Add FMT_CUSTOM_ASSERT_FAIL #4505

HazardyKnusperkeks · 2025-07-29T11:57:13Z

That way one can provide ones own implementation for assert_fail, which is moved out of the detail namespace. For binary compatibility the detail version stays to call the outer version.

HazardyKnusperkeks · 2025-07-29T11:59:03Z

I want to use libfmt in a bare metal project, with exceptions disabled and I don't want to pull in any of the printf-familiy, so I need my own implementation. Instead of a weak symbol one could hide the implementation behind an #if (e.g. FMT_USER_PROVIDED_ASSERT_FAIL), if that's more to your liking.

HazardyKnusperkeks · 2025-07-29T12:05:16Z

I want to use libfmt in a bare metal project, with exceptions disabled and I don't want to pull in any of the printf-familiy, so I need my own implementation. Instead of a weak symbol one could hide the implementation behind an #if (e.g. FMT_USER_PROVIDED_ASSERT_FAIL), if that's more to your liking.

Seeing the failing tests, that seems to be needed?

vitaut · 2025-08-01T14:00:49Z

Thanks for the PR but I don't think we should add yet another configuration macro. You can already control this via FMT_ASSERT / FMT_THROW.

HazardyKnusperkeks · 2025-08-02T14:41:20Z

I actually can not really. Because the functions I'd like to be called is not yet in scope, when fmt/format.h is included, even if using the classic assert, obviously the same for my own functions.

/opt/bin/g++ -g -O0 -Wall -Wextra -m64 -pipe -fexceptions -fvisibility=default -fPIC -DFMT_USE_NOEXCEPT '-DFMT_ASSERT(c,t)=assert(c)' -I.../libs/3rdParty/fmt/include -std=c++2b -o.../build/Desktop-Debug/Debug_Desktop_038b678e9426a45b/fmt.875c51b4/f5439ee4c1a5e132/os.cc.o -c .../libs/3rdParty/fmt/src/os.cc
.../libs/3rdParty/fmt/include/fmt/format.h: In function 'constexpr fmt::v11::detail::uint128_fallback fmt::v11::detail::operator*(const uint128_fallback&, uint32_t)':
<command-line>: error: 'assert' was not declared in this scope
.../libs/3rdParty/fmt/include/fmt/format.h:325:5: note: in expansion of macro 'FMT_ASSERT'
  325 |     FMT_ASSERT(lhs.hi_ == 0, "");
      |     ^~~~~~~~~~
In file included from .../libs/3rdParty/fmt/include/fmt/os.h:11,
                 from .../libs/3rdParty/fmt/src/os.cc:13:

But declaring it as weak, if and only if FMT_HEADER_ONLY is not defined would also work for me.

vitaut · 2025-08-03T20:32:02Z

Good point, let's reopen this.

vitaut · 2025-08-09T15:05:11Z

Have you considered wrapping {fmt} headers such that your custom assertion function is visible at the point of use?

HazardyKnusperkeks · 2025-08-11T13:19:32Z

I have not. But that would be hard for other libraries which also use fmt (which I don't have yet), and fmt's own source files. I did hope to land some form in the official code.

I understand your concern, and if we don't find an agreement here, I'll probably go with a fork.

vitaut

I think a weak symbol is slightly better than introducing a configuration macro. Also if we are making it overridable the symbol should go from detail to the fmt (with version) namespace. For ABI compatibility we should also have detail::assert_fail that can forward to the public API version.

vitaut · 2025-08-20T15:25:24Z

include/fmt/base.h

+#if (FMT_GCC_VERSION || FMT_CLANG_VERSION) && !defined(FMT_HEADER_ONLY) && \
+    !FMT_WIN32
+// Header only (which implies everything is inline) and weak symbols do not work
+// together. And on Windows weak symbols seem not to work properly.
+#  define FMT_WEAK __attribute__((weak))
+#endif
+#ifndef FMT_WEAK
+#  define FMT_WEAK
+#endif


Looking at all the caveats, I am now leaning towards your previous version with a configuration macro.

Also what's up with the test failures?

Naming-wise I suggest FMT_CUSTOM_ASSERT_FAIL.

