[v4] off64_t: prefer off_t for splice, etc.
Checks
Commit Message
For the few functions that come only in 64-bit off_t flavors,
document their APIs as using off_t instead of off64_t,
and say also that code should #define _FILE_OFFSET_BITS 64.
This documents what user code is (and should be) doing anyway,
if it needs to work on traditional x86 and ARM Linux.
---
man2/copy_file_range.2 | 20 +++++++++++++++++---
man2/readahead.2 | 11 ++++++++++-
man2/splice.2 | 17 +++++++++++++++--
man2/sync_file_range.2 | 12 ++++++++++--
man3/fopencookie.3 | 17 ++++++++++++++---
man7/feature_test_macros.7 | 12 ++++++++----
6 files changed, 74 insertions(+), 15 deletions(-)
Comments
Paul Eggert <eggert@cs.ucla.edu> writes:
> For the few functions that come only in 64-bit off_t flavors,
> document their APIs as using off_t instead of off64_t,
> and say also that code should #define _FILE_OFFSET_BITS 64.
> This documents what user code is (and should be) doing anyway,
> if it needs to work on traditional x86 and ARM Linux.
LGTM and thank you Paul.
I haven't checked for other prototypes/examples which need
changing.
Hi Paul, Sam, and Rich,
On 2023-07-09 08:16, Sam James wrote:
>
> Paul Eggert <eggert@cs.ucla.edu> writes:
>
>> For the few functions that come only in 64-bit off_t flavors,
>> document their APIs as using off_t instead of off64_t,
>> and say also that code should #define _FILE_OFFSET_BITS 64.
>> This documents what user code is (and should be) doing anyway,
>> if it needs to work on traditional x86 and ARM Linux.
>
> LGTM and thank you Paul.
>
> I haven't checked for other prototypes/examples which need
> changing.
Thanks, I'm going to apply the patch. Can you please confirm if I'm
correct in adding the following tags?
Reported-by: Rich Felker <dalias@libc.org>
Fixes: 9bebb17e5b57 ("splice.2: Use 'off64_t' instead of 'loff_t'")
Fixes: 76c5631fb442 ("copy_file_range.2: Document glibc wrapper instead of kernel syscall")
Fixes: 5cabfa06b407 ("man-pages 1.68")
Fixes: 3ca974e3988a ("New page for sync_file_range(2), new in kernel 2.6.17.")
Fixes: 9bebb17e5b57 ("sync_file_range.2: Document the architecture-specific sync_file_range2() system call")
Fixes: 79bf8cdcf36a ("Document fopencookie(3), a library function that allows custom implementation of a stdio stream.")
Signed-off-by: Paul Eggert <eggert@cs.ucla.edu>
Reviewed-by: Sam James <sam@gentoo.org>
Cc: Jonathan Wakely <jwakely@redhat.com>
Cc: Szabolcs Nagy <nsz@port70.net>
Cc: Jakub Wilk <jwilk@jwilk.net>
Cc: A. Wilcox <AWilcox@wilcox-tech.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
BTW, Rich, please note the commits that this fixes: most of them are
the initial commit that adds a page, which means that the function
had always been documented with off64_t in the "spec". Only splice(2)
and copy_file_range(2) have been adjusted afterwards, and in a manner
to be consistent with the rest of the pages, so I can only conclude
that we didn't break the spec, but rather fixed it.
Nevertheless, I'm sorry that it caused any problems to musl, and I'm
happy that you reported them and so we can now improve the pages.
Cheers,
Alex
On Sat, Jul 15, 2023 at 05:08:18PM +0200, Alejandro Colomar wrote:
> Hi Paul, Sam, and Rich,
>
> On 2023-07-09 08:16, Sam James wrote:
> >
> > Paul Eggert <eggert@cs.ucla.edu> writes:
> >
> >> For the few functions that come only in 64-bit off_t flavors,
> >> document their APIs as using off_t instead of off64_t,
> >> and say also that code should #define _FILE_OFFSET_BITS 64.
> >> This documents what user code is (and should be) doing anyway,
> >> if it needs to work on traditional x86 and ARM Linux.
> >
> > LGTM and thank you Paul.
> >
> > I haven't checked for other prototypes/examples which need
> > changing.
>
> Thanks, I'm going to apply the patch. Can you please confirm if I'm
> correct in adding the following tags?
