Re: On time64 and Large File Support

[Paul Eggert]

On 12/28/22 20:02, Zack Weinberg wrote:

Please revert that part of your follow-up patch.
OK, I reverted all that patch, except for the further changes you 
requested, plus some minor quoting and version-number fixes in comments.

Is there any
chance you could send a wdiff to the list, after restoring the text
you took out because you took out the _REQUIRED variants?

Sure, I'm attaching a proposed patch to Autoconf master documentation in 
two forms. The first is a simple "git format-patch" file; the second is 
the output of "git diff --color=always --word-diff=color".

I want it to be
crystal clear from the text of configure.ac that it bombed out because
package X considers 64-bit time_t and/or off_t a requirement.

Although I don't think this feature is useful, I don't have to use it so 
we can leave it in.

Here's why I'm not planning to use it. I help maintain (for example) GNU 
Tar, where file sizes and timestamps are crucial. But if I change GNU 
Tar to use AC_SYS_LARGEFILE_REQUIRED, people won't be able to build GNU 
Tar on ancient platforms that support file sizes only up to 2 GiB. 
Nobody will benefit from this: the very few users who'd care (computer 
museum curators, say) already know their systems are limited, and will 
simply build older versions of GNU Tar that don't use 
AC_SYS_LARGEFILE_REQUIRED, or will edit away the "_REQUIRED" before 
building. So adding the _REQUIRED will harm a very small set of users, 
will increase my maintenance burden a bit, and will benefit essentially 
nobody.

To put it another way: we've all gotten by without needing 
AC_SYS_LARGEFILE_REQUIRED for 25 years, and there's no reason for us to 
start needing it now even though it's imperative for any serious 
application that deals with files.

The situation for AC_SYS_YEAR2038_REQUIRED will likely be similar.

As things currently stand, I plan to use AC_SYS_YEAR2038 instead of 
AC_SYS_LARGEFILE in GNU Tar and similar applications. (They'll use 
Gnulib so they'll get Autoconf 2.72-compatible AC_SYS_YEAR2038 even with 
older Autoconf.) Perhaps other maintainers will want to use the 
*_REQUIRED macros, but as far as I can see my advice will be to avoid 
them as being more trouble than they're worth.


I *intended* to make it clear that they are not orthogonal in
practice, but it is not logically necessary for them to be coupled,
and it is, I think, easier to understand what
AC_SYS_{LARGEFILE,YEAR2038} do if we document them as abstractly

Unfortunately the documenation currently on Autoconf master makes for a 
lot of confusion and misimpression. It's more important for 
documentation to agree with what the code actually does, than for it to 
present an abstract picture of what we'd like the code to do eventually. 
The abovementioned patches fix this.


In the new year, I can look into the possibility of decoupling the
macros’ implementations.

I don't see how that would be possible. On the only platforms where 
--enable-year2038 matters, you cannot have year-2038 support without 
also having large-file support.

The platforms in question are glibc 2.34+ on 32-bit ARM and x86. The 
glibc community considered making the two things orthogonal but rejected 
that alternative as being considerably more trouble than it was worth. I 
think it unlikely for other 32-bit OS suppliers to decide differently.


I think you’re still writing documentation with application-colored
glasses on, making it sound like --enable-year2038 has no negative
implications whatsoever.

That's a bit unfair. The proposed documentation does mention the 
compatibility implications; it merely uses the same level of caution for 
both --enable-largefile and --enable-year2038, and it doesn't overhype 
the time_t issue with an undeserved big "Caution:" inbold.

Although I plead guilty in being close to users (many who need year 2038 
support now for obvious reasons) I'm well aware of the implication for 
libraries. When I implemented AC_SYS_LARGEFILE back in the 1990s, there 
were similar compatibility concerns with off_t that I also took 
seriously. In the end, the need for large-file support outweighed the 
backward-compatibility hassles and AC_SYS_LARGEFILE has been a success 
in that people have used AC_SYS_LARGEFILE for years and sure there are 
occasionally glitches but the benefits far outweigh the costs.

AC_SYS_YEAR2038 will be similar. Not identical of course, but similar. 
We've done this sort of thing before and have experience.


I do not believe we have consensus
for AC_SYS_LARGEFILE to become a synonym for AC_SYS_YEAR2038, not even
“in a future Autoconf version.”

