linux-kernel - Re: [PATCH] uapi/fcntl: conditionally define AT

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Message-ID: <9b2c8fe2-cf17-445b-abd7-a1ed44812a73@infradead.org>
Date: Sun, 24 Aug 2025 16:54:50 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-kernel@...r.kernel.org, Amir Goldstein <amir73il@...il.com>,
 Jeff Layton <jlayton@...nel.org>, Chuck Lever <chuck.lever@...cle.com>,
 Alexander Aring <alex.aring@...il.com>, Josef Bacik <josef@...icpanda.com>,
 Aleksa Sarai <cyphar@...har.com>, Jan Kara <jack@...e.cz>,
 Christian Brauner <brauner@...nel.org>, linux-fsdevel@...r.kernel.org,
 linux-api@...r.kernel.org
Subject: Re: [PATCH] uapi/fcntl: conditionally define AT_RENAME* macros



On 8/24/25 4:21 PM, Matthew Wilcox wrote:
> On Sun, Aug 24, 2025 at 03:10:55PM -0700, Randy Dunlap wrote:
>> Don't define the AT_RENAME_* macros when __USE_GNU is defined since
>> /usr/include/stdio.h defines them in that case (i.e. when _GNU_SOURCE
>> is defined, which causes __USE_GNU to be defined).
>>
>> Having them defined in 2 places causes build warnings (duplicate
>> definitions) in both samples/watch_queue/watch_test.c and
>> samples/vfs/test-statx.c.
> 
> It does?  What flags?
> 

for samples/vfs/test-statx.c:

In file included from ../samples/vfs/test-statx.c:23:
usr/include/linux/fcntl.h:159:9: warning: ‘AT_RENAME_NOREPLACE’ redefined
  159 | #define AT_RENAME_NOREPLACE     0x0001
In file included from ../samples/vfs/test-statx.c:13:
/usr/include/stdio.h:171:10: note: this is the location of the previous definition
  171 | # define AT_RENAME_NOREPLACE RENAME_NOREPLACE
usr/include/linux/fcntl.h:160:9: warning: ‘AT_RENAME_EXCHANGE’ redefined
  160 | #define AT_RENAME_EXCHANGE      0x0002
/usr/include/stdio.h:173:10: note: this is the location of the previous definition
  173 | # define AT_RENAME_EXCHANGE RENAME_EXCHANGE
usr/include/linux/fcntl.h:161:9: warning: ‘AT_RENAME_WHITEOUT’ redefined
  161 | #define AT_RENAME_WHITEOUT      0x0004
/usr/include/stdio.h:175:10: note: this is the location of the previous definition
  175 | # define AT_RENAME_WHITEOUT RENAME_WHITEOUT

for samples/watch_queue/watch_test.c:

In file included from usr/include/linux/watch_queue.h:6,
                 from ../samples/watch_queue/watch_test.c:19:
usr/include/linux/fcntl.h:159:9: warning: ‘AT_RENAME_NOREPLACE’ redefined
  159 | #define AT_RENAME_NOREPLACE     0x0001
In file included from ../samples/watch_queue/watch_test.c:11:
/usr/include/stdio.h:171:10: note: this is the location of the previous definition
  171 | # define AT_RENAME_NOREPLACE RENAME_NOREPLACE
usr/include/linux/fcntl.h:160:9: warning: ‘AT_RENAME_EXCHANGE’ redefined
  160 | #define AT_RENAME_EXCHANGE      0x0002
/usr/include/stdio.h:173:10: note: this is the location of the previous definition
  173 | # define AT_RENAME_EXCHANGE RENAME_EXCHANGE
usr/include/linux/fcntl.h:161:9: warning: ‘AT_RENAME_WHITEOUT’ redefined
  161 | #define AT_RENAME_WHITEOUT      0x0004
/usr/include/stdio.h:175:10: note: this is the location of the previous definition
  175 | # define AT_RENAME_WHITEOUT RENAME_WHITEOUT


> #define AT_RENAME_NOREPLACE     0x0001
> #define AT_RENAME_NOREPLACE     0x0001
> 
> int main(void)
> {
> 	return AT_RENAME_NOREPLACE;
> }
> 
> gcc -W -Wall testA.c -o testA
> 
> (no warnings)
> 
> I'm pretty sure C says that duplicate definitions are fine as long
> as they're identical.
The vales are identical but the strings are not identical.

We can't fix stdio.h, but we could just change uapi/linux/fcntl.h
to match stdio.h. I suppose.

-- 
~Randy