Re: [PATCH] Adjust printf(3) formats for 32-bit platforms

Lukasz Stelmach <[email protected]> Wed, 30 Jul 2025 11:20:16 +0200

Newsgroups	dev.linux.lists.connman
Message-ID	<oypijd5xfarq5r.fsf%[email protected]>

It was <2025-07-28 pon 15:37>, when Grant Erickson wrote:
> On Jul 21, 2025, at 7:17 AM, Łukasz Stelmach <[email protected]> wrote
>> On 32-bit platforms time_t may be defined as long long int to make it
>> 64-bit wide and y2038 compatible. On 64-bit platform with LP64 data model
>> ABI long long is also 64-bit, so it is convenient and safe to always
>> cast time_t to long long and use %lld/%llu format on all platforms.
>
> Łukasz:
>
> Had you considered using the mnemonics in inttypes.h to avoid the cast such as:
>
> 	selection_str = g_strdup_printf("%s:%” PRId64 "",
> 	device->last_user_selection_ident, device->last_user_selection_time)
>

Yes, I had. Alas, there is no mnemonic for time_t in glibc like for
example PRIdTT that matches time_t on current platform and time_t may be
different on different platforms. Using PRId64 would cause problems on
some 32-bit platforms. Even on the same hardware glibc may be configured
in different ways. With newer gcc even same sized types (e.g long and
long long on ILP64 platforms) trigger warnings/errors when checking
printf(3) formats and arguments. I figure casting "upwards" to long long
is the safest and most portable, albeit not too pretty, way to solve
this problem.

Take for example this code

--8<---------------cut here---------------start------------->8---
#include <stdio.h>
long a = 1L;
long long b = 1LL;

int main(int ac, char  *av[]) {
  printf("sizeof(a): %ld\n", sizeof(a));
  printf("sizeof(b): %ld\n", sizeof(b));

#ifdef WITH_ERRORS
  printf("%ld %ld\n", a, b);
  printf("%lld %lld\n", a, b);
#endif
}
--8<---------------cut here---------------end--------------->8---

Compile it once

    gcc -o longs longs.c

and run and then trigger the warnings

    gcc -Wformat -DWITH_ERRORS -o longs longs.c

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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