The proposed documentation patches contain a simple plan for how 
Autoconf can plausibly survive through the year 2038. Is there a better 
plan? If so, I'd like to hear it. If not, then let's use this plan. 
Given the long delay between Autoconf versions and end uses of 
downstream software, we need a plan now, not years from now. The plan 
doesn't have to be perfect, nor does it need to be cast in stone. But 
there needs to be a plan.
From 3e9f1159ae9145c50d048a74422dd8464a6a8f6f Mon Sep 17 00:00:00 2001
From: Paul Eggert <egg...@cs.ucla.edu>
Date: Fri, 30 Dec 2022 12:48:39 -0800
Subject: [PATCH] Improve year-2038 documentation
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* NEWS, doc/autoconf.texi (System Services):
Improve documentation for behavior of largefile and year-2038 support.
Say that in the current implementation, year-2038 support
requires largefile support.  Say that year-2038 support
matters only for GNU/Linux glibc 2.34+ on 32-bit x86 and ARM.
Prefer brevity when this does not hurt understandability;
for example, prefer active to passive voice.
Prefer “wider” to “larger” when talking about the number of
bits in an integer, as this terminology is more standard.
Tone down the wording in warnings about enabling year-2038 support,
use similar wording in warnings about enabling largefile support,
and warn also about disabling largefile and year-2038 support.
No need for @emph.  Also mention rlim_t.
Be a bit more careful about saying “2 GiB” rather than “2 GB”.
Mention that a future version of Autoconf might change
AC_SYS_LARGEFILE to default to --enable-year2038, since
something has gotta happen before 2038.
Coalesce descriptions of --enable-largefile and --enable-year2038
to simplify documentation.  Mention that the only system where
AC_SYS_LARGEFILE changes CC is IRIX and that these systems
are obsolete.  Say that ‘stat’ can fail due to time_t
overflow.  Say that you can’t portably print time_t with %ld.
Say that binary compatibilty problems also can occur when one
library is linking to amother; it’s not just apps vs libraries.
Mention the possibility of modifying libraries to support both
32- and 64-bit interfaces.  Warn more consistently about
ABI compatibility issues, but put the bulk of this text
in one location that the other locations refer to.
---
 NEWS              |  57 ++++++++--------
 doc/autoconf.texi | 165 ++++++++++++++++++++++++----------------------
 2 files changed, 113 insertions(+), 109 deletions(-)

diff --git a/NEWS b/NEWS
index eafd7d69..0f71628d 100644
--- a/NEWS
+++ b/NEWS
@@ -27,36 +27,33 @@ GNU Autoconf NEWS - User visible changes.
 
 ** New features
 
-*** New macros AC_SYS_YEAR2038 and AC_SYS_YEAR2038_REQUIRED.
-  These macros attempt to enlarge time_t to 64 bits, on systems where
-  it has historically been only 32 bits wide, and therefore (assuming
-  the usual Unix epoch) cannot represent dates after mid-January of
-  2038 (hence the names).  The difference between the two is that
-  AC_SYS_YEAR2038_REQUIRED unconditionally causes 'configure' to error
-  out if 64-bit time_t is not available.
-
-  Enlarging time_t to 64 bits is likely to have the side effect of
-  enlarging off_t and related types to 64 bits as well, as if you
-  had used AC_SYS_LARGEFILE.  See the manual for details.
-
-  Library authors should be cautious about adding these macros to
-  their configure scripts; they can break binary backward compatibility.
-
-*** New macro AC_SYS_LARGEFILE_REQUIRED.
-  This macro is the same as the existing AC_SYS_LARGEFILE except that
-  it will cause 'configure' to error out if 64-bit off_t is not available,
-  and it does not provide a --disable-largefile option.
-
-*** AC_SYS_LARGEFILE now optionally arranges to enlarge time_t.
-  As an experimental measure to make it easier to rebuild old programs
-  with support for dates after Jan 2038, if you regenerate any configure
-  script that uses AC_SYS_LARGEFILE (but not AC_SYS_YEAR2038) using
-  Autoconf 2.72, it will gain an --enable-year2038 option.  When the
-  program is configured with this option, time_t will be enlarged if
-  possible, as if AC_SYS_YEAR2038 had been used.
-
-  Using this option in a library build also potentially breaks binary
-  backward compatibility.
+*** New macro AC_SYS_YEAR2038.
+  This causes 'configure' to widen time_t if possible on systems where
+  time_t by default cannot represent file and other timestamps after
+  January 2038.  Widening is possible only on 32-bit GNU/Linux x86 and
+  ARM systems with glibc 2.34 or later.  To prevent widening,
+  configure with --disable-year2038.
+
+  This macro also has the effects as AC_SYS_LARGEFILE, because in
+  practice time_t cannot be widened without large-file sypport.
+
+  Application and library builders should take care that packages
+  configured with --enable-year2038 and --disable-year2038 options
+  are configured consistently, to avoid breaking binary compatibility.
+  This is similar to longstanding consistency requirements with
+  --enable-largefile and --disable-largefile.
+
+*** AC_SYS_LARGEFILE now optionally arranges to widen time_t.
+  It now acts like AC_SYS_YEAR2038, except 'configure' defaults to
+  --disable-year2038 unless AC_SYS_YEAR2038 is also present.
+  As with AC_SYS_YEAR2038, application and library builders should
+  configure consistently.
+
+*** New macros AC_SYS_LARGEFILE_REQUIRED and AC_SYS_YEAR2038_REQUIRED.
+  These act like AC_SYS_LARGEFILE and AC_SYS_YEAR2038 respectively,
+  except that they require large-file and year-2038 support respectively.
+  As with AC_SYS_YEAR2038, application and library builders should
+  configure consistently.
 
