Files Reference

tar.h File

Purpose

Contains definitions for flags used in the tar archive header.

Description

The /usr/include/tar.h file contains extended definitions used in the typeflag and mode fields of the tar archive header block. The file also provides values for the required POSIX entries.

tar Archive Header Block

Every file archived using the tar command is represented by a header block describing the file, followed by zero or more blocks that give the contents of the file. The end-of-archive indicator consists of two blocks filled with binary zeros. Each block is a fixed size of 512 bytes.

Blocks are grouped for physical I/O operations and groups can be written using a single write subroutine operation. On magnetic tape, the result of this write operation is a single tape record. The last record is always a full 512 bytes. Blocks after the end-of-archive zeros contain undefined data.

The header block structure is shown in the following table. All lengths and offsets are in decimal.

Names are preserved only if the characters are chosen from the POSIX portable file-name character set or if the same extended character set is used between systems. During a read operation, a file can be created only if the original file can be accessed using the open, stat, chdir, fcntl, or opendir subroutine.

Header Block Fields

Each field within the header block and each character on the archive medium are contiguous. There is no padding between fields. More information about the specific fields and their values follows:

name

The file's path name is created using this field, or by using this field in connection with the prefix field. If the prefix field is included, the name of the file is prefix/name. This field is null-terminated unless every character is non-null.

mode

Provides 9 bits for file permissions and 3 bits for SUID, SGID, and SVTX modes. All values for this field are in octal. During a read operation, the designated mode bits are ignored if the user does not have equal (or higher) permissions or if the modes are not supported. Numeric fields are terminated with a space and a null byte. The tar.h file contains the following possible values for this field:

Flag	Octal	Description
TSUID	04000	Set user ID on execution.
TSGID	02000	Set group ID on execution.
TSVTX	01000	Reserved.
TUREAD	00400	Read by owner.
TUWRITE	00200	Write by owner.
TUEXEC	00100	Execute or search by owner.
TGREAD	00040	Read by group.
TGWRITE	00020	Write by group.
TGEXEC	00010	Execute or search by group.
TOREAD	00004	Read by others.
TOWRITE	00002	Write by others.
TOEXEC	00001	Execute or search by other.

uid

Extracted from the corresponding archive fields unless a user with appropriate privileges restores the file. In that case, the field value is extracted from the password and group files instead. Numeric fields are terminated with a space and a null byte.

gid

Extracted from the corresponding archive fields unless a user with appropriate privileges restores the file. In that case, the field value is extracted from the password and group files instead. Numeric fields are terminated with a space and a null byte.

size

Value is 0 when the typeflag field is set to LNKTYPE. This field is terminated with a space only.

mtime

Value is obtained from the modification-time field of the stat subroutine. This field is terminated with a space only.

chksum

On calculation, the sum of all bytes in the header structure are treated as spaces. Each unsigned byte is added to an unsigned integer (initialized to 0) with at least 17-bits precision. Numeric fields are terminated with a space and a null byte.

typeflag

The tar.h file contains the following possible values for this field:

Flag	Value	Description
REGTYPE	'0'	Regular file.
AREGTYPE	'\0'	Regular file.
LNKTYPE	'1'	Link.
SYMTYPE	'2'	Reserved.
CHRTYPE	'3'	Character special.
BLKTYPE	'4'	Block special.
DIRTYPE	'5'	Directory. In this case, the size field has no meaning.
FIFOTYPE	'6'	FIFO special. Archiving a FIFO file archives its existence, not contents.
CONTTYPE	'7'	Reserved.

If other values are used, the file is extracted as a regular file and a warning issued to the standard error output. Numeric fields are terminated with a space and a null byte.

The LNKTYPE flag represents a link to another file, of any type, previously archived. Such linked-to files are identified by each file having the same device and file serial number. The linked-to name is specified in the linkname field, including a trailing null byte.

linkname

Does not use the prefix field to produce a path name. If the path name or linkname value is too long, an error message is returned and any action on that file or directory is canceled. This field is null-terminated unless every character is non-null.

magic

Contains the TMAGIC value, reflecting the extended tar archive format. In this case, the uname and gname fields will contain the ASCII representation for the file owner and the file group. If a file is restored by a user with the appropriate privileges, the uid and gid fields are extracted from the password and group files (instead of the corresponding archive fields). This field is null-terminated.

version

Represents the version of the tar command used to archive the file. This field is terminated with a space only.

uname

Contains the ASCII representation of the file owner. This field is null-terminated.

gname

Contains the ASCII representation of the file group. This field is null-terminated.

devmajor

Contains the device major number. Terminated with a space and a null byte.

devminor

Contains the device minor number. Terminated with a space and a null byte.

prefix

If this field is non-null, the file's path name is created using the prefix/name values together. Null-terminated unless every character is non-null.

Related Information

The tar command.