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GCC 15.1

(gcc.gnu.org)
270 points jrepinc | 3 comments | 25 Apr 25 10:53 UTC | HN request time: 0.001s | source
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Calavar ◴[25 Apr 25 12:39 UTC] No.43792948[source]▶
> {0} initializer in C or C++ for unions no longer guarantees clearing of the whole union (except for static storage duration initialization), it just initializes the first union member to zero. If initialization of the whole union including padding bits is desirable, use {} (valid in C23 or C++) or use -fzero-init-padding-bits=unions option to restore old GCC behavior.

This is going to silently break so much existing code, especially union based type punning in C code. {0} used to guarantee full zeroing and {} did not, and step by step we've flipped the situation to the reverse. The only sensible thing, in terms of not breaking old code, would be to have both {0} and {} zero initialize the whole union.

I'm sure this change was discussed in depth on the mailing list, but it's absolutely mind boggling to me

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nikic ◴[25 Apr 25 20:42 UTC] No.43798312[source]▶
Fun fact: GCC decided to adopt Clang's (old) behavior at the same time Clang decided to adopt GCC's (old) behavior.

So now you have this matrix of behaviors: * Old GCC: Initializes whole union. * New GCC: Initializes first member only. * Old Clang: Initializes first member only. * New Clang: Initializes whole union.

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1. augusto-moura ◴[25 Apr 25 23:06 UTC] No.43799320[source]▶
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

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2. Neywiny ◴[26 Apr 25 00:36 UTC] No.43799844[source]▶
I would argue the even deeper problem is that it's implementation defined. Should be in the spec and they should conform to the spec. That's why I'm so paranoid and zeroize things myself. Too much hassle to remember what is or isn't zero.
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3. flohofwoe ◴[26 Apr 25 08:47 UTC] No.43801951[source]▶
I wouldn't depend on that too much either though, or at least not depend on padding bytes being zeroed. The compiler is free to replace the memset call with code that only zeroes the struct members, but leaves junk in the padding bytes (and the same is true when copying/assigning a struct).