1TAR(5) File Formats Manual TAR(5)
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6 tar - tape archive file format
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9 Tar, (the tape archive command) dumps several files into one, in a
10 medium suitable for transportation.
11 A ``tar tape'' or file is a series of blocks. Each block is of size
13 TBLOCK. A file on the tape is represented by a header block which
14 describes the file, followed by zero or more blocks which give the con‐
15 tents of the file. At the end of the tape are two blocks filled with
16 binary zeros, as an end-of-file indicator.
17 The blocks are grouped for physical I/O operations. Each group of n
19 blocks (where n is set by the b keyletter on the tar(1) command line —
20 default is 20 blocks) is written with a single system call; on nine-
21 track tapes, the result of this write is a single tape record. The
22 last group is always written at the full size, so blocks after the two
23 zero blocks contain random data. On reading, the specified or default
24 group size is used for the first read, but if that read returns less
25 than a full tape block, the reduced block size is used for further
26 reads.
27 The header block looks like:
29 #define TBLOCK 512
31 #define NAMSIZ 100
32 union hblock {
34 char dummy[TBLOCK];
35 struct header {
36 char name[NAMSIZ];
37 char mode[8];
38 char uid[8];
39 char gid[8];
40 char size[12];
41 char mtime[12];
42 char chksum[8];
43 char linkflag;
44 char linkname[NAMSIZ];
45 } dbuf;
46 };
47 Name is a null-terminated string. The other fields are zero-filled
49 octal numbers in ASCII. Each field (of width w) contains w-2 digits, a
50 space, and a null, except size and mtime, which do not contain the
51 trailing null and chksum which has a null followed by a space. Name is
52 the name of the file, as specified on the tar command line. Files
53 dumped because they were in a directory which was named in the command
54 line have the directory name as prefix and /filename as suffix. Mode
55 is the file mode, with the top bit masked off. Uid and gid are the
56 user and group numbers which own the file. Size is the size of the
57 file in bytes. Links and symbolic links are dumped with this field
58 specified as zero. Mtime is the modification time of the file at the
59 time it was dumped. Chksum is an octal ASCII value which represents
60 the sum of all the bytes in the header block. When calculating the
61 checksum, the chksum field is treated as if it were all blanks. Link‐
62 flag is NULL if the file is ``normal'' or a special file, ASCII `1' if
63 it is an hard link, and ASCII `2' if it is a symbolic link. The name
64 linked-to, if any, is in linkname, with a trailing null. Unused fields
65 of the header are binary zeros (and are included in the checksum).
66 The first time a given i-node number is dumped, it is dumped as a regu‐
68 lar file. The second and subsequent times, it is dumped as a link
69 instead. Upon retrieval, if a link entry is retrieved, but not the
70 file it was linked to, an error message is printed and the tape must be
71 manually re-scanned to retrieve the linked-to file.
72 The encoding of the header is designed to be portable across machines.
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76 tar(1)
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79 Names or linknames longer than NAMSIZ produce error reports and cannot
80 be dumped.
814.2 Berkeley Distribution November 7, 1985 TAR(5)