 *** AC_USE_SYSTEM_EXTENSIONS now enables C23 Annex F extensions
   by defining __STDC_WANT_IEC_60559_EXT__.
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index df96280b..ce214284 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -8798,71 +8798,109 @@ if the system supports @samp{#!}, @samp{no} if not.
 @defmac AC_SYS_LARGEFILE
 @acindex{SYS_LARGEFILE}
 @cvindex _FILE_OFFSET_BITS
+@cvindex _TIME_BITS
 @ovindex CC
 @cindex Large file support
 @cindex LFS
-If the default @code{off_t} type is a 32-bit integer, and therefore
-cannot be used to work with files larger than 4 gigabytes, arrange to
-make a larger @code{off_t} available, if the system supports this.
-Several other types related to the sizes of files and file systems will
-also be enlarged: @code{ino_t}, @code{blkcnt_t}, @code{fsblkcnt_t},
-@code{fsfilcnt_t}, and possibly @code{dev_t}.
-
-If a large @code{off_t} is available (whether or not any arrangements
-were necessary), the shell variable @code{ac_have_largefile} will be set
-to @samp{yes}; if not, it will be set to @samp{no}.
-
-Preprocessor macros will be defined if necessary to make a larger
-@code{off_t} available.  (For example, on many systems the macro
-@code{_FILE_OFFSET_BITS} will be defined.)  Some of these macros only
-work if they are defined before the first system header is included;
+If the default @code{off_t} type is a 32-bit integer,
+and therefore cannot be used with files 2 GiB or larger,
+make a wider @code{off_t} available if the system supports it.
+Similarly, widen other types related to sizes of files and file systems
+if possible.  These types may include @code{blkcnt_t}, @code{dev_t},
+@code{ino_t}, @code{fsblkcnt_t}, @code{fsfilcnt_t}, and @code{rlim_t}.
+
+Also, arrange for a @command{configure} option @code{--enable-year2038}
+to request widening the type @code{time_t} as needed to represent file
+wand other timestamps after January 2038.  This widening is possible
+only on 32-bit GNU/Linux x86 and ARM systems with glibc 2.34 or later.
+If year-2038 support is requested but @command{configure} fails to find a way
+to widen @code{time_t} and inspection of the system suggests that
+this feature is available somehow, @command{configure} will error out.
+If you want the default to be @code{--enable-year2038}, you can use
+@code{AC_SYS_YEAR2038} instead of @code{AC_SYS_LARGEFILE}.
+In other words, older packages that have long used @code{AC_SYS_LARGEFILE}
+can have year-2038 support on 32-bit GNU/Linux x86 and ARM systems either by
+regenerating @file{configure} with current Autoconf and configuring with
+@option{--enable-year2038}, or by using @code{AC_SYS_YEAR2038} and
+configuring without @option{--disable-year2038}.
+A future version of Autoconf might change the @code{AC_SYS_LARGEFILE}
+default to @code{--enable-year2038}; if and when that happens,
+@code{AC_SYS_LARGEFILE} and @code{AC_SYS_YEAR2038} will become equivalent.
+@xref{AC_SYS_YEAR2038}.
+
+Set the shell variable @code{ac_have_largefile} to to @samp{yes} or
+@code{no} depending on whether a wide @code{off_t} is available,
+regardless of whether arrangements were necessary.
+Similarly, set the shell variable @code{ac_have_year2038} to @code{yes}
+or @code{no} depending on whether a wide-enough @code{time_t} is available.
+
+Define preprocessor macros if necessary to make types wider;
+for example, on GNU/Linux systems the macros @code{_FILE_OFFSET_BITS}
+and @code{_TIME_BITS} can be defined.  Some of these macros work only if
+defined before the first system header is included;
 therefore, when using this macro in concert with
 @code{AC_CONFIG_HEADERS}, make sure that @file{config.h} is included
 before any system headers.
 
