bus error

[Rahul]

Hi Everyone,

I read an article which claims that an invalid address ( not aligned
in power of two ) would cause a bus error exception, i tried the
following program and it didn't give any error,

int main()
{
int a = 10;
char *p = &a;
int *pp;
printf("%p\n",( void*)&a);
++p;
*p = 10;
pp = p;
printf("pp is %p and val %d\n",pp,*pp);
}

Am i missing something? How do i generate the bus error ?

Thanks in advance ! ! !

[Richard Tobin]

Re: bus error

In article <7a5a24d4-86eb-4545-9c10-915d26deaecf@a2 3g2000hsc.googl egroups.com>,
Rahul <sam_cit@yahoo. co.inwrote:

I read an article which claims that an invalid address ( not aligned
>in power of two ) would cause a bus error exception, i tried the
>following program and it didn't give any error,

You don't really mean "power of two". Processors that require
alignment usually require alignment on a multiple of the object size.

Some processors require alignment, some don't. Presumably you're
using one that doesn't, probably something in the x86 family.

Processors that require alignment do so for efficiency and simplicity
reasons: an aligned object can be read or written in a single access,
and can't cross page boundaries. From the viewpoint of most C
programmers, it has the advantage that it often catches certain errors
sooner than would happen otherwise. On the other hand, it has the
disadvantage of wasting space between objects of different sizes, and
making it hard to match data structures directly to external
representations .

-- Richard
--
:wq

[Ulrich Eckhardt]

Re: bus error

Rahul wrote:

I read an article which claims that an invalid address ( not aligned
in power of two ) would cause a bus error exception

That by itself is wrong. Rather, an unaligned access causes a bus error. An
unaligned access is e.g. accessing a 2-byte value on an odd address, but
not accessing a single byte on the same address. However, on the very
popular x86 platform, this error doesn't even come through to the OS, let
alone the program that caused it. Rather, the unaligned access is emulated
by the CPU, and the only way to see it is to watch for loss of performance.
I guess running on an x86 is the reason that you will never see this error.

That said, this behaviour is specific to both the CPU and the OS (it could
also emulate the access transparently in software!), none of which is
actually mandated by the C language, so I would suggest further discussion
in a different newsgroup.

Uli

[David Thompson]

Re: bus error

On 26 Mar 2008 15:38:11 GMT, richard@cogsci. ed.ac.uk (Richard Tobin)
wrote:

In article <7a5a24d4-86eb-4545-9c10-915d26deaecf@a2 3g2000hsc.googl egroups.com>,
Rahul <sam_cit@yahoo. co.inwrote:
>

I read an article which claims that an invalid address ( not aligned
in power of two ) would cause a bus error exception, i tried the
following program and it didn't give any error,

>
You don't really mean "power of two". Processors that require
alignment usually require alignment on a multiple of the object size.
>

Technically, but the sizes of primitive types almost always are powers
of two: 2, 4, 8, or 16 bytes. (On byte machines, which nowadays are
all 8 bits.) This is the sort of alignment that actually allows
hardware advantages; multiples of 5, while formally similar, would in
practice be worthless and thus not used.

Some processors require alignment, some don't. Presumably you're
using one that doesn't, probably something in the x86 family.
>

Yes. Or one that has a silent fixup in place, but that's rarer.

- formerly david.thompson1 || achar(64) || worldnet.att.ne t

[Richard Tobin]

Re: bus error

In article <78civ3d3jeur1m plnene9vmvi1988 27lca@4ax.com>,
David Thompson <dave.thompson2 @verizon.netwro te:

I read an article which claims that an invalid address ( not aligned
>in power of two ) would cause a bus error exception, i tried the
>following program and it didn't give any error,

>You don't really mean "power of two". Processors that require
>alignment usually require alignment on a multiple of the object size.

>Technically, but the sizes of primitive types almost always are powers
>of two: 2, 4, 8, or 16 bytes. (On byte machines, which nowadays are
>all 8 bits.) This is the sort of alignment that actually allows
>hardware advantages;

I wasn't suggesting that there might be non-power-of-two alignment,
but that you can't just use an arbitrary power of two; you have to
use whatever is appropriate to the object in question.

>multiples of 5, while formally similar, would in
>practice be worthless and thus not used.

Incidentally, 5 byte floats were common on some early microprocessor
systems, but of course they had no alignment restrictions as they
accessed memory in single bytes.

-- Richard
--
:wq