tar(5)
NAME
tar - tape (or other media) archive file format
DESCRIPTION
A ``tar tape'' or file contains a series of records. Each record
contains TRECORDSIZE bytes (see below). 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 indicator. A reasonable system should write a
record of zeros at the end, but must not assume that an end-of-file
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 -b 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 the header file <tar.h> as follows:
/*
* 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 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 */
All characters in header records are represented 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 or to be in any character set. The tar(5) format does not
distinguish text files from binary files, and no translation of file
contents should be performed.
The fields name, linkname, magic, uname, and gname are null-terminated
character strings. All other fields are zero-filled octal numbers in
ASCII. Each numeric field (of width w) contains w-2 digits, a space, and
a null, except size and mtime, which do not contain the trailing null.
The name field is the pathname of the file, with directory names (if any)
preceding the file name, separated by slashes.
The mode field provides nine bits specifying file permissions and three
bits to specify the Set UID, Set GID and Save Text (TSVTX) 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 an archive; e.g. the
group permission could be copied from the `other' permission.
The uid and gid fields are the user and group ID of the file owners,
respectively.
The size field is the size of the file in bytes; linked files are
archived with this field specified as zero.
The 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 in integer number of seconds
since January 1, 1970, 00:00 Coordinated Universal Time.
The chksum field is the ASCII representaion of the octal value of the
simple sum of all bytes in the header record. Each 8-bit byte in the
header is treated as an unsigned value. These values are added to an
unsigned integer, initialized to zero, the precision of which shall be no
less than seventeen bits. When calculating the checksum, the chksum
field is treated as if it were all blanks.
The 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,
tar issues a warning to the standard error.
LF_NORMAL or LF_OLDNORMAL
represents a regular file. For backward compatibility, a typeflag
value of LF_OLDNORMAL should be silently recognized as a regular
file. New archives should be created using LF_NORMAL. Also, for
backward compatability, tar treats a regular file whose name ends
with a slash as a directory.
LF_LINK
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 linkname field with a trailing null.
LF_SYMLINK
represents a symbolic link to another file. The linked-to name is
specified in the linkname field with a trailing null.
LF_CHR or LF_BLK
represent character special files and block special files
respectively. In this case the devmajor and 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.
LF_DIR
specifies a directory or sub-directory. The directory name in the
name field should end with a slash. On systems where disk
allocation is performed on a directory basis the 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
size field of zero indicates no such limiting. Systems which do not
support limiting in this manner should ignore the size field.
LF_FIFO
specifies a FIFO special file. Note that the archiving of a FIFO
file archives the existence of this file and not its contents.
LF_CONTIG
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.
`A' - `Z'
are reserved for custom implementations. None are used by this
version of the tar program.
other
values are reserved for specification in future revisions of the
P1003 standard, and should not be used by any tar program.
The magic field indicates that this archive was output in the P1003
archive format. If this field contains TMAGIC, then the uname and 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 contained within the uid
and gid fields. User names longer than TUNMLEN-1 or group names longer
than TGNMLEN-1 characters will be truncated.
SEE ALSO
tar(1), ar(5), cpio(5), dump(8), restor(8), restore(8)
BUGS
Names or link names longer than NAMSIZ-1 characters cannot be archived.
This format does not yet address multi-volume archives.
NOTES
This manual page was adapted by John Gilmore from Draft 6 of the P1003
specification