Comment by Calavar

That's funny and sad at the same time.

And it shows a deeper problem, even though they are willing to align behavior between each other, they failed to communicate and discuss what would be the best approach. That's a bit tragic, IMO

Since having multiple compilers is often touted as an advantage, how often do situations like what you're describing happen compared to the opposite — when a second compiler surfaces bugs in one's application or the other compiler?

It is like an era of average.

i will call it "webification" of C!

Ah, I can confirm what I see elsewhere in the thread, this is no longer true in Clang. That first clang was Apple Clang 17---who knows what version that actually is---and here is Clang 20:

    $ /opt/homebrew/opt/llvm/bin/clang-20 -O1 -c union.c -o union.o && objdump -d union.o

    union.o: file format mach-o arm64

    Disassembly of section __TEXT,__text:

    0000000000000000 <ltmp0>:
           0: f900001f      str xzr, [x0]
           4: d65f03c0      ret

    0000000000000008 <_create_d>:
           8: f900001f      str xzr, [x0]
           c: d65f03c0      ret

GCC has long been known to define undefined behavior in C unions. In particular, type punning in unions is undefined behavior under the C and C++ standards, but GCC (and Clang) define it.

When you have a big system many people rely on you generally try to look for ways to keep their code working - not look for the changes you’re contractually allowed to make.

GCC probably has a better justification than “we are allowed to”.

Undefined in the standard doesn't mean undefined in GCC. Type-punning through unions has always been a special case that GCC has taken care with beyond the standard.

C never had {} until C23. In C {0} was the only way to explicitly zero-initialize a structure in a generic manner. It works because in C initializer lists are applied to members as-if nested structures are flattened out lexically.

However, a long time ago C++ went in a completely different direction with initializer lists, and gcc and clang started emitting warnings (in C mode) about otherwise perfectly valid C code, thus the adoption of C++'s {} for C23. {0} is still technically valid C23, though, as well as valid C89, C90, C99, and C11. In fact, reading both C23 and C89 I'm struck by how little the language has changed:

C89 3.5.7p16:

> If the aggregate contains members that are aggregates or unions, or if the first member of a union is an aggregate or union, the rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the members of the subaggregate or the first member of the contained union. Otherwise, only enough initializers from the list are taken to account for the members of the first subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next member of the aggregate of which the current subaggregate or contained union is a part.

C23 6.7.10p21:

> If the aggregate or union contains elements or members that are aggregates or unions, these rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only enough initializers from the list are taken to account for the elements or members of the subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next element or member of the aggregate of which the current subaggregate or contained union is a part.

GCC now supports -ftrivial-auto-var-init=[zero|uninitialized|pattern] for stack variables https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#ind...

For malloc, you could use a custom allocator, or replace all the calls with calloc.

There are many low-level devices where initialization is very expensive. It may mean that you need two passes through memory instead of one, making whatever code you are running twice as slow.

C++26 has everything initialiied by default. The value is not specified though. Implementations are encourage to use something weird to detect using before explict initialization.

Devil's advocate: this would be unacceptable for os kernels and super performance critical code (e.g. hft).

Zero initializing often hides real and serious bugs, however. Say you have a function with an internal variable LEN that ought to get set to some dynamic length that internal operations will run over. Changes to the code introduce a path which skips the setting of LEN. Current compilers will (very likely) warn you about the potentially uninitialized use, valgrind will warn you (assuming the case gets triggered), and failing all that the program will potentially crash when some large value ends up in LEN-- alerting you to the issue.

Compare with default zero init: The compiler won't warn you, valgrind won't warn you, and the program won't crash. It will just be silently wrong in many cases (particularly for length/count variables).

Generally the attention to exploit safety can sometimes push us in directions that are bad for program correctness. There are many places where exploit safety is important, but also many cases where its irrelevant. For security it's generally 'safe' is a program erroneously shuts down or does less than it should but that is far from true for software generally.

I prefer this behavior: Use of an uninitialized variable is an error which the compiler will warn about, however, in code where the compiler cannot prove that it is not used the compiler's behavior is implementation defined and can include trapping on use, initializing to zero, or initializing to ~0 (the complement of zero) or other likely to crash pattern. The developer may annotate with _noinit which makes any use UB and avoids the cost of inserting a trap or ~0 initialization. ~0 init will usually fail but seldom in a silent way, so hopefully at least any user reports will be reproducible.

Similar to RESTRICT _noinit is a potential footgun, but its usage would presumably be quite rare and only in carefully maintained performance critical code. Code using _noinit like RESTRICT is at least still more maintainable than assembly.

This approach preserves the compiler's ability to detect programmer error, and lets the implementation pick the preferred way to handle the remaining error. In some contexts it's preferable to trap cleanly or crash reliably (init to ~0 or explicit trap), in others its better to be silently wrong (init 0).

Since C99 lets you declare variables wherever so it is often easy to just declare a variable where it is first set and that's probably best, of course. .. when you can.

As a retired Gentoo developer, I can say not really as far as I know. There could be static analysis tools that can find this, but I am not aware of anyone who runs them on the entire distribution.

Distributions tend to use shell-script-wrapped compilers that can inject additional flags desired by the distribution, and in all likelihood distributions will just add flags that force the old behaviour if there are problems.

> How much code actually uses unions this way?

I see this change caused Mbed-TLS to start failing its test suite when compiled with GCC 15: https://github.com/Mbed-TLS/mbedtls/issues/9814 (kinda scary since it's a security library). Hopefully other projects with less rigorous test suites aren't using {0} in that way. The Github issue mentions that Clang tried a similar optimization a while ago and backed it out after user complaints, so maybe the same thing will happen with GCC.

I would guess a lot. People aren't intimately familiar with the standard, and people are lazy when it comes to writing boilerplate like initialization code. And up until now, it just worked, so even a good test suite wouldn't catch it.

EDIT: I initially mentioned type punning for arithmetic, but this compiler change wouldn't affect that

No. This is how sum types are implemented.

And from a runtime perspective it’s going to be a struct with perhaps more padding. You’ll need more details about your specific threat model to explain why that’s bad.

> If the size of the new type is larger than the size of the last-written type, the contents of the excess bytes are unspecified (and may be a trap representation). Before C99 TC3 (DR 283) this behavior was undefined, but commonly implemented this way.

https://en.cppreference.com/w/c/language/union

> When initializing a union, the initializer list must have only one member, which initializes the first member of the union unless a designated initializer is used(since C99).

https://en.cppreference.com/w/c/language/struct_initializati...

→ = {0} initializes the first union variant, and bytes outside of that first variant are unspecified. Seems like GCC 15.1 follows the 26 year old standard correctly. (not sure how much has changed from C89 here)

Programming languages are products, that is like saying you want to keep using vi 1.0.

Maybe C should have stop at K&R C from UNIX V6, at least that would have spared the world in having it being adopted outside UNIX.

I suspect this change was motivated by standards conformance.

> I feel like once a language is standardized (or reaches 1.0), that's it. You're done. No more changes. You wanna make improvements? Try out some new ideas? Fine, do that in a new language.

Thank goodness this is not how the software world works overall. I'm not sure you understand the implications of what you ask for.

> if they aren't cheekily mutating over the years

You're complaining about languages mutating, then mention C++ which has added stuff but maintained backwards compatibility over the course of many standards (aside from a few hiccups like auto_ptr, which was also short lived), with a high aversion to modifying existing stuff.

Perl 6 and Python 3 joined the chat

It's careless development. Why think something in advance when you can fix it later. It works so well for Microsoft, Google and lately Apple. /s

The release cycle of a software speaks a lot about its quality. Move fast, break things has become the new development process.