-On a few older systems, the output variable @code{CC} will also be
-changed to add special compiler options that are needed to enable large
-@code{off_t}.
+On obsolete IRIX systems, also change the output variable @code{CC} to
+add compiler options needed for wide @code{off_t}.
 
 Large-file support can be disabled by configuring with the
-@option{--disable-largefile} option.  Note that this has no effect on
-systems where @code{off_t} is 64 bits or larger by default.  Disabling
-large-file support can have surprising effects, such as causing
-functions like @code{readdir} and @code{stat} to fail on small files
-(because their @emph{inode numbers} are unrepresentable).
+@option{--disable-largefile} option, and year-2038 support can
+be enabled and disabled via the @option{--enable-year2038} and
+@option{--disable-year2038} options.  These options have no effect on
+systems where types are wide enough by default.
+Large-file support is required for year-2038 support: if you configure
+with @option{--disable-largefile} on a platform with 32-bit
+@code{time_t}, then year-2038 support is not available.
+
+Disabling large-file or year-2038 support can have surprising effects,
+such as causing functions like @code{readdir} and @code{stat} to fail
+even on a small file because its inode number or timestamp is out of range.
 
 Regardless of whether you use this macro, portable programs should not
 assume that any of the types listed above fit into a @code{long int}.
-For example, it is not correct to print an arbitrary @code{off_t} value
-@code{X} with @code{printf ("%ld", (long int) X)}.
+For example, it is not portable to print an arbitrary @code{off_t} or
+@code{time_t} value @code{X} with @code{printf ("%ld", (long int) X)}.
 
-Note that the standard C library functions @code{fseek} and @code{ftell}
+The standard C library functions @code{fseek} and @code{ftell}
 do not use @code{off_t}.  If you need to use either of these functions,
 you should use @code{AC_FUNC_FSEEKO} as well as @code{AC_SYS_LARGEFILE},
-and then use their Posix replacements @code{fseeko} and @code{ftello},
-which @emph{do} use @code{off_t}, when available.  @xref{AC_FUNC_FSEEKO}.
-
-As of Autoconf 2.72, @code{AC_SYS_LARGEFILE} also @emph{optionally}
-arranges to enlarge @code{time_t}.  This is to make it easier to build
-programs that support timestamps after 2038; many configure scripts will
-not need to be modified, only regenerated with newer Autoconf.  When
-@code{AC_SYS_LARGEFILE} is used, and @code{AC_SYS_YEAR2038} is
-@emph{not} used, @code{time_t} will normally be left at the system's
-default size, but you can request it be enlarged by configuring with the
-@option{--enable-year2038} option.  (When @code{AC_SYS_YEAR2038} is also
-used, @code{time_t} is enlarged if possible.  @xref{AC_SYS_YEAR2038}.)
+and then use their Posix replacements @code{fseeko} and @code{ftello}.
+@xref{AC_FUNC_FSEEKO}.
+
+When using @code{AC_SYS_LARGEFILE} in different packages that are linked
+together and that have interfaces that depend on the width of @code{off_t},
+@code{time_t} or related types, the simplest thing is to configure all
+components the same way.  For example, if an application uses
+@code{AC_SYS_LARGEFILE} and is configured with
+@option{--enable-year2038}, libraries it links to with an @code{off_t}-
+or @code{time_t}-dependent interface should be configured equivalently.
+Alternatively, you can modify libraries to support both 32- and 64-bit
+interfaces though this is more work and few libraries other than the C
+library itself are modified in this way.
+
+Applications and libraries should be configured compatibly.
+If @code{off_t}, @code{time_t} or related types appear in a library's
+public interface, enabling or disabling the library's large-file or
+year-2038 support may break binary compatibility with applications or
+with other libraries.  Similarly, if an application links to a such a
+library, enabling or disabling the application's large-file support may
+break binary compatibility with that library.
 @end defmac
 
