Editing Bus error (section)

==Example==
This is an example of unaligned memory access, written in the [[C (programming language)|C programming language]] with [[AT&T syntax|AT&T assembly syntax]].

<syntaxhighlight lang="c">
#include <stdlib.h>

int main(int argc, char **argv) 
{
    int *iptr;
    char *cptr;
    
#if defined(__GNUC__)
# if defined(__i386__)
    /* Enable Alignment Checking on x86 */
    __asm__("pushf\norl $0x40000,(%esp)\npopf");
# elif defined(__x86_64__) 
     /* Enable Alignment Checking on x86_64 */
    __asm__("pushf\norl $0x40000,(%rsp)\npopf");
# endif
#endif

    /* malloc() always provides memory which is aligned for all fundamental types */
    cptr = malloc(sizeof(int) + 1);
    
    /* Increment the pointer by one, making it misaligned */
    iptr = (int *) ++cptr;

    /* Dereference it as an int pointer, causing an unaligned access */
    *iptr = 42;

    /*
       Following accesses will also result in sigbus error.
       short *sptr;
       int    i;

       sptr = (short *)&i;
       // For all odd value increments, it will result in sigbus.
       sptr = (short *)(((char *)sptr) + 1);
       *sptr = 100;
    
    */

    return 0;
}
</syntaxhighlight>

Compiling and running the example on a [[POSIX]] compliant OS on [[x86]] demonstrates the error:
<syntaxhighlight lang="console">
$ gcc -ansi sigbus.c -o sigbus
$ ./sigbus 
Bus error
$ gdb ./sigbus
(gdb) r
Program received signal SIGBUS, Bus error.
0x080483ba in main ()
(gdb) x/i $pc
0x80483ba <main+54>:    mov    DWORD PTR [eax],0x2a
(gdb) p/x $eax
$1 = 0x804a009
(gdb) p/t $eax & (sizeof(int) - 1)
$2 = 1
</syntaxhighlight>
The [[GDB]] [[debugger]] shows that the [[constant (programming)|immediate value]] 0x2a is being stored at the location stored in the [[IA-32|EAX]] [[processor register|register]], using [[X86 assembly language]]. This is an example of [[addressing mode#Register indirect|register indirect]] addressing.

Printing the [[least significant bit|low order bits]] of the address shows that it is not [[data structure alignment|aligned to a word boundary]] ("dword" using x86 terminology).