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Add FLATCC_ENFORCE_ALIGNED_EMPTY_VECTORS #289

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Merged
mikkelfj merged 5 commits into from
Aug 19, 2024
Merged

Add FLATCC_ENFORCE_ALIGNED_EMPTY_VECTORS #289

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5ac3d68
sketch FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS
bkietz Aug 16, 2024
9fb9a1d
revert reader changes, handle align better
bkietz Aug 17, 2024
007e1f1
decls before stmts, add comment
bkietz Aug 19, 2024
5be03be
TOLERATE_MISALIGNED -> ENFORCE_ALIGNED
bkietz Aug 19, 2024
41f5ee4
amend README.md
bkietz Aug 19, 2024
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amend README.md
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@bkietz
bkietz committed Aug 19, 2024
commit 41f5ee48d8d1e66c975eff05ad00cfea9fd66c4d
27 changes: 15 additions & 12 deletions README.md
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Expand Up @@ -1164,18 +1164,20 @@ sufficient, but for example standard 32-bit Windows only allocates on an
aligned fields.

NOTE: as of August 2024 it has been discovered that C++ writer code has been
aligning empty vectors to the size field only, even if elements where larger,
like the double type of size 8. This would cause the FlatCC verifier to
(correctly) reject these vectors because it would result in an invalid C pointer
type on some architectures. However, because this has been in effect for over
10 years, the consensus is to have verifiers tolerate this behaviour even if
C++ will eventually fix this issue. The FlatCC verifier will be updated to
accept such buffers by default with a compile time flag to enforce the strict
behaviour as well. In principle the misaligned vectors can lead potentially
lead to undefined behaviour in agressively optimized C compilers. As of now
it appears to be safe to read such buffers on common platforms and it is
preferably to avoid additional runtime reader overhead to deal with this.
For more, see [FlatCC #287], [Google Flatbuffers #8374].
aligning empty vectors to the size field only, even if elements require
greater alignment like the double type which requires 8. This would cause the
FlatCC verifier to (correctly) reject these vectors because it would result
in an invalid C pointer type on some architectures. However, because this has
been in effect for over 10 years, the consensus is to have verifiers tolerate
this behaviour even if C++ will eventually fix this issue. The FlatCC
verifier has been updated to accept such buffers by default with an optional
compile time flag to enforce the strict behaviour as well
(`FLATCC_ENFORCE_ALIGNED_EMPTY_VECTORS`). In principle the misaligned vectors
can lead potentially lead to undefined behaviour in agressively optimized C
compilers. As of now it appears to be safe to read such buffers on common
platforms and it is preferable to avoid additional runtime reader overhead to
deal with this. For more, see [FlatCC #287], [Google Flatbuffers #8374],
[FlatCC #289].


### Potential Name Conflicts
Expand Down Expand Up @@ -2718,3 +2720,4 @@ See [Benchmarks]
[WebKit Style]: https://webkit.org/code-style-guidelines/
[FlatCC #287]: https://github.com/dvidelabs/flatcc/issues/287
[Google Flatbuffers #8374]: https://github.com/google/flatbuffers/issues/8374
[FlatCC #289]: https://github.com/dvidelabs/flatcc/issues/289