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GCC 15.1

(gcc.gnu.org)
270 points jrepinc | 1 comments | 25 Apr 25 10:53 UTC | HN request time: 0.204s | 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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1. not2b ◴[26 Apr 25 04:35 UTC] No.43800975[source]▶
I'm skeptical of the claim that this change will "silently break so much existing code". For it to change the behavior of code, the first member would have to be smaller than other members, someone would have to use this construct to initialize union objects, and it would have to affect the behavior. In any case, it's standard for the Fedora, Ubuntu, and Debian developers to go through all the packages and test with new GCC versions before they come out, so that issues are fixed before the new compiler is released.