable of type text. You would never declare a variable of this type. However, you can use TTextRec to typecast a text file variable to access the internal data fields, such as the file name.

TextBuf is the type of the Buffer entry in the TTextRec record, and indicates the size of the buffer used.

Note: Do not use this unless you are familiar with writing Object Pascal text file device drivers.

type
PTextBuf = ^TTextBuf;
TTextBuf = array[0..127] of Char;

TTextRec =
packed record
    Handle: Integer;
    Mode: Integer;
    BufSize: Cardinal;
    BufPos: Cardinal;
    BufEnd: Cardinal;
    BufPtr: PChar;
    OpenFunc: Pointer;
    InOutFunc: Pointer;
    FlushFunc: Pointer;
    CloseFunc: Pointer;
    UserData: array[1..32] of Byte;
    Name: array[0..259] of Char;
    Buffer: TTextBuf;
end;

SysUtils

Truncate

Call Truncate to cause the current file position to become end-of-file (Eof(F) is True). F is a file variable of any type except a text file. Truncate does not work on text files. F must be open.

System

Write/WriteLn

See
- example under Append in File I/O
- College Student Guide to Reading and Writing Files, Borland's TI 641D

System