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Document timestamp resolution more accurately

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Paul Eggert
2026-02-18 12:41:19 -08:00
parent 678dbc679a
commit c8f2800b9b
1 changed files with 24 additions and 13 deletions
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doc/tar.texi
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@@ -9615,7 +9615,7 @@ The most frequently used formats are (in alphabetical order):
@table @asis
@item gnu
Format used by @GNUTAR{} versions up to 1.13.25. This format derived
Format used by @GNUTAR{} versions starting with 1.13. This format derived
from an early @acronym{POSIX} standard, adding some improvements such as
sparse file handling and incremental archives. Unfortunately these
features were implemented in a way incompatible with other archive
@@ -9623,6 +9623,9 @@ formats.
Archives in @samp{gnu} format are able to hold file names of unlimited
length.
However, file timestamps in this format are integers, so the timestamps
have only one-second resolution; any subsecond information is lost.
Also, UIDs and GIDs are limited to 64 bits, and device numbers to 63 bits.
@item oldgnu
Format used by @GNUTAR{} of versions prior to 1.12.
@@ -9638,7 +9641,7 @@ File names and symbolic links can contain at most 100 bytes.
@item
File sizes must be less than 8 GiB (@math{2^33} bytes = 8,589,934,592 bytes).
@item
File timestamps must be nonnegative and less than @math{2^33} seconds
File timestamps must be nonnegative integers less than @math{2^33} seconds
since the Epoch, corresponding to the range from 1970-01-01 00:00:00
(inclusive) to 2242-03-16 12:56:32 (exclusive) UTC, ignoring leap seconds.
@item
@@ -9674,7 +9677,7 @@ Symbolic links can contain at most 100 bytes.
@item
File sizes must be less than 8 GiB (@math{2^33} bytes = 8,589,934,592 bytes).
@item
File timestamps must be nonnegative and less than @math{2^33} seconds
File timestamps must nonnegative integers less than @math{2^33} seconds
since the Epoch, corresponding to the range from 1970-01-01 00:00:00
(inclusive) to 2242-03-16 12:56:32 (exclusive) UTC, ignoring leap seconds.
@item
@@ -9690,7 +9693,8 @@ does not produce them.
@item posix
The format defined by @acronym{POSIX.1-2001} and later. This is the
most flexible and feature-rich format. It does not impose arbitrary
restrictions on file sizes or timestamps or file name lengths.
restrictions on file sizes, file timestamp range or resolution,
file name lengths, and number of bits in UIDs or device numbers.
This format is more
recent, so some @command{tar} implementations cannot handle it properly.
However, any @command{tar} implementation able to read @samp{ustar}
@@ -9703,16 +9707,23 @@ of @GNUTAR{}.
@end table
The following table summarizes the limitations of each of these
formats:
The following table summarizes the limits of each of these formats.
``Size'' and ``Date'' stands for file size and date,
``Res'' for file timestamp resolution (where ``1 s'' means one second),
``Name'' for length of file names,
``Sym'' for length of symbolic links,
``Un'' for the number of bits in a UID or GID, and
``Dn'' for the number of bits in a device number.
A ``-'' means the archive format imposes no practical limits,
though the operating system may have limits of its own.
@multitable {Format} {no limit} {File Size} {File Date} {File Name} {no limit}
@headitem Format @tab UID @tab File Size @tab File Date @tab File Name @tab Devn
@item gnu @tab 1.8e19 @tab no limit @tab no limit @tab no limit @tab 63
@item oldgnu @tab 1.8e19 @tab no limit @tab no limit @tab no limit @tab 63
@item v7 @tab 2097151 @tab < 8 GiB @tab 1970--2242 @tab 99 @tab n/a
@item ustar @tab 2097151 @tab < 8 GiB @tab 1970--2242 @tab 255 @tab 21
@item posix @tab no limit @tab no limit @tab no limit @tab no limit @tab no limit
@multitable {Format} {< 8 GiB} {1970--2242} {Res} {Name} {Sym} {Un} {Dn}
@headitem Format @tab Size @tab Date @tab Res @tab Name @tab Sym @tab Un @tab Dn
@item v7 @tab < 8 GiB @tab 1970--2242 @tab 1 s @tab 100 @tab 100 @tab 21 @tab n/a
@item ustar @tab < 8 GiB @tab 1970--2242 @tab 1 s @tab 255 @tab 100 @tab 21 @tab 21
@item oldgnu @tab - @tab - @tab 1 s @tab - @tab - @tab 64 @tab 63
@item gnu @tab - @tab - @tab 1 s @tab - @tab - @tab 64 @tab 63
@item posix @tab - @tab - @tab - @tab - @tab - @tab - @tab -
@end multitable
The default format for @GNUTAR{} is defined at compilation