NAME

tar − tape (or other media) archive file format

DESCRIPTION

A “tar tape” or file contains a series of records.  Each record contains TRECORDSIZE bytes (see below).  Although this format may be thought of as being on magnetic tape, other media are often used.  Each file archived is represented by a header record which describes the file, followed by zero or more records which give the contents of the file.  At the end of the archive file there may be a record filled with binary zeros as an end-of-file indicator.  A reasonable system should write a record of zeros at the end, but must not assume that an end-of-file record exists when reading an archive. 
 
The records may be blocked for physical I/O operations.  Each block of N records (where N is set by the −b option to tar) is written with a single write() operation.  On magnetic tapes, the result of such a write is a single tape record.  When writing an archive, the last block of records should be written at the full size, with records after the zero record containing all zeroes.  When reading an archive, a reasonable system should properly handle an archive whose last block is shorter than the rest, or which contains garbage records after a zero record. 
 
The header record is defined in the header file <tar.h> as follows:

/∗
 ∗ Standard Archive Format - Standard TAR - USTAR
 ∗/
#defineRECORDSIZE512
#defineNAMSIZ100
#defineTUNMLEN32
#defineTGNMLEN32
 union record {
charcharptr[RECORDSIZE];
struct header {
charname[NAMSIZ];
charmode[8];
charuid[8];
chargid[8];
charsize[12];
charmtime[12];
charchksum[8];
charlinkflag;
charlinkname[NAMSIZ];
charmagic[8];
charuname[TUNMLEN];
chargname[TGNMLEN];
chardevmajor[8];
chardevminor[8];
} header;
};
 /∗ The checksum field is filled with this while the checksum is computed. ∗/
#defineCHKBLANKS"        "/∗ 8 blanks, no null ∗/
 /∗ The magic field is filled with this if uname and gname are valid. ∗/
#defineTMAGIC"ustar  "/∗ 7 chars and a null ∗/
 /∗ The linkflag defines the type of file ∗/
#defineLF_OLDNORMAL ’\0’/∗ Normal disk file, Unix compatible ∗/
#defineLF_NORMAL’0’/∗ Normal disk file ∗/
#defineLF_LINK’1’/∗ Link to previously dumped file ∗/
#defineLF_SYMLINK’2’/∗ Symbolic link ∗/
#defineLF_CHR’3’/∗ Character special file ∗/
#defineLF_BLK’4’/∗ Block special file ∗/
#defineLF_DIR’5’/∗ Directory ∗/
#defineLF_FIFO’6’/∗ FIFO special file ∗/
#defineLF_CONTIG’7’/∗ Contiguous file ∗/
/∗ Further link types may be defined later. ∗/
 /∗ Bits used in the mode field - values in octal ∗/
#defineTSUID04000/∗ Set UID on execution ∗/
#defineTSGID02000/∗ Set GID on execution ∗/
#defineTSVTX01000/∗ Save text (sticky bit) ∗/
 /∗ File permissions ∗/
#defineTUREAD00400/∗ read by owner ∗/
#defineTUWRITE00200/∗ write by owner ∗/
#defineTUEXEC00100/∗ execute/search by owner ∗/
#defineTGREAD00040/∗ read by group ∗/
#defineTGWRITE00020/∗ write by group ∗/
#defineTGEXEC00010/∗ execute/search by group ∗/
#defineTOREAD00004/∗ read by other ∗/
#defineTOWRITE00002/∗ write by other ∗/
#defineTOEXEC00001/∗ execute/search by other ∗/

All characters in header records are represented using 8-bit characters in the local variant of ASCII.  Each field within the structure is contiguous; that is, there is no padding used within the structure.  Each character on the archive medium is stored contiguously. 
 
Bytes representing the contents of files (after the header record of each file) are not translated in any way and are not constrained to represent characters or to be in any character set. The tar(5) format does not distinguish text files from binary files, and no translation of file contents should be performed. 
 
The fields name, linkname, magic, uname, and gname are null-terminated character strings.  All other fields are zero-filled octal numbers in ASCII.  Each numeric field (of width w) contains w-2 digits, a space, and a null, except size and mtime, which do not contain the trailing null. 
 