>
> Reported-by: Rich Felker <dalias@libc.org>
> Fixes: 9bebb17e5b57 ("splice.2: Use 'off64_t' instead of 'loff_t'")
> Fixes: 76c5631fb442 ("copy_file_range.2: Document glibc wrapper instead of kernel syscall")
> Fixes: 5cabfa06b407 ("man-pages 1.68")
> Fixes: 3ca974e3988a ("New page for sync_file_range(2), new in kernel 2.6.17.")
> Fixes: 9bebb17e5b57 ("sync_file_range.2: Document the architecture-specific sync_file_range2() system call")
> Fixes: 79bf8cdcf36a ("Document fopencookie(3), a library function that allows custom implementation of a stdio stream.")
> Signed-off-by: Paul Eggert <eggert@cs.ucla.edu>
> Reviewed-by: Sam James <sam@gentoo.org>
> Cc: Jonathan Wakely <jwakely@redhat.com>
> Cc: Szabolcs Nagy <nsz@port70.net>
> Cc: Jakub Wilk <jwilk@jwilk.net>
> Cc: A. Wilcox <AWilcox@wilcox-tech.com>
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
>
>
> BTW, Rich, please note the commits that this fixes: most of them are
> the initial commit that adds a page, which means that the function
> had always been documented with off64_t in the "spec". Only splice(2)
> and copy_file_range(2) have been adjusted afterwards, and in a manner
> to be consistent with the rest of the pages, so I can only conclude
> that we didn't break the spec, but rather fixed it.
>
> Nevertheless, I'm sorry that it caused any problems to musl, and I'm
> happy that you reported them and so we can now improve the pages.
While I like off_t, I am still unhappy that this seems to have been a
unilateral action from documentation side without even hearing input
from any major implementors other than myself. Is "you can't use these
interfaces without -D_FILE_OFFSET_BITS=64" an acceptable outcome to
the glibc folks?
Rich
On 2023-07-15 11:35, Rich Felker wrote:
> Is "you can't use these
> interfaces without -D_FILE_OFFSET_BITS=64" an acceptable outcome to
> the glibc folks?
You can ask on libc-alpha if you like. I'll follow up if necessary.
[CC += glibc, enh]
Hi Rich,
On 2023-07-15 20:35, Rich Felker wrote:
> On Sat, Jul 15, 2023 at 05:08:18PM +0200, Alejandro Colomar wrote:
>> Hi Paul, Sam, and Rich,
>>
>> On 2023-07-09 08:16, Sam James wrote:
>>>
>>> Paul Eggert <eggert@cs.ucla.edu> writes:
>>>
>>>> For the few functions that come only in 64-bit off_t flavors,
>>>> document their APIs as using off_t instead of off64_t,
>>>> and say also that code should #define _FILE_OFFSET_BITS 64.
>>>> This documents what user code is (and should be) doing anyway,
>>>> if it needs to work on traditional x86 and ARM Linux.
>>>
>>> LGTM and thank you Paul.
>>>
>>> I haven't checked for other prototypes/examples which need
>>> changing.
>>
>> Thanks, I'm going to apply the patch. Can you please confirm if I'm
>> correct in adding the following tags?
>>
>> Reported-by: Rich Felker <dalias@libc.org>
>> Fixes: 9bebb17e5b57 ("splice.2: Use 'off64_t' instead of 'loff_t'")
>> Fixes: 76c5631fb442 ("copy_file_range.2: Document glibc wrapper instead of kernel syscall")
>> Fixes: 5cabfa06b407 ("man-pages 1.68")
>> Fixes: 3ca974e3988a ("New page for sync_file_range(2), new in kernel 2.6.17.")
>> Fixes: 9bebb17e5b57 ("sync_file_range.2: Document the architecture-specific sync_file_range2() system call")
>> Fixes: 79bf8cdcf36a ("Document fopencookie(3), a library function that allows custom implementation of a stdio stream.")
