This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: Derek M. Jones, WG14
Date: 1992-12-10
Reference document: X3J11/90-056
Submitted against: C90
Status: Fixed
Fixed in: C90 TC1
Converted from: dr.htm, dr_017.html
Pointer to multidimensional array
Given the declaration:
char a[3][4], (*p)[4]=a[1];
Does the behavior become undefined when:
p no longer points within the slice of the array, orp no longer points within the object a?This case should be explicitly stated.
Arguments for/against:
The standard refers to a pointed-to object. There does not appear to be any concept of a slice of an array being an independent object.
Comment from WG14 on 1997-09-23:
For an array of arrays, the permitted pointer arithmetic in subclause 6.3.6, page 47, lines 12-40 is to be understood by interpreting the use of the word “object” as denoting the specific object determined directly by the pointer's type and value, not other objects related to that one by contiguity. Therefore, if an expression exceeds these permissions, the behavior is undefined. For example, the following code has undefined behavior:
int a[4][5];
a[1][7] = 0; /* undefined */
Some conforming implementations may choose to diagnose an “array bounds violation,” while others may choose to interpret such attempted accesses successfully with the “obvious” extended semantics.
Add to subclause G.2, page 201:
- An array subscript is out of range, even if an object is apparently
accessible with the given subscript (as in the lvalue expression a[1][7] given
the declaration int a[4][5]) (6.3.6).