Issue 0016.02: Can you implicitly initialize a union when null pointers have nonzero bit patterns?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Sam Kendall, WG14
Date: 1992-12-10
Reference document: X3J11/90-052
Submitted against: C90
Status: Fixed
Fixed in: C90 TC1
Converted from: dr.htm, dr_016.html

This one is relevant only for hardware on which either null pointer or floating point zero is not represented as all zero bits.

Consider this sentence in subclause 6.5.7 (starting on page 71, line 41):

If an object that has static storage duration is not initialized explicitly, it is initialized implicitly as if every member that has arithmetic type were assigned 0 and every member that has pointer type were assigned a null pointer constant.

This implies that you cannot implicitly initialize a union object that could contain overlapping members with different representations for zero/null pointer. For example, given this translation unit:

union { char *p; int i; } x;

If the null pointer is represented as, say, 0x80000000, then there is no way to implicitly initialize this object. Either the p member contains the null pointer, or the i member contains 0, but not both. So the behavior of this translation unit is undefined.

This is a bad state of affairs. I assume it was not the Committee's intention to prohibit a large class of implicitly initialized unions; this would render a great deal of existing code nonconforming.

The right thing - although I can find no support for this idea in the draft - is to implicitly initialize only the first member of a union, by analogy with explicit initialization. Here is a proposed new sentence; perhaps it can be saved for the next time we make a C standard. (This sentence also tries to get around the difficulty of the old “as if ... assigned” language in dealing with const items; Dave Prosser tipped me off there.)

1. If an object that has static storage duration is not initialized explicitly, it is initialized implicitly according to these rules:
2. if it is a scalar with pointer type, it is initialized implicitly to a null pointer constant;
3. if it is a scalar with non-pointer type, it is initialized implicitly to zero;
4. if it is an aggregate, every member is initialized (recursively) according to these rules;
5. if it is a union, the first member is initialized (recursively) according to these rules.

Comment from WG14 on 1997-09-23:

Correction

In subclause 6.5.7, page 71, line 41 through page 72, line 2, change:

If an object that has static storage duration is not initialized explicitly, it is initialized implicitly as if every member that has arithmetic type were assigned 0 and every member that has pointer type were assigned a null pointer constant.

to:

1. If an object that has static storage duration is not initialized explicitly, then:
2. if it has pointer type, it is initialized to a null pointer;
3. if it has arithmetic type, it is initialized to zero;
4. if it is an aggregate, every member is initialized (recursively) according to these rules;
5. if it is a union, the first named member is initialized (recursively) according to these rules.