>> Signed-off-by: Paul Eggert <eggert@cs.ucla.edu>
>> Reviewed-by: Sam James <sam@gentoo.org>
>> Cc: Jonathan Wakely <jwakely@redhat.com>
>> Cc: Szabolcs Nagy <nsz@port70.net>
>> Cc: Jakub Wilk <jwilk@jwilk.net>
>> Cc: A. Wilcox <AWilcox@wilcox-tech.com>
>> Signed-off-by: Alejandro Colomar <alx@kernel.org>
>>
>>
>> BTW, Rich, please note the commits that this fixes: most of them are
>> the initial commit that adds a page, which means that the function
>> had always been documented with off64_t in the "spec". Only splice(2)
>> and copy_file_range(2) have been adjusted afterwards, and in a manner
>> to be consistent with the rest of the pages, so I can only conclude
>> that we didn't break the spec, but rather fixed it.
>>
>> Nevertheless, I'm sorry that it caused any problems to musl, and I'm
>> happy that you reported them and so we can now improve the pages.
>
> While I like off_t, I am still unhappy that this seems to have been a
> unilateral action from documentation side without even hearing input
> from any major implementors other than myself.
Since Paul Eggert is a glibc maintainer, I consider his input as being
representative enough of glibc, even with the list not in CC. He
usually produces very high quality patches, and I know he has special
care about very odd platforms where type sizes are not usual.
We also had the review and approval of a distribution maintainer, Sam,
so I wouldn't call this unilateral.
It's true we didn't ask the entire glibc list. I'm fixing that by
adding libc-alpha@ to the loop; let's see if they have anything to say
about the patch, which BTW I pushed a few hours ago. And hell, while
we're at it, I'm CCing enh from bionic too, just in case he has any
opinion (although he probably read this thread from the linux-man@
list).
For future times, when opening a thread like this where input from
glibc (or kernel) maintainers is not only welcome but essential, it's
usually better to include the relevant list in CC right from the very
first email, to provide them with full context, as I suggest in the
CONTRIBUTING file.
And while at it, I'm thinking that maybe we should mention musl's
list in CONTRIBUTING too. Should I apply the patch below? Would you
mind reading that file, and suggesting anything you want for
preventing similar conflicts with musl in the future?
Thank you all!
Alex
diff --git a/CONTRIBUTING b/CONTRIBUTING
index 80052c38e..a030b54a5 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -27,12 +27,13 @@ Description
discussed in a man-pages email, please identify yourself as such.
Relevant mailing lists may include:
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: Linux API <linux-api@vger.kernel.org>
Cc: Glibc <libc-alpha@sourceware.org>
+ Cc: musl libc <musl@lists.openwall.com>
For other kernel mailing lists and maintainers, check the
<MAINTAINERS> file in the Linux kernel repository.
Please don't send HTML email; it will be discarded by the list.
@@ -186,13 +187,13 @@ Description
Reporting bugs
Report bugs to the mailing list, following the instructions above
for sending mails to the list. If you can write a patch (see
instructions for sending patches above), it's preferred.
If you're unsure if the bug is in the manual page or in the code
- being documented (kernel, glibc, ...), it's best to send the
+ being documented (kernel, libc, ...), it's best to send the
report to both at the same time, that is, CC all the mailing
lists that may be concerned by the report.
Some distributions (for example Debian) apply patches to the
upstream manual pages. If you suspect the bug is in one of those
patches, report it to your distribution maintainer.
> Is "you can't use these
> interfaces without -D_FILE_OFFSET_BITS=64" an acceptable outcome to
> the glibc folks?
>
> Rich
I forgot to refer to the start of the thread:
<https://lore.kernel.org/linux-man/20230628191947.GE3630668@port70.net/T/#t>
The patch applied was this one:
<https://lore.kernel.org/linux-man/20230628191947.GE3630668@port70.net/T/#md5cb63f26575d9ecf5753027e7a87c2782d0d872>
On 2023-07-16 02:35, Alejandro Colomar wrote:
> [CC += glibc, enh]
>
> Hi Rich,
>
> On 2023-07-15 20:35, Rich Felker wrote:
>> On Sat, Jul 15, 2023 at 05:08:18PM +0200, Alejandro Colomar wrote:
>>> Hi Paul, Sam, and Rich,
>>>
>>> On 2023-07-09 08:16, Sam James wrote:
>>>>
>>>> Paul Eggert <eggert@cs.ucla.edu> writes:
>>>>
>>>>> For the few functions that come only in 64-bit off_t flavors,
>>>>> document their APIs as using off_t instead of off64_t,
>>>>> and say also that code should #define _FILE_OFFSET_BITS 64.
