https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502
Bug ID: 61502
Summary: == comparison on "one-past" pointer gives wrong result
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: Peter.Sewell at cl dot cam.ac.uk
Created attachment 32934
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=32934&action=edit
C code as pasted into bug report
The following code can produce a pointer to one-past the x object. When it
does, according to the C11 standard text, the result of the pointer comparison
should be true, but gcc gives false.
#include <stdio.h>
int y = 2, x=1;
int main()
{
int *p;
p = &x +1 ;
printf("&x=%p &y=%p p=%p\n",(void*)&x, (void*)&y, (void*)p);
_Bool b1 = (p==&y);
printf("(p==&y) = %s\n", b1?"true":"false");
return 0;
}
gcc-4.8 -std=c11 -pedantic -Wall -Wextra -O2 -o a.out
pointer_representation_1e.c && ./a.out
&x=0x601020 &y=0x601024 p=0x601024
(p==&y) = false
gcc-4.8 --version
gcc-4.8 (Ubuntu 4.8.1-2ubuntu1~12.04) 4.8.1
The pointer addition is licensed by 6.5.6 "Additive operators", where:
6.5.6p7 says "For the purposes of these operators, a pointer to an object that
is not an element of an array behaves the same as a pointer to the first
element of an array of length one with the type of the object as its element
type.", and
6.5.6p8 says "[...] Moreover, if the expression P points to the last element of
an array object, the expression (P)+1 points one past the last element of the
array object [...]".
The pointer comparison is licensed by 6.5.9 "Equality operators", where:
6.5.9p7 says "For the purposes of these operators, a pointer to an object that
is not an element of an array behaves the same as a pointer to the first
element of an array of length one with the type of the object as its element
type.",
6.5.9p6 says "Two pointers compare equal if and only if [...] or one is a
pointer to one past the end of one array object and the other is a pointer to
the start of a different array object that happens to immediately follow the
first array object in the address space.109)", and
Footnote 109 says "Two objects may be adjacent in memory because they are
adjacent elements of a larger array or adjacent members of a structure with no
padding between them, or because the implementation chose to place them so,
even though they are unrelated. [...]".