 @defmac AC_SYS_LARGEFILE_REQUIRED
 @acindex{SYS_LARGEFILE_REQUIRED}
 This macro has the same effect as @code{AC_SYS_LARGEFILE},
 but also declares that the program being configured
-@emph{requires} support for large files.
+requires support for large files.
 If a large @code{off_t} is unavailable,
 @command{configure} will error out.
 The @option{--disable-largefile} option will not be available.
-@end defmac
 
+Large-file and year-2038 support for applications and libraries should
+be configured compatibly.  @xref{AC_SYS_LARGEFILE}.
+@end defmac
 
 @anchor{AC_SYS_LONG_FILE_NAMES}
 @defmac AC_SYS_LONG_FILE_NAMES
@@ -8885,55 +8923,24 @@ system.  If so, set the shell variable @code{ac_cv_sys_posix_termios} to
 @anchor{AC_SYS_YEAR2038}
 @defmac AC_SYS_YEAR2038
 @acindex{SYS_YEAR2038}
-@cvindex _TIME_BITS
 @cindex Year 2038
-If the default @code{time_t} type is a signed 32-bit integer,
-and therefore (assuming the usual Unix epoch) cannot represent
-timestamps after mid-January of 2038, arrange to make a larger
-@code{time_t} available, if the system supports this.
-
-If a large @code{time_t} is available (whether or not any arrangements
-were necessary), the shell variable @code{ac_have_year2038} will be set
-to @samp{yes}; if not, it will be set to @samp{no}.
-
-Preprocessor macros will be defined if necessary to make a larger
-@code{time_t} available.  (For example, on some systems the macro
-@code{_TIME_BITS} will be defined.)  Some of these macros only work if
-they are defined before the first system header is included; therefore,
-when using this macro in concert with @code{AC_CONFIG_HEADERS}, make
-sure that @file{config.h} is included before any system headers.
-
-Support for timestamps after 2038 can be disabled by configuring with
-the @option{--disable-year2038} option.  Note that this has no effect on
-systems where @code{time_t} is 64 bits or larger by default.
-If this option is @emph{not} given, and @command{configure} fails to
-find a way to enable a large @code{time_t}, but inspection of the
-system suggests that this feature is available @emph{somehow}, it will
-error out.
-
-Regardless of whether you use this macro, portable programs should not
-assume that @code{time_t} fits into @code{long int}.  For example, it is
-not correct to print an arbitrary @code{time_t} value @code{X} with
-@code{printf ("%ld", (long int) X)}.
-
-@strong{Caution:} If you are developing a shared library, and
-@code{time_t} appears anywhere in your library's public interface, use
-of this macro may break binary compatibility with older executables.
+This is like @code{AC_SYS_LARGEFILE} except it defaults to enabling
+instead of disabling year-2038 support.  Year-2038 support for
+applications and libraries should be configured compatibly.
+@xref{AC_SYS_LARGEFILE}.
 @end defmac
 
 @defmac AC_SYS_YEAR2038_REQUIRED
 @acindex{SYS_YEAR2038_REQUIRED}
 This macro has the same effect as @code{AC_SYS_YEAR2038},
 but also declares that the program being configured
-@emph{requires} support for timestamps after mid-January of 2038.
+requires support for timestamps after mid-January of 2038.
 If a large @code{time_t} is unavailable,
-@command{configure} will @emph{unconditionally} error out
-(unlike the behavior of @code{AC_SYS_YEAR2038}).
+@command{configure} will unconditionally error out.
 The @option{--disable-year2038} option will not be available.
 
-@strong{Caution:} If you are developing a shared library, and
-@code{time_t} appears anywhere in your library's public interface, use
-of this macro may break binary compatibility with older executables.
+Year-2038 support for applications and libraries should be configured
+compatibly.  @xref{AC_SYS_YEAR2038}.
 @end defmac
 
 @node C and Posix Variants
-- 
2.38.1

diff --git a/NEWS b/NEWS
index eafd7d69..0f71628d 100644
--- a/NEWS
+++ b/NEWS
@@ -27,36 +27,33 @@ GNU Autoconf NEWS - User visible changes.