I had some update trouble with my Linux machine to investigate, my gut feeling is, that the assert tries to jump to an invalid address. I had something like on my Windows machine too, but thought that was because (as I've been told), that weak symbols currently do not work with binutils.

vitaut · 2025-08-20T15:29:35Z

include/fmt/base.h

+#if (FMT_GCC_VERSION || FMT_CLANG_VERSION) && !defined(FMT_HEADER_ONLY) && \
+    !FMT_WIN32
+// Header only (which implies everything is inline) and weak symbols do not work
+// together. And on Windows weak symbols seem not to work properly.
+#  define FMT_WEAK __attribute__((weak))
+#endif
+#ifndef FMT_WEAK
+#  define FMT_WEAK
+#endif


Naming-wise I suggest FMT_CUSTOM_ASSERT_FAIL.

vitaut · 2025-08-24T14:50:30Z

include/fmt/base.h

+FMT_BEGIN_EXPORT
+FMT_NORETURN FMT_API void assert_fail(const char* file, int line,
+                                      const char* message);
+FMT_END_EXPORT


I don't think it needs to be exported from the module. The reason why it is in the fmt namespace is that it can be replaced by users, but it's not supposed to be called directly.

vitaut · 2025-08-24T15:51:54Z

Overall looks good but please address the remaining comment and rebase.

That way one can provide ones own implementation for assert_fail, which is moved out of the detail namespace. For binary compatibility the detail version stays to call the outer version.

HazardyKnusperkeks · 2025-08-24T21:06:07Z

Done, sorry for the noise.

vitaut · 2025-08-25T16:02:07Z

Merged, thanks!

@fyrsta7

- Update from version 11.2.0 to 12.1.0 - Update of rootfile - so-bump so mpd requires shipping - Changelog 12.1.0 - Optimized `buffer::append`, resulting in up to ~16% improvement on spdlog benchmarks (fmtlib/fmt#4541). Thanks @fyrsta7. - Worked around an ABI incompatibility in `std::locale_ref` between clang and gcc (fmtlib/fmt#4573). - Made `std::variant` and `std::expected` formatters work with `format_as` (fmtlib/fmt#4574, fmtlib/fmt#4575). Thanks @phprus. - Made `fmt::join<string_view>` work with C++ modules (fmtlib/fmt#4379, fmtlib/fmt#4577). Thanks @Arghnews. - Exported `fmt::is_compiled_string` and `operator""_cf` from the module (fmtlib/fmt#4544). Thanks @CrackedMatter. - Fixed a compatibility issue with C++ modules in clang (fmtlib/fmt#4548). Thanks @tsarn. - Added support for cv-qualified types to the `std::optional` formatter (fmtlib/fmt#4561, fmtlib/fmt#4562). 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HazardyKnusperkeks force-pushed the weak-assert branch from 9fd399a to 5ad7b5f Compare July 29, 2025 12:08

HazardyKnusperkeks changed the title ~~Mark detail::assert_fail as weak symbol~~ Add FMT_USER_PROVIDED_ASSERT_FAIL Jul 29, 2025

vitaut closed this Aug 1, 2025

vitaut reopened this Aug 3, 2025

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HazardyKnusperkeks force-pushed the weak-assert branch from 5ad7b5f to 95aeea7 Compare August 18, 2025 22:08

HazardyKnusperkeks changed the title ~~Add FMT_USER_PROVIDED_ASSERT_FAIL~~ Add fmt::assert_fail as a weak symbol Aug 18, 2025

vitaut reviewed Aug 20, 2025

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HazardyKnusperkeks force-pushed the weak-assert branch from 95aeea7 to 5f63582 Compare August 23, 2025 20:31

HazardyKnusperkeks changed the title ~~Add fmt::assert_fail as a weak symbol~~ Add FMT_CUSTOM_ASSERT_FAIL Aug 23, 2025

vitaut reviewed Aug 24, 2025

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HazardyKnusperkeks force-pushed the weak-assert branch 2 times, most recently from 17d715e to 96f28a9 Compare August 24, 2025 20:58

Add FMT_CUSTOM_ASSERT_FAIL

82f2158

That way one can provide ones own implementation for assert_fail, which is moved out of the detail namespace. For binary compatibility the detail version stays to call the outer version.

HazardyKnusperkeks force-pushed the weak-assert branch from 96f28a9 to 82f2158 Compare August 24, 2025 21:05

vitaut approved these changes Aug 24, 2025

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vitaut merged commit e181e94 into fmtlib:master Aug 25, 2025
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Add FMT_CUSTOM_ASSERT_FAIL #4505

Add FMT_CUSTOM_ASSERT_FAIL #4505

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