TAR(5) TAR(5)
NAME
tar - tape archive file format
DESCRIPTION
Tar, (the tape archive command) dumps several files into one, in a
medium suitable for transportation.
A ``tar tape'' or file is a series of blocks. Each block is of size
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 con-
tents of the file. At the end of the tape are two blocks filled with
binary zeros, as an end-of-file indicator.
The blocks are grouped for physical I/O operations. Each group of n
blocks (where n is set by the b keyletter on the tar(1) command line --
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.
The header block looks like:
#define TBLOCK 512
#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;
};
Name is a null-terminated string. The other fields are zero-filled
octal numbers in ASCII. Each field (of width w) contains w-2 digits, a
space, and a null, except size and mtime, which do not contain the
trailing null. Name is the name of the file, as specified on the 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 /file-
name as suffix. Mode is the file mode, with the top bit masked off.
Uid and gid are the user and group numbers which own the file. Size is
the size of the file in bytes. Links and symbolic links are dumped
with this field specified as zero. Mtime is the modification time of
the file at the time it was dumped. Chksum is a decimal ASCII value
which represents the sum of all the bytes in the header block. When
calculating the checksum, the chksum field is treated as if it were all
blanks. Linkflag is ASCII `0' if the file is ``normal'' or a special
file, ASCII `1' if it is an hard link, and ASCII `2' if it is a sym-
bolic link. The name linked-to, if any, is in linkname, with a trail-
ing null. Unused fields of the header are binary zeros (and are
included in the checksum).
The first time a given i-node number is dumped, it is dumped as a regu-
lar 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.
The encoding of the header is designed to be portable across machines.
SEE ALSO
tar(1)
BUGS
Names or linknames longer than NAMSIZ produce error reports and cannot
be dumped.
15 January 1983 TAR(5)
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