Mis-aligned pointers on x86

Question

Can someone provide an example were casting a pointer from one type to another fails due to mis-alignment?

In the comments to this answer, bothie states that doing s

Answer 1

#include 

int main(int argc, char **argv)
{
  char c[] = "a";

  printf("%d\n", *(int*)(c));
}

This gives me a SIGBUS after setting set $ps |= (1<<18) in gdb, which apparently is thrown when address alignment is incorrect (amongst other reasons).

EDIT: It's fairly easy to raise SIGBUS:

int main(int argc, char **argv)
{
    /* EDIT: enable AC check */
    asm("pushf; "
        "orl $(1<<18), (%esp); "
        "popf;");

    char c[] = "1234567";
    char d[] = "12345678";
    return 0;
}

Looking at main's disassembly in gdb:

Dump of assembler code for function main:
....
0x08048406 :   mov    0x8048510,%eax
0x0804840b :   mov    0x8048514,%edx
0x08048411 :   mov    %eax,-0x10(%ebp)
0x08048414 :   mov    %edx,-0xc(%ebp)
0x08048417 :   movl   $0x34333231,-0x19(%ebp)   <== BAM! SIGBUS
0x0804841e :   movl   $0x38373635,-0x15(%ebp)
0x08048425 :   movb   $0x0,-0x11(%ebp)

Anyhow, Christoph your test program fails under Linux raising a SIGBUS as it should. It's probably a Windows thing?

---

You can enable the Alignment Check bit in code using this snippet:

/* enable AC check */
asm("pushf; "
    "orl $(1<<18), (%esp); "
    "popf;");

Also, ensure that the flag was indeed set:

unsigned int flags;
asm("pushf; "
    "movl (%%esp), %0; "
    "popf; " : "=r"(flags));
fprintf(stderr, "%d\n", flags & (1<<18));