** New features

*** New macros AC_SYS_YEAR2038 and AC_SYS_YEAR2038_REQUIRED.
  These macros attemptmacro AC_SYS_YEAR2038.
  This causes 'configure' to enlargewiden time_t 
to 64 bits,if possible on systems where
  it has historically been only 32 bits wide, and therefore (assuming
  the usual Unix epoch)time_t by default cannot represent 
datesfile and other timestamps after
  mid-January of
  2038 (hence the names).  The difference between the twoJanuary 
2038.  Widening is that
  AC_SYS_YEAR2038_REQUIRED unconditionally causes 'configure' to error
  out if 64-bit time_t is not available.

  Enlarging time_t to 64 bits is likely to have the side effect of
  enlarging off_tpossible only on 32-bit GNU/Linux x86 and
  related types to 64 bits as well, as if you
  had used AC_SYS_LARGEFILE.  See the manual for details.

  Library authors should be cautious about adding these macros to
  theirARM systems with glibc 2.34 or later.  To prevent 
widening,
  configure scripts; they can break binary backward compatibility.

*** New macro AC_SYS_LARGEFILE_REQUIRED.with --disable-year2038.

  This macro isalso has the sameeffects as 
the existing AC_SYS_LARGEFILE exceptAC_SYS_LARGEFILE, because in
  practice time_t cannot be widened without large-file sypport.

  Application and library builders should take care that it will 
cause 'configure'packages
  configured with --enable-year2038 and --disable-year2038 options
  are configured consistently, to error out if 64-bit 
off_tavoid breaking binary compatibility.
  This is not available,similar to longstanding 
consistency requirements with
  --enable-largefile and it does not provide a --disable-largefile 
option.--disable-largefile.

*** AC_SYS_LARGEFILE now optionally arranges to enlargewiden 
time_t.
  As an experimental measure to make it easierIt now acts like 
AC_SYS_YEAR2038, except 'configure' defaults to
  rebuild old programs--disable-year2038 unless AC_SYS_YEAR2038 is 
also present.
  As with support for dates after Jan 2038, if you regenerate 
anyAC_SYS_YEAR2038, application and library builders should
  configure script that usesconsistently.

*** New macros AC_SYS_LARGEFILE_REQUIRED and AC_SYS_YEAR2038_REQUIRED.
  These act like AC_SYS_LARGEFILE (but not AC_SYS_YEAR2038) 
using
  Autoconf 2.72, it will gain an --enable-year2038 option.  When the
  program is configured with this option, time_t will be enlarged if
  possible, as ifand AC_SYS_YEAR2038 had been used.

  Using this option in arespectively,
  except that they require large-file and year-2038 support 
respectively.
  As with AC_SYS_YEAR2038, application and library build also 
potentially breaks binary
  backward compatibility.builders should
  configure consistently.

*** AC_USE_SYSTEM_EXTENSIONS now enables C23 Annex F extensions
  by defining __STDC_WANT_IEC_60559_EXT__.
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index df96280b..ce214284 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -8798,71 +8798,109 @@ if the system supports @samp{#!}, @samp{no} 
if not.
@defmac AC_SYS_LARGEFILE
@acindex{SYS_LARGEFILE}
@cvindex _FILE_OFFSET_BITS
@cvindex _TIME_BITS
@ovindex CC
@cindex Large file support
@cindex LFS
If the default @code{off_t} type is a 32-bit integer,
and therefore cannot be usedto work with files larger than 4 
gigabytes, arrange to2 GiB or larger,
make a largerwider @code{off_t} 
available,available if the system supports this.
Severalit.
Similarly, widen other types related tothe sizes of files and 
file systems
will
also be enlarged: @code{ino_t},if possible.  These types may 
include @code{blkcnt_t}, @code{dev_t},
@code{ino_t}, @code{fsblkcnt_t}, @code{fsfilcnt_t}, and possibly 
@code{dev_t}.@code{rlim_t}.

