4.4BSD/usr/src/old/tar/tarformat.5

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.\"     @(#)tarformat.5	6.4 (Berkeley) 5/10/91
.\"
.Dd May 10, 1991
.Dt TAR 5
.Os BSD 4.2
.Sh NAME
.Nm tar
.Nd tape archive file format
.Sh DESCRIPTION
The
.Nm tar
tape archive command
dumps several files into one, in a medium suitable for transportation.
.Pp
A ``tar tape'' or file is a series of blocks.  Each block is of size
.Dv TBLOCK .
A file on the tape is represented by a header block which describes
the file, followed by zero or more blocks which give the contents of the
file.  At the end of the tape are two blocks filled with binary
zeros, as an end-of-file indicator.  
.Pp
The blocks are grouped for physical
.Tn I/O
operations.  Each group of
.Ar n
functions
blocks (where
.Ar n
is set by the 
.Cm b
keyletter on the 
.Xr tar 1
command line \(em default is 20 blocks) is written with a single system
call; on nine-track tapes, the result of this write is a single tape
record.  The last group is always written at the full size, so blocks after
the two zero blocks contain random data.  On reading, the specified or
default group size is used for the
first read, but if that read returns less than a full tape block, the reduced
block size is used for further reads.
.Pp
The header block looks like:
.Bd -literal -offset indent
#define TBLOCK	512
#define NBLOCK	20
#define NAMSIZ	100

union hblock {
        char dummy[TBLOCK];
        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];
        } dbuf;
};
.Ed
.Pp
The
.Fa name
field
is a null-terminated string.
The other fields are zero-filled octal numbers in
.Tn ASCII . 
Each field
(of width w) contains w\-2 digits, a space, and a null, except
.Xr size
and
.Fa mtime ,
which do not contain the trailing null and
.Fa chksum
which has a null followed by a space.
.Fa Name
is the name of the file, as specified on the 
.Nm tar
command line.
Files dumped because they were in a directory which
was named in the command line have the directory name as prefix and
.Pa /filename
as suffix.
.Fa Mode
is the file mode, with the top bit masked off.
.Fa Uid
and
.Fa gid
are the user and group numbers which own the file.
.Fa Size
is the size of the file in bytes.  Links and symbolic links are dumped
with this field specified as zero.
.Fa Mtime
is the modification time of the file at the time it was dumped.
.Fa Chksum
is an octal
.Tn ASCII
value which represents the sum of all the bytes in the
header block.  When calculating the checksum, the 
.Fa chksum
field is treated as if it were all blanks.
.Fa Linkflag
is
.Dv NULL
if the file is ``normal'' or a special file,
.Tn ASCII
`1'
if it is an hard link, and
.Tn ASCII
`2'
if it is a symbolic link.  The name linked-to, if any, is in
.Fa linkname ,
with a trailing null.
Unused fields of the header are binary zeros (and are included in the
checksum).
.Pp
The first time a given i-node number is dumped, it is dumped as a regular
file.  The second and subsequent times, it is dumped as a link instead.
Upon retrieval, if a link entry is retrieved, but not the file it was
linked to, an error message is printed and the tape must be manually
re-scanned to retrieve the linked-to file.
.Pp
The encoding of the header is designed to be portable across machines.
.Sh SEE ALSO
.Xr tar 1
.Sh BUGS
Names or linknames longer than
.Dv NAMSIZ
produce error reports and cannot be
dumped.
.Sh HISTORY
The
.Nm
file format manual appeared in
.Bx 4.2 .