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Date:   Thu, 02 Dec 2021 09:55:20 -0500
From:   "Zack Weinberg" <zack@owlfolio.org>
To:     "Arnd Bergmann" <arnd@arndb.de>, "Cyril Hrubis" <chrubis@suse.cz>
Cc:     "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        libc-alpha@sourceware.org,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "David Howells" <dhowells@redhat.com>,
        "David Laight" <David.Laight@aculab.com>,
        "ltp@lists.linux.it" <ltp@lists.linux.it>
Subject: Re: [PATCH] uapi: Make __{u,s}64 match {u,}int64_t in userspace
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On Mon, Nov 29, 2021, at 9:34 AM, Arnd Bergmann wrote:
> On Mon, Nov 29, 2021 at 12:58 PM Cyril Hrubis <chrubis@suse.cz> wrote:
>>
>> What about guarding the change with __STDINT_COMPATIBLE_TYPES__

In user space, I don't see a compelling need for backward compatibility?  User space's expectation is that the types are *already* the same and we (glibc) regularly get bug reports because they aren't.

I could be persuaded otherwise with an example of a program for which changing
__s64 from 'long long' to 'long' would break *binary* backward compatibility, or
similarly for __u64.

> I don't think we can include stdint.h here, the entire point of the custom
> kernel types is to ensure the other kernel headers can use these types
> without relying on libc headers.

If __KERNEL__ is not defined, though, there should be no issue, right?

From user space's perspective, it's an ongoing source of problems whenever __uN isn't exactly the same "underlying type" as uintN_t, same for __sN and intN_t.  We would really like it if the uapi headers, when included from user space, deferred to the C library for the definitions of these types.

<stdint.h> does define a lot of things beyond just the fixed-width types, and it defines names in the application namespace (i.e. with no __ prefix).  Perhaps we could come to some agreement on a private header, provided by libcs, that *only* defined __{u,}int{8,16,32,64}_t.  glibc already has <bits/types.h> which promises
to define only __-prefix names, but it defines a lot of other types as well (__dev_t, __uid_t, __pid_t, __time_t, etc etc etc).

zw