PC World 2000 February/PCWorld_2000-02_cd.bin/Software/Servis/FFE/ARCHIVES.SWG/0013_TAR EXPLANATION.pas

Pascal/Delphi Source File | 1996-09-03 | 12KB | 270 lines

--------A-TAR-G-----------------------------

The  Unix  Tape ARchives mostly have  the  extension TAR. The info about
this comes from a magic file, thus useful only for identification.

--------A-TAR-------------------------------

The  Unix  TAR  program is an archiver  program  which stores files in a
single archive without compression.

OFFSET Count TYPE Description
@section The Standard Format

A  @dfn{tar  tape}  or file contains  a  series  of records. Each record
contains @code{RECORDSIZE} bytes. Although this format may be thought of
as being on magnetic tape, other media are often used.

Each file archived is represented by a header record which describes the
file,  followed  by zero or more records  which give the contents of the
file.  At the end of the archive file  there may be a record filled with
binary  zeros as an end-of-file marker. A reasonable system should write
a  record  of zeros at the end, but  must  not assume that such a record
exists when reading an archive.

The records may be @dfn{blocked} for physical I/O operations. Each block
of  @var{N}  records  (where @var{N} is  set  by the @samp{-b} option to
@code{tar})  is  written  with  a  single  @code{write()}  operation. On
magnetic tapes, the result of such a write is a single tape record. When
writing  an archive, the last block of  records should be written at the
full  size,  with records after the  zero  record containing all zeroes.
When  reading an archive, a reasonable  system should properly handle an
archive  whose  last block is shorter  than  the rest, or which contains
garbage records after a zero record.

The header record is defined in C as follows:

@example
/*
 * Standard Archive Format - Standard TAR - USTAR
 */
#define  RECORDSIZE  512
#define  NAMSIZ      100
#define  TUNMLEN      32
#define  TGNMLEN      32

union record @{
    char        charptr[RECORDSIZE];
    struct header @{
        char    name[NAMSIZ];
        char    mode[8];
        char    uid[8];
        char    gid[8];
        char    size[12];
        char    mtime[12];
        char    chksum[8];
        char    linkflag;
        char    linkname[NAMSIZ];
        char    magic[8];
        char    uname[TUNMLEN];
        char    gname[TGNMLEN];
        char    devmajor[8];
        char    devminor[8];
    @} header;
@};

/* The checksum field is filled with this while the checksum is computed. */
#define    CHKBLANKS    "        "        /* 8 blanks, no null */

/* The magic field is filled with this if uname and gname are valid. */
#define    TMAGIC    "ustar  "        /* 7 chars and a null */

/* The magic field is filled with this if this is a GNU format dump entry */
#define    GNUMAGIC  "GNUtar "        /* 7 chars and a null */

/* The linkflag defines the type of file */
#define  LF_OLDNORMAL '\0'       /* Normal disk file, Unix compatible */
#define  LF_NORMAL    '0'        /* Normal disk file */
#define  LF_LINK      '1'        /* Link to previously dumped file */
#define  LF_SYMLINK   '2'        /* Symbolic link */
#define  LF_CHR       '3'        /* Character special file */
#define  LF_BLK       '4'        /* Block special file */
#define  LF_DIR       '5'        /* Directory */
#define  LF_FIFO      '6'        /* FIFO special file */
#define  LF_CONTIG    '7'        /* Contiguous file */

/* Further link types may be defined later. */

/* Bits used in the mode field - values in octal */
#define  TSUID    04000        /* Set UID on execution */
#define  TSGID    02000        /* Set GID on execution */
#define  TSVTX    01000        /* Save text (sticky bit) */

/* File permissions */
#define  TUREAD   00400        /* read by owner */
#define  TUWRITE  00200        /* write by owner */
#define  TUEXEC   00100        /* execute/search by owner */
#define  TGREAD   00040        /* read by group */
#define  TGWRITE  00020        /* write by group */
#define  TGEXEC   00010        /* execute/search by group */
#define  TOREAD   00004        /* read by other */
#define  TOWRITE  00002        /* write by other */
#define  TOEXEC   00001        /* execute/search by other */
@end example

All  characters  in  header  records  are  represented  by  using  8-bit
characters  in  the  local  variant  of  ASCII.  Each  field  within the
structure  is  contiguous; that is, there  is no padding used within the
structure. Each character on the archive medium is stored contiguously.

Bytes  representing  the contents of files  (after  the header record of
each  file)  are  not translated in any  way  and are not constrained to
represent  characters  in any character  set. The @code{tar} format does
not distinguish text files from binary files, and no translation of file
contents is performed.

The   @code{name},  @code{linkname},   @code{magic},  @code{uname},  and
@code{gname} are null-terminated character strings. All other fileds are
zero-filled  octal numbers in ASCII. Each numeric field of width @var{w}
contains   @var{w}@minus{}  2  digits,  a  space,  and  a  null,  except
@code{size}, and @code{mtime}, which do not contain the trailing null.

The  @code{name} field is the pathname of the file, with directory names
(if any) preceding the file name, separated by slashes.

The @code{mode} field provides nine bits specifying file permissions and
three  bits  to specify the Set UID,  Set GID, and Save Text (``stick'')
modes. Values for these bits are defined above. When special permissions
are  required to create a file with a given mode, and the user restoring
files  from the archive does not  hold such permissions, the mode bit(s)
specifying  those  special permissions are  ignored. Modes which are not
supported  by the operating system restoring files from the archive will
be  ignored.  Unsupported  modes  should be  faked  up  when creating or
updating  an archive; e.g. the group permission could be copied from the
@code{other} permission.