Also, arrange for a @command{configure} option @code{--enable-year2038}
to request widening the type @code{time_t} as needed to represent file
wand other timestamps after January 2038.  This widening is possible
only on 32-bit GNU/Linux x86 and ARM systems with glibc 2.34 or later.
If year-2038 support is requested but @command{configure} fails to find 
a large @code{off_t}way
to widen @code{time_t} and inspection of the system suggests that
this feature is available (whethersomehow, 
@command{configure} will error out.
If you want the default to be @code{--enable-year2038}, you can use
@code{AC_SYS_YEAR2038} instead of @code{AC_SYS_LARGEFILE}.
In other words, older packages that have long used 
@code{AC_SYS_LARGEFILE}
can have year-2038 support on 32-bit GNU/Linux x86 and ARM systems either 
by
regenerating @file{configure} with current Autoconf and configuring with
@option{--enable-year2038}, or not any arrangements
were necessary),by using @code{AC_SYS_YEAR2038} and
configuring without @option{--disable-year2038}.
A future version of Autoconf might change the @code{AC_SYS_LARGEFILE}
default to @code{--enable-year2038}; if and when that happens,
@code{AC_SYS_LARGEFILE} and @code{AC_SYS_YEAR2038} will become 
equivalent.
@xref{AC_SYS_YEAR2038}.

Set the shell variable @code{ac_have_largefile}will be set to 
@samp{yes}; if not, it will beto @samp{yes} or
@code{no} depending on whether a wide @code{off_t} is available,
regardless of whether arrangements were necessary.
Similarly, set the shell variable @code{ac_have_year2038} to 
@samp{no}.

Preprocessor@code{yes}
or @code{no} depending on whether a wide-enough @code{time_t} is 
available.

Define preprocessor macroswill be defined if necessary to make 
a larger
@code{off_t} available.  (Fortypes wider;
for example, on manyGNU/Linux systems the 
macromacros @code{_FILE_OFFSET_BITS}
willand @code{_TIME_BITS} can be 
defined.)defined.  Some of these macrosonly work 
only ifthey are
defined before the first system header is included;
therefore, when using this macro in concert with
@code{AC_CONFIG_HEADERS}, make sure that @file{config.h} is included
before any system headers.

On a few olderobsolete IRIX systems, also change the 
output variable @code{CC}will also be
changed to
addspecial compiler optionsthat are needed to enable 
largefor wide @code{off_t}.

Large-file support can be disabled by configuring with the
@option{--disable-largefile} option.  Note that this hasoption, 
and year-2038 support can
be enabled and disabled via the @option{--enable-year2038} and
@option{--disable-year2038} options.  These options have no effect on
systems where @code{off_t} is 64 bits or largertypes are wide 
enough by default.
Large-file support is required for year-2038 support: if you configure
with @option{--disable-largefile} on a platform with 32-bit
@code{time_t}, then year-2038 support is not available.

Disabling large-file or year-2038 support can have surprising effects,
such as causing functions like @code{readdir} and @code{stat} to fail
even on a small files
(because their @emph{inode numbers} are unrepresentable).file 
because its inode number or timestamp is out of range.

Regardless of whether you use this macro, portable programs should not
assume that any of the types listed above fit into a @code{long int}.
For example, it is not correctportable to print an arbitrary 
@code{off_t} or
@code{time_t} value @code{X} with @code{printf ("%ld", (long int) X)}.

Note that theThe standard C library functions @code{fseek} and 
@code{ftell}
do not use @code{off_t}.  If you need to use either of these functions,
you should use @code{AC_FUNC_FSEEKO} as well as @code{AC_SYS_LARGEFILE},
and then use their Posix replacements @code{fseeko} and @code{ftello},
which @emph{do} use @code{off_t}, when available.@code{ftello}.
@xref{AC_FUNC_FSEEKO}.

As of Autoconf 2.72,When using @code{AC_SYS_LARGEFILE} 
also @emph{optionally}
arranges to enlarge @code{time_t}.  Thisin different packages that 
are linked
together and that have interfaces that depend on the width of 
@code{off_t},
@code{time_t} or related types, the simplest thing is tomake it 
easier to build
programs that support timestamps after 2038; many configure 
scripts will
not need to be modified, only regenerated with newer Autoconf.  
Whenall
components the same way.  For example, if an application uses
@code{AC_SYS_LARGEFILE}is used, and@code{AC_SYS_YEAR2038} is 
@emph{not} used, @code{time_t} will normallyconfigured with
@option{--enable-year2038}, libraries it links to with an @code{off_t}-
or @code{time_t}-dependent interface should be left at the 
system's
default size, butconfigured equivalently.
Alternatively, you can request it be enlarged by configuring with 
the
@option{--enable-year2038} option.  (When 
@code{AC_SYS_YEAR2038}modify libraries to support both 32- and 64-bit
interfaces though this is also
used,more work and few libraries other than the C
library itself are modified in this way.

