TAR(5)TAR(5)NAMEtar - 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
contents 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 and chksum which has a null followed by a space. 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 /filename 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 an octal 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 NULL if the file is ``normal'' or a special file, ASCII `1'
if it is an hard link, and ASCII `2' if it is a symbolic link. The
name linked-to, if any, is in linkname, with a trailing 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
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.
The encoding of the header is designed to be portable across machines.
SEE ALSOtar(1)BUGS
Names or linknames longer than NAMSIZ produce error reports and cannot
be dumped.
4.2 Berkeley Distribution November 7, 1985 TAR(5)
