Is using a structure without all members assigned undefined?

Question

Consider this code in block scope:

struct foo { unsigned char a; unsigned char b; } x, y;
x.a = 0;
y = x;

C [N1570] 6.3.2.1 2 says “If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.”

Although a member of x has been assigned a value, no assignment to x has been performed, and its address has not been taken. Thus, it appears 6.3.2.1 2 tells us the behavior of x in y = x is undefined.

However, if we had assigned a value to every member of x, it would seem unreasonable to consider x to be uninitialized for the purposes of 6.3.2.1 2.

(1) Is there anything in the standard which, strictly speaking, causes 6.3.2.1 2 not to apply to (make undefined) the code above?

(2) Supposing we were modifying the standard or determining a reasonable modification to 6.3.2.1 2, are there reasons to prefer one of the following over the others? (a) 6.3.2.1 2 does not apply to structures. (b) If at least one member of a structure has been assigned a value, the structure is not uninitialized for purposes of 6.3.2.1 2. (c) If all named¹ members of a structure have been assigned a value, the structure is not uninitialized for purposes of 6.3.2.1 2.

Footnote

¹ Structures may have unnamed members, so it is not always possible to assign a value to every member of a structure. (Unnamed members have indeterminate value even if the structure is initialized, per 6.7.9 9.)

Answer 1

Copying a partially-written structure falls in the category of actions which quality implementations will process in consistent fashion absent a good reason to do otherwise, specialized implementations might process differently because they have a good reason to do so, and poor-quality-but-conforming implementations may use as an excuse to behave nonsensically.

Note that copying uninitialized values of an automatic-duration or malloc-created character array would fall in a similar category of actions, except that implementations that would trap on such an action (e.g. to help programmers identify and track down potential information leaks) would not be allowed to describe themselves as "conforming".

An implementation which is specialized to diagnose accidental information leaks might sensibly trap efforts to copy a partially-written structure. On an implementation where using an unitialized value of some type could result in strange behavior, copying a structure with an unitialized member of that type and then attempting to use that member of the copy might sensibly do likewise.

The Standard doesn't particularly say whether a partially-written structure counts as having been written or not, because people seeking to produce quality implementations shouldn't care. Quality implementations specialized for detecting potential information leakage should squawk at any attempt to copy uninitialized data, without regard for when the Standard would or would not allow such behavior (provided that they describe themselves as non-conforming). Quality general-purpose implementations designed to support a wide variety of programs should allow partially-initialized structures to be copied in cases where programs don't look at the uninitialized portions outside the context of whole-structure copying (such treatment is useful and generally costs nothing in non-contrived cases). The Standard could be construed as granting poor-quality-but-conforming implementations the right treat copying of partially-written structures as an excuse to behave nonsensically, but such implementations could use almost anything as such an excuse. Quality implementations won't do anything unusual when copying structures unless they document a good reason for doing so.