Submitter: Clark Nelson
Submission Date: 2002-09-18
Subject: Accessing a non-current union member ("type punning")
[This report isolates one of the points from DR257.]Problem
In the paragraph corresponding to 188.8.131.52#5, C89 contained this sentence:
With one exception, if a member of a union object is accessed after a value has been stored in a different member of the object, the behavior is implementation-defined.Associated with that sentence was this footnote:
The "byte orders" for scalar types are invisible to isolated programs that do not indulge in type punning (for example, by assigning to one member of a union and inspecting the storage by accessing another member that is an appropriately sixed array of character type), but must be accounted for when conforming to externally imposed storage layouts.The only corresponding verbiage in C99 is 184.108.40.206#7:
When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values, but the value of the union object shall not thereby become a trap representation.It is not perfectly clear that the C99 words have the same implications as the C89 words.
Suggested Technical Corrigendum
[Essentially verbatim from DR257]
Attach a new footnote 78a to the words "named member" in 220.127.116.11#3:
78a If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called "type punning"). This might be a trap representation.
Proposed Committee Response
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