Is zero initialization of structures guaranteed to wipe padded areas?

Question

Suppose I have the following structure:

typedef struct
{
    unsigned field1 :1;
    unsigned field2 :1;
    unsigned field3 :1;
} mytype;

Answer 1

Yes and no. The actual standard, C11, specifies:

If an object that has static or thread storage duration is not initialized explicitly, then:

• ....

• if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

So this only holds for objects of static storage, at a first view. But then later it says in addition:

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

So this means that padding inside sub-structures that are not initialized explicitly is zero-bit initialized.

In summarry, some padding in a structure is guaranteed to be zero-bit initialized, some isn't. I don't think that such a confusion is intentional, I will file a defect report for this.

Older versions didn't have that at all. So with most existing compilers you'd have to be even more careful, since they don't implement C11, yet. But AFAIR, clang already does on that behalf.

Also be aware that this only holds for initialization. Padding isn't necessarily copied on assignment.

Answer 2

My question is, are these padding bits guaranteed by the standard to be zero initialized by the statement:

No.

The value of the padding is unspecified:

(C99, 6.2.6.1p6) "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 values"

EDIT: See Jens Gustedt answer, C11 now guarantees the padding is set to 0 in (rare) certain circumstances

Answer 3

The C99 standard doesn't specify the padding bits would be set to zero. In fact, it specifically mentions that the values of any padding bits are unspecified, so that padding need not be copied in an assignment.

Footnote 51 to 6.2.6.1 (6) (n1570):

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

The new C2011 standard - thanks to Jens Gustedt for sharing that knowledge - specifies that padding bits in objects of static or thread storage duration without explicit initialisation are initialised to 0.

There are still no guarantees for assignment.