>>>>> This documents what user code is (and should be) doing anyway,
>>>>> if it needs to work on traditional x86 and ARM Linux.
>>>>
>>>> LGTM and thank you Paul.
>>>>
>>>> I haven't checked for other prototypes/examples which need
>>>> changing.
>>>
>>> Thanks, I'm going to apply the patch. Can you please confirm if I'm
>>> correct in adding the following tags?
>>>
>>> Reported-by: Rich Felker <dalias@libc.org>
>>> Fixes: 9bebb17e5b57 ("splice.2: Use 'off64_t' instead of 'loff_t'")
>>> Fixes: 76c5631fb442 ("copy_file_range.2: Document glibc wrapper instead of kernel syscall")
>>> Fixes: 5cabfa06b407 ("man-pages 1.68")
>>> Fixes: 3ca974e3988a ("New page for sync_file_range(2), new in kernel 2.6.17.")
>>> Fixes: 9bebb17e5b57 ("sync_file_range.2: Document the architecture-specific sync_file_range2() system call")
>>> Fixes: 79bf8cdcf36a ("Document fopencookie(3), a library function that allows custom implementation of a stdio stream.")
>>> Signed-off-by: Paul Eggert <eggert@cs.ucla.edu>
>>> Reviewed-by: Sam James <sam@gentoo.org>
>>> Cc: Jonathan Wakely <jwakely@redhat.com>
>>> Cc: Szabolcs Nagy <nsz@port70.net>
>>> Cc: Jakub Wilk <jwilk@jwilk.net>
>>> Cc: A. Wilcox <AWilcox@wilcox-tech.com>
>>> Signed-off-by: Alejandro Colomar <alx@kernel.org>
>>>
>>>
>>> BTW, Rich, please note the commits that this fixes: most of them are
>>> the initial commit that adds a page, which means that the function
>>> had always been documented with off64_t in the "spec". Only splice(2)
>>> and copy_file_range(2) have been adjusted afterwards, and in a manner
>>> to be consistent with the rest of the pages, so I can only conclude
>>> that we didn't break the spec, but rather fixed it.
>>>
>>> Nevertheless, I'm sorry that it caused any problems to musl, and I'm
>>> happy that you reported them and so we can now improve the pages.
>>
>> While I like off_t, I am still unhappy that this seems to have been a
>> unilateral action from documentation side without even hearing input
>> from any major implementors other than myself.
>
> Since Paul Eggert is a glibc maintainer, I consider his input as being
> representative enough of glibc, even with the list not in CC. He
> usually produces very high quality patches, and I know he has special
> care about very odd platforms where type sizes are not usual.
>
> We also had the review and approval of a distribution maintainer, Sam,
> so I wouldn't call this unilateral.
>
> It's true we didn't ask the entire glibc list. I'm fixing that by
> adding libc-alpha@ to the loop; let's see if they have anything to say
> about the patch, which BTW I pushed a few hours ago. And hell, while
> we're at it, I'm CCing enh from bionic too, just in case he has any
> opinion (although he probably read this thread from the linux-man@
> list).
>
> For future times, when opening a thread like this where input from
> glibc (or kernel) maintainers is not only welcome but essential, it's
> usually better to include the relevant list in CC right from the very
> first email, to provide them with full context, as I suggest in the
> CONTRIBUTING file.
>
> And while at it, I'm thinking that maybe we should mention musl's
> list in CONTRIBUTING too. Should I apply the patch below? Would you
> mind reading that file, and suggesting anything you want for
> preventing similar conflicts with musl in the future?
>
> Thank you all!
> Alex
>
> diff --git a/CONTRIBUTING b/CONTRIBUTING
> index 80052c38e..a030b54a5 100644
> --- a/CONTRIBUTING
> +++ b/CONTRIBUTING
> @@ -27,12 +27,13 @@ Description
> discussed in a man-pages email, please identify yourself as such.
> Relevant mailing lists may include:
>
> Cc: LKML <linux-kernel@vger.kernel.org>
> Cc: Linux API <linux-api@vger.kernel.org>
> Cc: Glibc <libc-alpha@sourceware.org>
> + Cc: musl libc <musl@lists.openwall.com>
>
> For other kernel mailing lists and maintainers, check the
> <MAINTAINERS> file in the Linux kernel repository.
