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BSD 2.11 - man page for tar (bsd section 5)

TAR(5)						 File Formats Manual					       TAR(5)

tar - tape archive file format
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 repre- sented 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.
Names or linknames longer than NAMSIZ produce error reports and cannot be dumped. 4.2 Berkeley Distribution November 7, 1985 TAR(5)

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