[GitHub] [arrow] emkornfield commented on a change in pull request #12599: ARROW-15846: [Format] Clarify presence of struct validity bitmap

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emkornfield commented on a change in pull request #12599:
URL: https://github.com/apache/arrow/pull/12599#discussion_r824114223



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File path: docs/source/format/Columnar.rst
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@@ -208,17 +208,19 @@ right-to-left: ::
               0  0  1  0  1  0  1  1
 
 Arrays having a 0 null count may choose to not allocate the validity
-bitmap. Implementations may choose to always allocate one anyway as a
-matter of convenience, but this should be noted when memory is being
-shared.
+bitmap; how this is represented depends on the implementation (for
+example, a C++ implementation may represent such an "absent" validity
+bitmap using a NULL pointer). Implementations may choose to always allocate
+a validity bitmap anyway as a matter of convenience. Consumers of Arrow
+arrays should be ready to handle those two possibilities.
 
-Nested type arrays except for union types have their own validity bitmap and
-null count regardless of the null count and valid bits of their child arrays.
+Nested type arrays (except for union types as noted above) have their own
+top-level validity bitmap and null count, regardless of the null count and
+valid bits of their child arrays.
 
-Array slots which are null are not required to have a particular
-value; any "masked" memory can have any value and need not be zeroed,
-though implementations frequently choose to zero memory for null
-values.
+Array slots which are null are not required to have a particular value;
+any "masked" memory can have any value and need not be zeroed, though

Review comment:
       can you clarify what you mean by UB here? Dereferencing a pointer should always be defined given other constraints but tools like ASAN might complain if you try to take actions on the memory location. 




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