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

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.