Defect Report #178

Submission Date: 06 Feb 96
Submittor: WG14
Source: Frank Farance
Question
Is the following conforming?
struct x
{
char y[1];
};
struct x *z;

z = (struct x *) malloc(sizeof (*z) + 100);
z- y[5] = '?';
Defect Report #051 states that this isn't conforming behavior because the pointer arithmetic for the larger structure might not be compatible with a smaller structure. Thus, it recommends the safer idiom:
#define HUGE_ARR 1000 /* or bigger than ever needed */
struct x
{
char y[HUGE_ARR];
};
struct x *z;

z = (struct x *) malloc(sizeof (*z) + 100);
z- y[5] = '?';
However, Defect Report #073 states that the safer idiom is undefined behavior because it is possible to implement the operator - as first fetching all of *z, then selecting y[5] from it. This approach would cause access to unallocated memory. Thus, the operation produces undefined behavior.
These responses are inconsistent. At the Oct 95 meeting in Nashua NH, WG14 indicated that it wanted to designate this as undefined behavior.
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