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Document base-256 representation in GNU format

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Problem reported by Rodrigo Queiro in:
https://lists.gnu.org/r/bug-tar/2017-11/msg00018.html
* doc/intern.texi (Standard, Extensions):
Document base-256 representations.
This commit is contained in:
Paul Eggert
2017-11-18 08:39:33 -08:00
parent 9ed0a6ba35
commit 37d7ce16c5
1 changed files with 18 additions and 6 deletions
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doc/intern.texi
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@@ -87,6 +87,8 @@ The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
@code{gname} are null-terminated character strings. All other fields
are zero-filled octal numbers in ASCII. Each numeric field of width
@var{w} contains @var{w} minus 1 digits, and a null.
(In the extended @acronym{GNU} format, the numeric fields can take
other forms.)
The @code{name} field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.
@@ -112,14 +114,12 @@ be ignored.
The @code{size} field is the size of the file in bytes; linked files
are archived with this field specified as zero.
The @code{mtime} field is the data 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's contents were modified, represented
as an integer number of
The @code{mtime} field represents the data modification time of the file at
the time it was archived. It represents the integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.
The @code{chksum} field is the ASCII representation of the octal value
of the simple sum of all bytes in the header block. Each 8-bit
The @code{chksum} field represents
the simple sum of all bytes in the header block. Each 8-bit
byte in the header is added to an unsigned integer, initialized to
zero, the precision of which shall be no less than seventeen bits.
When calculating the checksum, the @code{chksum} field is treated as
@@ -310,6 +310,18 @@ of an archive should have this type.
@end table
For fields containing numbers or timestamps that are out of range for
the basic format, the @acronym{GNU} format uses a base-256
representation instead of an ASCII octal number. If the leading byte
is 0xff (255), all the bytes of the field (including the leading byte)
are concatenated in big-endian order, with the result being a negative
number expressed in two's complement form. If the leading byte is
0x80 (128), the non-leading bytes of the field are concatenating in
big-endian order, with the result being a positive number expressed in
binary form. Leading bytes other than 0xff, 0x80 and ASCII octal
digits are reserved for future use, as are base-256 representations of
values that would be in range for the basic format.
You may have trouble reading a @acronym{GNU} format archive on a
non-@acronym{GNU} system if the options @option{--incremental} (@option{-G}),
@option{--multi-volume} (@option{-M}), @option{--sparse} (@option{-S}), or @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) were