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GNU Info File
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1995-06-17
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679 lines
This is Info file tar.info, produced by Makeinfo-1.61 from the input
file tar.texinfo.
START-INFO-DIR-ENTRY
* tar: (tar). Making tape (or disk) archives.
END-INFO-DIR-ENTRY
This file documents GNU `tar', a utility used to store, backup, and
transport files.
Copyright (C) 1992, 1994, 1995 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.
File: tar.info, Node: Standard, Next: Extensions, Prev: Archive Format, Up: Archive Format
The Standard Format
===================
*(This message will disappear, once this node revised.)*
A `tar' archive file contains a series of records. Each record
contains `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 "blocked" for physical I/O operations. Each
block of N records (where N is set by the `--block-size=512-SIZE' (`-b
512-SIZE') option to `tar') is written with a single `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. In the GNU `tar'
distribution, this is part of file `src/tar.h':
/* Standard Archive Format - Standard TAR - USTAR. */
/* Header block on tape.
We use traditional DP naming conventions here. A "block" is a big chunk
of stuff that we do I/O on. A "record" is a piece of info that we care
about. Typically many "record"s fit into a "block". */
#define RECORDSIZE 512
#define NAMSIZ 100
#define TUNMLEN 32
#define TGNMLEN 32
#define SPARSE_EXT_HDR 21
#define SPARSE_IN_HDR 4
struct sparse
{
char offset[12];
char numbytes[12];
};
union record
{
char charptr[RECORDSIZE];
struct header
{
char arch_name[NAMSIZ];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char chksum[8];
char linkflag;
char arch_linkname[NAMSIZ];
char magic[8];
char uname[TUNMLEN];
char gname[TGNMLEN];
char devmajor[8];
char devminor[8];
/* The following fields were added for GNU and are not standard. */
char atime[12];
char ctime[12];
char offset[12];
char longnames[4];
/* Some compilers would insert the pad themselves, so pad was
once autoconfigured. It is simpler to always insert it! */
char pad;
struct sparse sp[SPARSE_IN_HDR];
char isextended;
char realsize[12]; /* true size of the sparse file */
#if 0
char ending_blanks[12]; /* number of nulls at the end of the file,
if any */
#endif
}
header;
struct extended_header
{
struct sparse sp[21];
char isextended;
}
ext_hdr;
};
/* 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 value if uname and gname are valid,
marking the archive as being in standard POSIX format (though GNU tar
itself is not POSIX conforming). */
#define TMAGIC "ustar " /* 7 chars and a null */
/* The magic field is filled with this if this is a GNU format dump entry.
But I suspect this is not true anymore. */
#define GNUMAGIC "GNUtar " /* 7 chars and a null */
/* The linkflag defines the type of file. */
#define LF_OLDNORMAL '\0' /* normal disk file, Unix compat */
#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. */
/* Note that the standards committee allows only capital A through
capital Z for user-defined expansion. This means that defining
something as, say '8' is a *bad* idea. */
/* This is a dir entry that contains the names of files that were in the
dir at the time the dump was made. */
#define LF_DUMPDIR 'D'
/* Identifies the NEXT file on the tape as having a long linkname. */
#define LF_LONGLINK 'K'
/* Identifies the NEXT file on the tape as having a long name. */
#define LF_LONGNAME 'L'
/* This is the continuation of a file that began on another volume. */
#define LF_MULTIVOL 'M'
/* For storing filenames that didn't fit in 100 characters. */
#define LF_NAMES 'N'
/* This is for sparse files. */
#define LF_SPARSE 'S'
/* This file is a tape/volume header. Ignore it on extraction. */
#define LF_VOLHDR 'V'
#if 0
/* The following two blocks of #define's are unused in GNU tar. */
/* 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 */
#endif
/* End of Standard Archive Format description. */
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