[Bug c/63233] New: Valid out of bounds access leads to undefined behavior
leis at in dot tum.de
gcc-bugzilla@gcc.gnu.org
Thu Sep 11 21:15:00 GMT 2014
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
Bug ID: 63233
Summary: Valid out of bounds access leads to undefined behavior
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: leis at in dot tum.de
Consider the following (minimized) program:
#include <stdlib.h>
#include <stdio.h>
struct Foo {
int a[1];
int b;
};
int main(int argc, char** argv) {
int index = atoi(argv[1]);
struct Foo foo;
foo.a[1] = 99;
printf("%d\n", foo.a[index]);
return 0;
}
When compiled with -O1 or higher and called with 1 as command line argument an
undefined value instead of 99 is printed. In my understanding of the standard,
foo.a is a pointer and foo.a[1] is simply (foo.a+1), which is a perfectly fine
memory address.
This program always prints the expected value (99) with clang (3.5) and icc
(14) on all optimization levels. In gcc 4.3.4 I get the expected result on -O0
and -O1 but not on -O2 or -O3.
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