Do a union or struct permit assignment from an uninitialised instance?

Question

This question is about the definedness or otherwise of assigning an uninitalised automatic variable to another one of the same type.

Consider

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Answer 1

The behavior is undefined in (1) and (2).

Per the C standard, the value of an object with automatic storage duration that is not initialized is indeterminate (C 2011 [N1570] 6.7.9 10). Nominally, this means it has some value, but we do not know what it is while writing the program.

However, the standard also 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” (6.3.2.1 2). In your sample code, the address of a is never taken, and it is not initialized, and using it in an expression is an lvalue. Therefore, the behavior is undefined.

(This passage, 6.3.2.1 2, was designed to accommodate processors that can detect use of an uninitialized register. Nonetheless, the rule in the C standard applies to all implementations.)

(3) is not clearly addressed by the C standard. Although a member of the union has been assigned a value, and hence is not uninitialized for purposes of 6.3.2.1 2, the object being used in b = a is the union, not its member. Obviously, our intuitive notion is that, if a member of a union is assigned a value, the union has a value. However, I do not see this specified in the C standard.

We can infer 6.3.2.1 2 is not intended to consider a union or structure to be uninitialized, at least if part of it has been assigned a value, because:

1. Structures can have unnamed members, such as unnamed bit fields.

2. Per C 6.7.9 9, unnamed members of structures have indeterminate value, even after initialization (of the structures).

3. If 6.3.2.1 2 applied to structures in which not every member had been assigned a value, then b = a would always be undefined if a were a structure with an unnamed member and had automatic storage duration.

4. That seems unreasonable and not what the standard intended.

However, there is some wiggle room here. The standard could have specified that a structure is not uninitialized only if it were initialized or all of its named members have been assigned values. In that case (3) would be undefined if a were a structure in which only one member had been assigned a value. I do not think this wiggle room exists with a union; if a member of the union has been assigned a value, it is only reasonable to consider the union not to be uninitialized.

Answer 2

In general, assigning from an uninitialized object isn't undefined behavior, it only makes the result unspecified.

But the code you show indeed has undefined behavior -- for a different reason than you assume. Citing N1570 (latest C11 draft), §6.3.2.1 p2 here:

[...] 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.

Explaining this a bit: The C standard is prepared to handle values that aren't stored in an addressable location. This is typically the case when they are held in one of the CPU's registers. Explicitly giving an object the register storage class is only a hint to the compiler that it should, if sensible, hold that object in a register. The other way around, a compiler is free to hold any object with automatic storage duration in a register as long as the code doesn't need to address it (by taking a pointer).

In your code, you have uninitialized objects with automatic storage duration that never have their address taken, so the compiler would be free to place them in registers. This means there is no value for the object (not even an unspecified one) before it is initialized. Therefore, using this potentially non-existent value to initialize another object (or, for other purposes) is undefined behavior.

If your code would take a pointer to the respective a in all these examples, the result of the assignment would be unspecified (of course), but the behavior would be defined.

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It's worth to add that structs and unions have nothing to do with the answer to your question. The rules are the same for all kind of objects with automatic storage duration. That said, in your third example, a isn't uninitialized any more, after you assign one member of the union. So for your third example, the behavior is well-defined. It doesn't matter what's in the other member of the union, a union can only hold a value for one of its members at a time.