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.
Table 1. Header Block Structure| Field Name |
Structure |
|---|
| name |
- Offset:
- 0
- Length in Bytes:
- 100
- Contents:
- File name without a / (slash)
|
| mode |
- Offset:
- 100
- Length in Bytes:
- 8
- Contents:
- File mode
|
| uid |
- Offset:
- 108
- Length in Bytes:
- MAXIMPL_LOGIN_NAME_MAX
- Contents:
- User ID
|
| gid |
- Offset:
- 116
- Length in Bytes:
- MAXIMPL_LOGIN_NAME_MAX
- Contents:
- Group ID
|
| size |
- Offset:
- 124
- Length in Bytes:
- 12
- Contents:
- Size in bytes
|
| mtime |
- Offset:
- 136
- Length in Bytes:
- 12
- Contents:
- Latest modification time
|
| cksum |
- Offset:
- 148
- Length in Bytes:
- 8
- Contents:
- File and header checksum
|
| typeflag |
- Offset:
- 156
- Length in Bytes:
- 1
- Contents:
- File type
|
| linkname |
- Offset:
- 157
- Length in Bytes:
- 100
- Contents:
- Linked path name or file name
|
| magic |
- Offset:
- 257
- Length in Bytes:
- 6
- Contents:
- Format representation for tar
|
| version |
- Offset:
- 263
- Length in Bytes:
- 3
- Contents:
- Version representation for tar
|
| uname |
- Offset:
- 265
- Length in Bytes:
- 32
- Contents:
- User name
|
| gname |
- Offset:
- 297
- Length in Bytes:
- 32
- Contents:
- Group name
|
| devmajor |
- Offset:
- 329
- Length in Bytes:
- 8
- Contents:
- Major device representation
|
| devminor |
- Offset:
- 337
- Length in Bytes:
- 8
- Contents:
- Minor device representation
|
| prefix |
- Offset:
- 345
- Length in Bytes:
- 155
- Contents:
- Path name without trailing slashes
|
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.