The name field is the pathname of the file, with directory names (if any) preceding the file name, separated by slashes. 
 
The mode field provides nine bits specifying file permissions and three bits to specify the Set UID, Set GID and Save Text (TSVTX) modes.  Values for these bits are defined above.  When special permissions are required to create a file with a given mode, and the user restoring files from the archive does not hold such permissions, the mode bit(s) specifying those special permissions are ignored.  Modes which are not supported by the operating system restoring files from the archive will be ignored.  Unsupported modes should be faked up when creating an archive; e.g.  the group permission could be copied from the ‘other’ permission. 
 
The uid and gid fields are the user and group ID of the file owners, respectively. 
 
The size field is the size of the file in bytes; linked files are archived with this field specified as zero. 
 
The mtime field is the modification time of the file at the time it was archived.  It is the ASCII representation of the octal value of the last time the file was modified, represented as in integer number of seconds since January 1, 1970, 00:00 Coordinated Universal Time. 
 
The chksum field is the ASCII representaion of the octal value of the simple sum of all bytes in the header record.  Each 8-bit byte in the header is treated as an unsigned value.  These values are added to an unsigned integer, initialized to zero, the precision of which shall be no less than seventeen bits.  When calculating the checksum, the chksum field is treated as if it were all blanks. 
 
The typeflag field specifies the type of file archived.  If a particular implementation does not recognize or permit the specified type, the file will be extracted as if it were a regular file.  As this action occurs, tar issues a warning to the standard error. 

LF_NORMAL or LF_OLDNORMAL
represents a regular file. For backward compatibility, a typeflag value of LF_OLDNORMAL should be silently recognized as a regular file.  New archives should be created using LF_NORMAL.  Also, for backward compatability, tar treats a regular file whose name ends with a slash as a directory. 

LF_LINK
represents a file linked to another file, of any type, previously archived.  Such files are identified in Unix by each file having the same device and inode number.  The linked-to name is specified in the linkname field with a trailing null. 

LF_SYMLINK
represents a symbolic link to another file.  The linked-to name is specified in the linkname field with a trailing null. 

LF_CHR or LF_BLK
represent character special files and block special files respectively. In this case the devmajor and devminor fields will contain the major and minor device numbers respectively.  Operating systems may map the device specifications to their own local specification, or may ignore the entry. 

LF_DIR
specifies a directory or sub-directory.  The directory name in the name field should end with a slash.  On systems where disk allocation is performed on a directory basis the size field will contain the maximum number of bytes (which may be rounded to the nearest disk block allocation unit) which the directory may hold.  A size field of zero indicates no such limiting.  Systems which do not support limiting in this manner should ignore the size field. 

LF_FIFO
specifies a FIFO special file.  Note that the archiving of a FIFO file archives the existence of this file and not its contents.

LF_CONTIG
specifies a contiguous file, which is the same as a normal file except that, in operating systems which support it, all its space is allocated contiguously on the disk.  Operating systems which do not allow contiguous allocation should silently treat this type as a normal file.

‘A’ − ‘Z’
are reserved for custom implementations.  None are used by this version of the tar program. 

othervalues are reserved for specification in future revisions of the P1003 standard, and should not be used by any tar program. 

The magic field indicates that this archive was output in the P1003 archive format.  If this field contains TMAGIC, then the uname and gname fields will contain the ASCII representation of the owner and group of the file respectively.  If found, the user and group ID represented by these names will be used rather than the values contained within the uid and gid fields.  User names longer than TUNMLEN-1 or group names longer than TGNMLEN-1 characters will be truncated. 

SEE ALSO

tar(1), ar(5), cpio(5), dump(8), restor(8), restore(8)

BUGS

Names or link names longer than NAMSIZ-1 characters cannot be archived. 
 
This format does not yet address multi-volume archives.

NOTES

This manual page was adapted by John Gilmore from Draft 6 of the P1003 specification

INTEGRATED SOLUTIONS 4.3 BSD  —  15 October 1987