Defect Report #222

Submitter: Clive Feather (UK)
Submission Date: 2000-04-04
Source:
Reference Document: N/A
Version: 1.4
Date: 2001-09-18 16:03:01
Subject: Partially initialized structures

Summary
Consider the code extract:

   struct listheader
     {
         struct item *head;
         struct item *tail;
     };
     // The following is at block scope    struct listheader h1;
     h1.head = NULL;
     struct listheader h2;
     h2 = h1;

The value of h1.tail is indeterminate throughout, but provided that the code never accesses it this is not a problem. However, if it holds a trap representation, the assignment to h2 involves assigning a trap representation, which is undefined behaviour.

There are two possible resolutions I can think of:

1. Say that the code is defined. Any implementation that uses memberwise copying of structures now has to explicitly disable detection of trap values.
2. Say that the code is undefined. This is going to surprise a number of people. In particular, it becomes impossible to assign any structure where the complete list of fields is unknown (e.g. any structure defined in a Standard header).

Committee Discussion (for history only)

A TC should remove the notion of objects of struct or union type having a trap representation. Changes need to be made to 6.2.6.1 paragraphs 6 and 7, and footnote 42. It was observed that the point of the original footnote was primarily to illustrate one reason why padding bits might not be copied: because member-by-member assignment might be performed. But member-by-member assignment would imply that struct assignment could produce undefined behavior if a member of the struct had a value that was a trap representation. Instead of adding further text explaining that member values that were trap representations were not permitted to render assignment of a containing struct or union object undefined (e.g. if member-by-member copying were used), it was decided that the footnote should simply clarify the issue of padding bits directly.

Technical Corrigendum
Change 6.2.6.1 paragraph #6 to:

When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified value.⁴²⁾ The value of a struct or union object is never a trap representation, even though the value of a member of a struct or union object may be a trap representation.

42) Thus, for example, structure assignment need not copy any padding bits.

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.

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