Fix #14150 (Add unionzeroinit check) #7870
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Second attempt of #7843 which accidently got merged. |
danmar
reviewed
Oct 8, 2025
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I hope all feedback have been addressed at this point. |
This has bitten me before and is definitely a foot gun. GCC 15[1] changed their
semantics related to zero initialization of unions; from initializing the
complete union (sizeof union) to only zero initializing the first member. If the
same first member is not the largest one, the state of the remaining storage is
considered undefined and in practice most likely stack garbage.
The unionzeroinit checker can detect such scenarios and emit a warning. It does
not cover the designated initializers as I would interpret those as being
intentional.
Example output from one of my projects:
```
x86-decoder.c:294:7: warning: Zero initializing union Evex does not guarantee its complete storage to be zero initialized as its largest member is not declared as the first member. Consider making u32 the first member or favor memset(). [unionzeroinit-unionzeroinit]
Evex evex = {0};
^
```
[1] https://trofi.github.io/posts/328-c-union-init-and-gcc-15.html
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danmar
approved these changes
Oct 12, 2025
|
@mptre thanks.. now I merged it by intention :-) please close the ticket. |
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This has bitten me before and is definitely a foot gun. GCC 15[1] changed their semantics related to zero initialization of unions; from initializing the complete union (sizeof union) to only zero initializing the first member. If the same first member is not the largest one, the state of the remaining storage is considered undefined and in practice most likely stack garbage.
The unionzeroinit checker can detect such scenarios and emit a warning. It does not cover the designated initializers as I would interpret those as being intentional.
Example output from one of my projects:
[1] https://trofi.github.io/posts/328-c-union-init-and-gcc-15.html