The  @code{uid} and @code{gid} fields are  the numeric user and group ID
of  the  file  owners, respectively. If  the  operating  system does not
support numeric user or group IDs, these fields should be ignored.

The @code{size} field is the size of the file in bytes; linked files are
archived  with this field specified  as zero. @xref{Extraction Options};
in particular the @samp{-G} option.@refill

The  @code{mtime} field is the modification time of the file at the time
it  was  archived. It is the ASCII  representation of the octal value of
the last time the file was modified, represented as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

The  @code{chksum} field is the ASCII  representation of the octal value
of  the simple sum of all bytes in the header record. Each 8-bit byte in
the  header  is added to an  unsigned  integer, initialized to zero, the
precision   of  which  shall  be  no  less  than  seventeen  bits.  When
calculating  the  checksum, the @code{chksum} field  is treated as if it
were all blanks.

The  @code{typeflag}  field  specifies the type  of  file archived. If a
particular  implementation  does not recognize  or  permit the specified
type,  the file will be extracted as if  it were a regular file. As this
action occurs, @code{tar} issues a warning to the standard error.

@table @code
@item LF_NORMAL
@itemx LF_OLDNORMAL
These  represent  a regular file. In  order  to be compatible with older
versions  of @code{tar}, a  @code{typeflag} value of @code{LF_OLDNORMAL}
should  be silently recognized as a regular file. New archives should be
created   using  @code{LF_NORMAL}.  Also,  for  backward  compatibility,
@code{tar}  treats  a  regular file whose name  ends  with  a slash as a
directory.

@item LF_LINK
This  represents a file linked to  another file, of any type, previously
archived. Such files are identified in Unix by each file having the same
device  and  inode  number.  The  linked-to  name  is  specified  in the
@code{linkname} field with a trailing null.

@item LF_SYMLINK
This  represents a symbolic link to  another file. The linked-to name is
specified in the @code{linkname} field with a trailing null.

@item LF_CHR
@itemx LF_BLK
These   represent  character  special  files  and  block  special  files
respectively.  In  this  case  the  @code{devmajor}  and @code{devminor}
fields  will  contain the major  and  minor device numbers respectively.
Operating  systems may map the device  specifications to their own local
specification, or may ignore the entry.

@item LF_DIR
This  specifies a directory or sub-directory.  The directory name in the
@code{name}  field  should  end  with a  slash.  On  systems  where disk
allocation  is performed on a directory basis the @code{size} field will
contain the maximum number of bytes (which may be rounded to the nearest
disk  block allocation unit) which the directory may hold. A @code{size}
field  of zero indicates no such  limiting. Systems which do not support
limiting in this manner should ignore the @code{size} field.

@item LF_FIFO
This  specifies  a FIFO special file. Note  that the archiving of a FIFO
file archives the existence of this file and not its contents.

@item LF_CONTIG
This  specifies  a contiguous file, which is  the  same as a normal file
except  that,  in operating systems which  support  it, all its space is
allocated contiguously on the disk. Operating systems which do not allow
contiguous allocation should silently treat this type as a normal file.

@item 'A' @dots{}
@itemx 'Z'
These are reserved for custom implementations. Some of these are used in
the GNU modified format, as described below. @end table

Other  values are reserved for specification  in future revisions of the
P1003 standard, and should not be used by any @code{tar} program.

The  @code{magic}  field indicates that this  archive  was output in the
P1003   archive  format.  If  this  field  contains  @code{TMAGIC},  the
@code{uname}   and   @code{gname}   fields   will   contain   the  ASCII
representation  of  the  owner and group  of  the  file respectively. If
found,  the  user and group ID represented  by  these names will be used
rather than the values within the @code{uid} and @code{gid} fields.

@section  GNU  Extensions  to  the Archive  Format  The  GNU format uses
additional  file  types  to describe new  types  of files in an archive.
These are listed below.

@table @code
@item LF_DUMPDIR
@itemx 'D'
This represents a directory and a list of files created by the @samp{-G}
option.  The  @code{size} field gives the  total  size of the associated
list  of  files. Each filename is  preceded  by either a @code{'Y'} (the
file  should  be  in  this  archive) or  an  @code{'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.

@item LF_MULTIVOL
@itemx 'M'
This  represents a file continued from  another volume of a multi-volume
archive created with the @samp{-M} option. The original type of the file
is  not given here. The @code{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  @code{offset}  field  gives  the  offset  from  the
beginning  of  the  file  where  this  part  of  the  file  begins. Thus
@code{size}  plus  @code{offset} should equal  the  original size of the
file.

@item LF_VOLHDR
@itemx 'V'
This file type is used to mark the volume header that was given with the
@samp{-V}  option  when the archive  was  created. The @code{name} field
contains   the  @code{name}  given  after   the  @samp{-V}  option.  The
@code{size}  field  is  zero. Only the first  file  in each volume of an
archive should have this type.

@end table
EXTENSION:
OCCURENCES:
PROGRAMS:
REFERENCE:
SEE ALSO:
VALIDATION:
OFFSET              Count TYPE   Description
0000h                 256 byte   Other header info ?
0100h                   6 char   ID='ustar',0
EXTENSION:TAR
OCCURENCES:PC, Unix
PROGRAMS:TAR