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.