LibTar

Native ObjectPascal Library for accessing tar (Tape Archive) files

Author: Stefan Heymann, E-Mail stefan@destructor.de

Purpose

LibTar is a Library which contains the ObjectPascal class TTarArchive. Using this class, you can read the contents (Directory entries and file contents) of a tar file.

About tar

tar is a format widely used on Unix and Linux systems. "tar" is short for "Tape Archive". The format was intended primarily for use with tape backup devices. However, tar files are also stored on disks. The common extension for tar files is ".tar".

A tar file contains several files, making one file out of them. There is no compression. The tar file just contains a directory entry for each file (which holds information like the name of the file, size, last modified date, access permissions, etc.). and the file itself.

TTarArchive

Using the TTarArchive class, you scan through the tar file, thereby reading out the directory entries and – if you want – the files.

Step Code
Choose a constructor

You can either pass a TStream instance or a filename

 
constructor Create (Stream : TStream);  overload;
constructor Create (Filename : STRING); overload;
Make an instance of TTarArchive 
TA := TTarArchive.Create (Filename);
Prepare your scan 
TA.Reset;
Scan through the archive 
while TA.FindNext (DirRec) do begin
Evaluate the DirRec for each file 
  ListBox.Items.Add (DirRec.Name);
Read out the current file

You can leave this step out if you don't want to read the file

 
  TA.ReadFile (DestFilename);
  END;
You're done 
TA.Free

FindNext, TTarDirRec

The FindNext function returns two values:

The function return value (boolean) is true, when there was another file found in the archive and false when the end of the archive has been reached. So you can use it in a "while FindNext () do" loop. Using this loop, you hop from file to file, accessing one at a time.

There is a var parameter, named DirRec of type TTarDirRec. This record gives all the informations about the current file in the archive has the following fields:

Field Name Content
Name The file's path and file name. The path is usually divided with slash characters, because tar is a Unix format
Size The size of the file in bytes
DateTime Last last modification date and time. Note that this time is given in UTC (GMT). You must probably convert it to your local time before using it for comparisons.
Permissions A set telling who had which access rights to the file at the time it was written to the archive:
tpReadByOwner,
tpWriteByOwner,
tpExecuteByOwner		 The file owner has permission to read/write/execute it
tpReadByGroup,
tpWriteByGroup,
tpExecuteByGroup		 The user's group has permission to read/write/execute the file
tpReadByOther,
tpWriteByOther,
tpExecuteByOther		 All other users have permission to read/write/execute the file
FileType The type of the file:
TFileType Description
ftNormal A regular file
ftLink A link to another, previously archived, file. There is no data stored for a link. The name of the file which the link refers to is stored in the "LinkName" field of TTarDirRec
ftSymbolicLink A symbolic link to another file. There is no data stored for a link. The name of the file which the link refers to is stored in the "LinkName" field of TTarDirRec
ftCharacter, ftBlock Character/Block special files. Unix specific. Can be treated as a regular file by other OSes
ftDirectory A subdirectory entry. There is no data stored for a directory. The "Size" field of TTarDirRec is zero (unlimited) or gives the maximum number of bytes which may be stored in the directory.
ftFifo FIFO special file. There is no data stored for a FIFO file.
ftContiguous If supported by the OS, the file shall be stored in contiguous blocks of the storage medium. Otherwise, it can be treated like a regular file.
ftDumpDir This represents a directory and a list of files created by the -G option of tar. The Size field gives the total size of the associated list of files. Each filename is preceded by either a 'Y' (meaning the file should be in this archive) or an 'N' (The file is a directory, or is not stored in the archive). Each filename is terminated by a null. There is an additional null after the last filename.
ftMultivolume This represents a file continued from another volume of a multi-volume archive created with the -M option of tar. The original type of the file is not given here. Size field gives the maximum size of this piece of the file (assuming the volume does not end before the file is written out). The offset field gives the offset from the beginning of the file where this part of the file begins. Thus size plus offset should equal the original size of the file.
ftVolumeHeader This file type is used to mark the volume header that was given with the -V option when the archive was created. The Name field contains the name given after the -V option. The Size field is zero. Only the first file in each volume of an archive should have this type.
LinkName If the FileType is ftLink or ftSymbolicLink, the LinkName field contains the name of the file where the link is going to.
UID, GID Numerical User ID and Group ID
UserName, GroupName User Name and Group Name
CheckSumOK True, if the checksum of the tar header was checked OK, false if not.
Mode I don't know enough about tar to tell you what the heck this means. Sorry.
Magic Contains the string in the "magic" field of the tar header. Possible values are 'ustar' and 'GNUtar'. If it is 'ustar', then the UserName and GroupName field can be considered valid. If it is 'GNUtar' then there is a GNU format dump entry.
MajorDevNo, MinorDevNo Major and Minor Device Number for Files of type ftCharacter or ftBlock
FilePos The Position in the tar file. This corresponds to the byte position of the first byte of the current header record. You can use the "SetFilePos" method to return to this position at any time later.

File Position

There are several methods you can use to retrieve or set the Current Position in the tar file during or after scanning:

Source, Legals ("Licence")

The official site to get this code is http://www.destructor.de

Usage and Distribution of this Source Code is ruled by the "Destructor.de Source code Licence" (DSL) which comes with this file or can be downloaded at http://www.destructor.de

IN SHORT: Usage and distribution of this source code is free. You use it completely on your own risk.

Support

Please send support requests to stefan@destructor.de. I'll do my best to help you.

Postcardware

If you like this code, please send a postcard of your city to my address (anonymous if you want):

	Stefan Heymann
	Eschenweg 3
	72076 Tübingen
	GERMANY

2001-04-28, Stefan Heymann