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* [glibc] doc: _TIME_BITS defaults may change
@ 2021-06-23 16:05 Paul Eggert
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From: Paul Eggert @ 2021-06-23 16:05 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=03caacbc7f3004ad21fc00bf883f00421a211130
commit 03caacbc7f3004ad21fc00bf883f00421a211130
Author: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed Jun 23 08:57:35 2021 -0700
doc: _TIME_BITS defaults may change
* NEWS: Don't imply the default will always be 32-bit.
* manual/creature.texi (Feature Test Macros):
Say that _TIME_BITS and _FILE_OFFSET_BITS defaults
may change in future releases.
Diff:
---
NEWS | 2 +-
manual/creature.texi | 57 ++++++++++++++++++++++++++--------------------------
2 files changed, 29 insertions(+), 30 deletions(-)
diff --git a/NEWS b/NEWS
index cadc40262f..d64be288da 100644
--- a/NEWS
+++ b/NEWS
@@ -34,7 +34,7 @@ Major new features:
* On powerpc64*, glibc can now be compiled without scv support using the
--disable-scv configure option.
-* Add support for 64-bit time_t for ABIs with defaults to 32-bit time_t.
+* Add support for 64-bit time_t for ABIs with defaults currently unchanged.
This is enabled with the _TIME_BITS preprocessor set to 64 and only
supported when LFS (_FILE_OFFSET_BITS=64) is also enabled. It is only
enabled for Linux and the full support requires a minimum version of 5.1.
diff --git a/manual/creature.texi b/manual/creature.texi
index 9fa658e9a7..d3c4fa4702 100644
--- a/manual/creature.texi
+++ b/manual/creature.texi
@@ -146,8 +146,8 @@ bit} interface available as an additional interface,
@code{_FILE_OFFSET_BITS} allows the @w{64 bit} interface to
replace the old interface.
-If @code{_FILE_OFFSET_BITS} is undefined, or if it is defined to the
-value @code{32}, nothing changes. The @w{32 bit} interface is used and
+If @code{_FILE_OFFSET_BITS} is defined to the
+value @code{32}, the @w{32 bit} interface is used and
types like @code{off_t} have a size of @w{32 bits} on @w{32 bit}
systems.
@@ -157,6 +157,11 @@ under different names (as they are with @code{_LARGEFILE64_SOURCE}).
Instead the old function names now reference the new functions, e.g., a
call to @code{fseeko} now indeed calls @code{fseeko64}.
+If the macro is not defined it currently defaults to @code{32}, but
+this default is planned to change due to a need to update
+@code{time_t} for Y2038 safety, and applications should not rely on
+the default.
+
This macro should only be selected if the system provides mechanisms for
handling large files. On @w{64 bit} systems this macro has no effect
since the @code{*64} functions are identical to the normal functions.
@@ -166,47 +171,41 @@ This macro was introduced as part of the Large File Support extension
@end defvr
@defvr Macro _TIME_BITS
-This macro determines the bit size of @code{time_t} (and therefore the
-bit size of all @code{time_t} derived types and the prototypes of all
-related functions). If @code{_TIME_BITS} is undefined, the bit size of
-@code{time_t} is architecture dependent.
+This macro determines the bit size of @code{time_t}, and therefore the
+bit size of all @code{time_t}-derived types and the prototypes of all
+related functions.
-Possible values of @code{_TIME_BITS}:
@enumerate
-@item
-@code{_TIME_BITS=64} and port from the outset uses 64-bit
-@code{time_t} and word size equals to @w{64 bits} (e.g. x86_64) - no
-action
@item
-@code{_TIME_BITS=32} and port from the outset uses 32-bit
-@code{time_t} and word size equals to @w{64 bits} (e.g. ARM) - no
-action
+If @code{_TIME_BITS} is undefined, the bit size of @code{time_t} is
+architecture dependent. Currently it defaults to 64 bits on most
+architectures. Although it defaults to 32 bits on some traditional
+architectures (i686, ARM), this is planned to change and applications
+should not rely on this.
@item
-@code{_TIME_BITS=64} and port from the outset uses 64-bit
-@code{time_t} and word size equals to @w{32 bits} (e.g. ARC, RV32)
-- no action
+If @code{_TIME_BITS} is defined to be 64, @code{time_t} is defined
+to be a 64-bit integer. On platforms where @code{time_t} was
+traditionally 32 bits, calls to proper syscalls depend on the
+Linux kernel version on which the system is running. For Linux kernel
+version above @b{5.1} syscalls supporting 64-bit time are used. Otherwise,
+a fallback code is used with legacy (i.e. 32-bit) syscalls.
@item
-@code{_TIME_BITS=64} and port from the outset uses 32-bit
-@code{time_t} and word size equals to @w{32 bits} (e.g. ARM)
-- the @code{time_t} is modified to be able to hold 64-bit time.
+If @code{_TIME_BITS} is defined to be 32, @code{time_t} is defined to
+be a 32-bit integer where that is supported. This is not recommended,
+as 32-bit @code{time_t} stops working in the year 2038.
@item
-For any other use case the compile-time error is emitted.
+For any other use case a compile-time error is emitted.
@end enumerate
-The @code{_TIME_BITS} can be only used when @code{_FILE_OFFSET_BITS=64}
-is also defined.
-
-For the point @b{4} above, calls to proper syscalls depend on the
-Linux kernel version on which the system is running. For Linux kernel
-version above @b{5.1} syscalls supporting 64-bit time are used. Otherwise,
-a fallback code is used with legacy (i.e. 32-bit) syscalls.
+@code{_TIME_BITS=64} can be defined only when
+@code{_FILE_OFFSET_BITS=64} is also defined.
By using this macro certain ports gain support for 64-bit time and as
-a result become immune to Y2038 problem.
+a result become immune to the Y2038 problem.
@end defvr
@defvr Macro _ISOC99_SOURCE
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