>
> Please don't send HTML email; it will be discarded by the list.
>
> @@ -186,13 +187,13 @@ Description
> Reporting bugs
> Report bugs to the mailing list, following the instructions above
> for sending mails to the list. If you can write a patch (see
> instructions for sending patches above), it's preferred.
>
> If you're unsure if the bug is in the manual page or in the code
> - being documented (kernel, glibc, ...), it's best to send the
> + being documented (kernel, libc, ...), it's best to send the
> report to both at the same time, that is, CC all the mailing
> lists that may be concerned by the report.
>
> Some distributions (for example Debian) apply patches to the
> upstream manual pages. If you suspect the bug is in one of those
> patches, report it to your distribution maintainer.
>
>
>> Is "you can't use these
>> interfaces without -D_FILE_OFFSET_BITS=64" an acceptable outcome to
>> the glibc folks?
>>
>> Rich
>
@@ -11,10 +11,11 @@ Standard C library
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
+.B #define _FILE_OFFSET_BITS 64
.B #include <unistd.h>
.PP
-.BI "ssize_t copy_file_range(int " fd_in ", off64_t *_Nullable " off_in ,
-.BI " int " fd_out ", off64_t *_Nullable " off_out ,
+.BI "ssize_t copy_file_range(int " fd_in ", off_t *_Nullable " off_in ,
+.BI " int " fd_out ", off_t *_Nullable " off_out ,
.BI " size_t " len ", unsigned int " flags );
.fi
.SH DESCRIPTION
@@ -224,6 +225,18 @@ gives filesystems an opportunity to implement "copy acceleration" techniques,
such as the use of reflinks (i.e., two or more inodes that share
pointers to the same copy-on-write disk blocks)
or server-side-copy (in the case of NFS).
+.PP
+.B _FILE_OFFSET_BITS
+should be defined to be 64 in code that uses non-null
+.I off_in
+or
+.I off_out
+or that takes the address of
+.BR copy_file_range ,
+if the code is intended to be portable
+to traditional 32-bit x86 and ARM platforms where
+.BR off_t 's
+width defaults to 32 bits.
.SH BUGS
In Linux 5.3 to Linux 5.18,
cross-filesystem copies were implemented by the kernel,
@@ -234,6 +247,7 @@ the call failed to copy, while still reporting success.
.\" SRC BEGIN (copy_file_range.c)
.EX
#define _GNU_SOURCE
+#define _FILE_OFFSET_BITS 64
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
@@ -244,7 +258,7 @@ int
main(int argc, char *argv[])
{
int fd_in, fd_out;
- off64_t len, ret;
+ off_t len, ret;
struct stat stat;
\&
if (argc != 3) {
@@ -14,9 +14,10 @@ Standard C library
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
+.B #define _FILE_OFFSET_BITS 64
.B #include <fcntl.h>
.PP
-.BI "ssize_t readahead(int " fd ", off64_t " offset ", size_t " count );
+.BI "ssize_t readahead(int " fd ", off_t " offset ", size_t " count );
.fi
.SH DESCRIPTION
.BR readahead ()
@@ -73,6 +74,14 @@ Linux.
.SH HISTORY
Linux 2.4.13,
glibc 2.3.
+.SH NOTES
+.B _FILE_OFFSET_BITS
+should be defined to be 64 in code that uses a pointer to
+.BR readahead ,
+if the code is intended to be portable
+to traditional 32-bit x86 and ARM platforms where
+.BR off_t 's
+width defaults to 32 bits.
.SH BUGS
.BR readahead ()
attempts to schedule the reads in the background and return immediately.
@@ -12,10 +12,11 @@ Standard C library
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
+.B "#define _FILE_OFFSET_BITS 64
.B #include <fcntl.h>
.PP
-.BI "ssize_t splice(int " fd_in ", off64_t *_Nullable " off_in ,
-.BI " int " fd_out ", off64_t *_Nullable " off_out ,
+.BI "ssize_t splice(int " fd_in ", off_t *_Nullable " off_in ,
+.BI " int " fd_out ", off_t *_Nullable " off_out ,
.BI " size_t " len ", unsigned int " flags );
.\" Return type was long before glibc 2.7
.fi
@@ -242,6 +243,18 @@ only pointers are copied, not the pages of the buffer.
