Re: [libc-coord] Re: regression in man pages for interfaces using loff_t

Paul Eggert
2023-06-30T19:44:28Z

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On 2023-06-30 01:02, Jonathan Wakely wrote:

> For APIs like copy_file_range(2) and splice(2) the arguments are loff_t* so
> you can't just "pass arguments that fit in off_t" to them.

Sorry, I missed that detail. Still, the argument stands. On legacy
32-bit platforms without -D_FILE_OFFSET_BITS=64, calls will still work
if they pass null pointers to copy_file_range, a common case in my
experience. The calls that don't, will get typecheck errors or warnings,
and that's good enough to address the issue.

> And in C++ it won't even compile unless you get the pointer
> types exactly right (C compilers will typically allow the mismatch with
> just a warning).

That's good! People should be using -D_FILE_OFFSET_BITS=64 if they use
these functions, and the typecheck errors and/or warnings will remind
them. The man pages don't need to (and shouldn't) document what happens
if you call these functions on legacy 32-bit platforms without first
defining _FILE_OFFSET_BITS to be 64.

> People miss footnotes.

OK, let's make the point more prominently, at the start of the man page.
Proposed patch attached. This patch should work for musl as well as for
glibc.

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From 38bfd1ecda2014955c701f7658a4ab55fa5c8b9d Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 30 Jun 2023 12:25:53 -0700
Subject: [PATCH] off64_t: prefer off_t for splice, etc.

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 legacy 32-bit Linux.
---
man2/copy_file_range.2 | 17 ++++++++++++++---
man2/readahead.2 | 8 +++++++-
man2/splice.2 | 14 ++++++++++++--
man2/sync_file_range.2 | 9 +++++++--
man3/fopencookie.3 | 14 +++++++++++---
man7/feature_test_macros.7 | 12 ++++++++----
6 files changed, 59 insertions(+), 15 deletions(-)

diff --git a/man2/copy_file_range.2 b/man2/copy_file_range.2
index 6f3aa4971..bb5aa2223 100644
--- a/man2/copy_file_range.2
+++ b/man2/copy_file_range.2
@@ -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 +225,15 @@ 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 legacy 32-bit platforms.
.SH BUGS
In Linux 5.3 to Linux 5.18,
cross-filesystem copies were implemented by the kernel,
@@ -234,6 +244,7 @@ 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 +255,7 @@ 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) {
diff --git a/man2/readahead.2 b/man2/readahead.2
index d69795979..62b9e6786 100644
--- a/man2/readahead.2
+++ b/man2/readahead.2
@@ -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 +74,11 @@ 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 legacy 32-bit platforms.
.SH BUGS
.BR readahead ()
attempts to schedule the reads in the background and return immediately.
diff --git a/man2/splice.2 b/man2/splice.2
index dd78e8cd4..829d2e336 100644
--- a/man2/splice.2
+++ b/man2/splice.2
@@ -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 +243,15 @@ 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 legacy 32-bit platforms.
.SH EXAMPLES
See
.BR tee (2).
diff --git a/man2/sync_file_range.2 b/man2/sync_file_range.2
index d633b08ff..0bf17f824 100644
--- a/man2/sync_file_range.2
+++ b/man2/sync_file_range.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 +199,10 @@ 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 legacy 32-bit platforms.
.SH SEE ALSO
.BR fdatasync (2),
.BR fsync (2),
diff --git a/man3/fopencookie.3 b/man3/fopencookie.3
index 409a3c81a..08b190394 100644
--- a/man3/fopencookie.3
+++ b/man3/fopencookie.3
@@ -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 +452,13 @@ 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 legacy 32-bit platforms.
.SH SEE ALSO
.BR fclose (3),
.BR fmemopen (3),
diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7
index f1620611c..462fd4abb 100644
--- a/man7/feature_test_macros.7
+++ b/man7/feature_test_macros.7
@@ -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 in cases where feature macros
+expose the function declaration with the correct type,
+and these 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
--
2.41.0