Applications and libraries should be configured compatibly.
If @code{off_t}, @code{time_t} is enlargedor related types 
appear in a library's
public interface, enabling or disabling the library's large-file or
year-2038 support may break binary compatibility with applications or
with other libraries.  Similarly, if possible.  
@xref{AC_SYS_YEAR2038}.)an application links to a such a
library, enabling or disabling the application's large-file support may
break binary compatibility with that library.
@end defmac

@defmac AC_SYS_LARGEFILE_REQUIRED
@acindex{SYS_LARGEFILE_REQUIRED}
This macro has the same effect as @code{AC_SYS_LARGEFILE},
but also declares that the program being configured
@emph{requires}requires support for large files.
If a large @code{off_t} is unavailable,
@command{configure} will error out.
The @option{--disable-largefile} option will not be available.
@end defmac

Large-file and year-2038 support for applications and libraries should
be configured compatibly.  @xref{AC_SYS_LARGEFILE}.
@end defmac

@anchor{AC_SYS_LONG_FILE_NAMES}
@defmac AC_SYS_LONG_FILE_NAMES
@@ -8885,55 +8923,24 @@ system.  If so, set the shell variable 
@code{ac_cv_sys_posix_termios} to
@anchor{AC_SYS_YEAR2038}
@defmac AC_SYS_YEAR2038
@acindex{SYS_YEAR2038}
@cvindex _TIME_BITS
@cindex Year 2038
If the default @code{time_t} typeThis is a signed 32-bit 
integer,
and therefore (assuming the usual Unix epoch) cannot represent
timestamps after mid-January of 2038, arrange to make a larger
@code{time_t} available, if the system supports this.

If a large @code{time_t} is available (whether or not any arrangements
were necessary), the shell variable @code{ac_have_year2038} will be set
to @samp{yes}; if not,like @code{AC_SYS_LARGEFILE} except it 
will be setdefaults to @samp{no}.

Preprocessor macros will be defined if necessary to make a larger
@code{time_t} available.  (For example, on some systems the macro
@code{_TIME_BITS} will be defined.)  Someenabling
instead of these macros only work if
they are defined before the first system header is included; therefore,
when using this macro in concert with @code{AC_CONFIG_HEADERS}, make
sure that @file{config.h} is included before any system headers.

Supportdisabling year-2038 support.  Year-2038 support for
timestamps after 2038 can be disabled by configuring with
the @option{--disable-year2038} option.  Note that this has no effect on
systems where @code{time_t} is 64 bits or larger by default.
If this option is @emph{not} given,applications and 
@command{configure} fails to
find a way to enable a large @code{time_t}, but inspection of the
system suggests that this feature is available @emph{somehow}, it will
error out.

Regardless of whether you use this macro, portable 
programslibraries should not
assume that @code{time_t} fits into @code{long int}.  For example, it is
not correct to print an arbitrary @code{time_t} value @code{X} with
@code{printf ("%ld", (long int) X)}.

@strong{Caution:} If you are developing a shared library, and
@code{time_t} appears anywhere in your library's public interface, use
of this macro may break binary compatibility with older 
executables.be configured compatibly.
@xref{AC_SYS_LARGEFILE}.
@end defmac

@defmac AC_SYS_YEAR2038_REQUIRED
@acindex{SYS_YEAR2038_REQUIRED}
This macro has the same effect as @code{AC_SYS_YEAR2038},
but also declares that the program being configured
@emph{requires}requires support for timestamps after 
mid-January of 2038.
If a large @code{time_t} is unavailable,
@command{configure} will @emph{unconditionally}unconditionally 
error out
(unlike the behavior of @code{AC_SYS_YEAR2038}).out.
The @option{--disable-year2038} option will not be available.

@strong{Caution:} If you are developing a shared library,Year-2038 
support for applications and @code{time_t} appears anywhere in your 
library's public interface, use
of this macro may break binary compatibility with older 
executables.libraries should be configured
compatibly.  @xref{AC_SYS_YEAR2038}.
@end defmac

@node C and Posix Variants