.\" the data and choose to forward it to two or more different
.\" users - for things like logging etc.).
.\"
+.PP
+.B _FILE_OFFSET_BITS
+should be defined to be 64 in code that uses non-null
+.I off_in
+or
+.I off_out
+or that takes the address of
+.BR splice ,
+if the code is intended to be portable
+to traditional 32-bit x86 and ARM platforms where
+.BR off_t 's
+width defaults to 32 bits.
.SH EXAMPLES
See
.BR tee (2).
@@ -16,9 +16,10 @@ Standard C library
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
+.B #define _FILE_OFFSET_BITS 64
.B #include <fcntl.h>
.PP
-.BI "int sync_file_range(int " fd ", off64_t " offset ", off64_t " nbytes ,
+.BI "int sync_file_range(int " fd ", off_t " offset ", off_t " nbytes ,
.BI " unsigned int " flags );
.fi
.SH DESCRIPTION
@@ -176,7 +177,7 @@ system call that orders the arguments suitably:
.in +4n
.EX
.BI "int sync_file_range2(int " fd ", unsigned int " flags ,
-.BI " off64_t " offset ", off64_t " nbytes );
+.BI " off_t " offset ", off_t " nbytes );
.EE
.in
.PP
@@ -198,6 +199,13 @@ glibc transparently wraps
under the name
.BR sync_file_range ().
.SH NOTES
+.B _FILE_OFFSET_BITS
+should be defined to be 64 in code that takes the address of
+.BR sync_file_range ,
+if the code is intended to be portable
+to traditional 32-bit x86 and ARM platforms where
+.BR off_t 's
+width defaults to 32 bits.
.SH SEE ALSO
.BR fdatasync (2),
.BR fsync (2),
@@ -13,6 +13,7 @@ Standard C library
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
+.B #define _FILE_OFFSET_BITS 64
.B #include <stdio.h>
.PP
.BI "FILE *fopencookie(void *restrict " cookie ", const char *restrict " mode ,
@@ -169,7 +170,7 @@ When called, it receives three arguments:
.IP
.in +4n
.EX
-int seek(void *cookie, off64_t *offset, int whence);
+int seek(void *cookie, off_t *offset, int whence);
.EE
.in
.IP
@@ -351,9 +352,9 @@ memfile_read(void *c, char *buf, size_t size)
}
\&
int
-memfile_seek(void *c, off64_t *offset, int whence)
+memfile_seek(void *c, off_t *offset, int whence)
{
- off64_t new_offset;
+ off_t new_offset;
struct memfile_cookie *cookie = c;
\&
if (whence == SEEK_SET)
@@ -451,6 +452,16 @@ main(int argc, char *argv[])
}
.EE
.\" SRC END
+.SH NOTES
+.B _FILE_OFFSET_BITS
+should be defined to be 64 in code that uses non-null
+.I seek
+or that takes the address of
+.BR fopencookie ,
+if the code is intended to be portable
+to traditional 32-bit x86 and ARM platforms where
+.BR off_t 's
+width defaults to 32 bits.
.SH SEE ALSO
.BR fclose (3),
.BR fmemopen (3),
@@ -113,15 +113,16 @@ feature test macro requirements (this example from
.RS +4
.EX
.B #define _GNU_SOURCE
+.B #define _FILE_OFFSET_BITS 64
.B #include <fcntl.h>
.PP
-.BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
+.BI "ssize_t readahead(int " fd ", off_t *" offset ", size_t " count );
.EE
.RE
.PP
-This format is employed in cases where only a single
-feature test macro can be used to expose the function
-declaration, and that macro is not defined by default.
+This format is employed when the feature test macros ensure
+that the proper function declarations are visible,
+and the macros are not defined by default.
.SS Feature test macros understood by glibc
The paragraphs below explain how feature test macros are handled
in glibc 2.\fIx\fP,
@@ -406,6 +407,9 @@ related to file I/O and filesystem operations into references to
their 64-bit counterparts.
This is useful for performing I/O on large files (> 2 Gigabytes)
on 32-bit systems.
+It is also useful when calling functions like
+.BR copy_file_range (2)
+that were added more recently and that come only in 64-bit flavors.
(Defining this macro permits correctly written programs to use
large files with only a recompilation being required.)
.IP