is NULL-checking redundant in accessor-functions? - Programming Languages

Hello!

Consider the following simple accessor function:

typedef struct {
int i;
char name[32];
} MY_TYPE;

const char *
getName(const MY_TYPE *p)
{
return p->name;
}

I always assumed, that checking the passed pointer `p' against NULL is
redundant, because, if it happens to be NULL at run-time (as a result of
programming error), trying to dereference it would result in SEGFAULT
anyway.

I'm now looking at a situation, where an accessor function, instead of
crashing, returns the offset of the field `name' from 0 (0x4 in the above
example). As if there is a legal value 0x00000000 at the NULL pointer...

Any comments? This is on a Sparc machine under Solaris-9. Thanks!

-mi
--
comp.lang.c.moderated - moderation address: [email]clcm@plethora.net[/email]

26: 1,2
[...else...]

Ye shall eat the flesh of your sons, and the flesh of your daughters ye shall eat.
Leviticus 26:29


Roast Child with Cornbread Stuffing

Turkey may be substituted for this classic holiday feast.
Although time consuming, this dish seems to take longer than it actually does;
as the entire house is filled with such a heavenly aroma,
the waiting becomes almost unbearable.

1 whole child, cleaned and de-headed
1 batch cornbread stuffing (see index)
½ cup melted butter

Remove the giblets from the infant and set aside.
Stuff the cavity where the child?s genitals and anus were located
using ½ cup per pound of meat.
Tie the arms flat to the body, then pull the skin flaps up to close the cavity.
Now tie the thighs up tight to hold it all together.
Place breast side up in a large metal roasting pan.
Bake in 325° oven covered for 2 hours.
Remove cover, stick a cooking thermometer deep into one of the
baby?s buttocks and cook uncovered till thermo

"Mikhail Teterin" <usenet@aldan.algebra.com> wrote[color=blue]
>
> I'm now looking at a situation, where an accessor function, instead of
> crashing, returns the offset of the field `name' from 0 (0x4 in the above
> example). As if there is a legal value 0x00000000 at the NULL pointer...
>
> Any comments? This is on a Sparc machine under Solaris-9. Thanks!
>[/color]
Dereferencing a null pointer is undefined, so if you are totally relying on
any particular behaviour, then you must make sure you don't do it.
One problem with C is that accessing an illegal, though not null, pointer is
also undefined, but there is no way to protect your function from being
passed such a pointer.
In many environments, simply accepting that the function will probably crash
but might just return garbage if passed NULL is OK. However if it is an
issue for you, then you should trap the null pointer and consider what to do
if passed it.
--
comp.lang.c.moderated - moderation address: [email]clcm@plethora.net[/email]

Mikhail Teterin wrote on 25/12/04 :[color=blue]
> Consider the following simple accessor function:
>
> typedef struct {
> int i;
> char name[32];
> } MY_TYPE;
>
> const char *
> getName(const MY_TYPE *p)
> {
> return p->name;
> }
>
> I always assumed, that checking the passed pointer `p' against NULL is
> redundant,[/color]

I didn't :

char *getName (const MY_TYPE *this)
{
char *s = 0;

if (this != NULL)
{
s = this->name;
}
return s;
}

[color=blue]
> because, if it happens to be NULL at run-time (as a result of
> programming error), trying to dereference it would result in SEGFAULT
> anyway.[/color]

Who knows? The standard says that dereferencing a NULL pointer invokes
an undefined behaviour. It may crash or not.
[color=blue]
> I'm now looking at a situation, where an accessor function, instead of
> crashing, returns the offset of the field `name' from 0 (0x4 in the above
> example). As if there is a legal value 0x00000000 at the NULL pointer...
>
> Any comments? This is on a Sparc machine under Solaris-9. Thanks![/color]

Now, you know better what an undefined behaviour is.

--
Emmanuel
The C-FAQ: [url]http://www.eskimo.com/~scs/C-faq/faq.html[/url]
The C-library: [url]http://www.dinkumware.com/refxc.html[/url]

"C is a sharp tool"
--
comp.lang.c.moderated - moderation address: [email]clcm@plethora.net[/email]

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[Note: missing F'up2 reduction of Xpost fixed...]

In comp.lang.c.moderated Mikhail Teterin <usenet@aldan.algebra.com> wrote:[color=blue]
> Hello![/color]
[color=blue]
> Consider the following simple accessor function:[/color]
[color=blue]
> typedef struct {
> int i;
> char name[32];
> } MY_TYPE;[/color]
[color=blue]
> const char *
> getName(const MY_TYPE *p)
> {
> return p->name;
> }[/color]
[color=blue]
> I always assumed, that checking the passed pointer `p' against NULL is
> redundant, because, if it happens to be NULL at run-time (as a result of
> programming error), trying to dereference it would result in SEGFAULT
> anyway.[/color]

That assumption is dangerously incorrect. Passing NULL (or any other
wild pointer) into this function will result in undefined behaviour.
Period. That undefined behaviour may typically be a SIGSEGV, on your
platform, but there's nothing even remotely resembling a guarantee for
that.

It's your responsibility to protect your code from invalid pointer
values being dereferenced, not the platform's or anyone else's.

--
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.
--
comp.lang.c.moderated - moderation address: [email]clcm@plethora.net[/email] -- you must
have an appropriate newsgroups line in your header for your mail to be seen,
or the newsgroup name in square brackets in the subject line. Sorry.

Mikhail Teterin wrote:[color=blue]
> Hello!
>
> Consider the following simple accessor function:
>
> typedef struct {
> int i;
> char name[32];
> } MY_TYPE;
>
> const char *
> getName(const MY_TYPE *p)
> {
> return p->name;
> }
> I'm now looking at a situation, where an accessor function, instead of
> crashing, returns the offset of the field `name' from 0 (0x4 in the above
> example). As if there is a legal value 0x00000000 at the NULL pointer...
>
> Any comments? This is on a Sparc machine under Solaris-9. Thanks![/color]

You could write getName as:

const char *
getName(const MY_TYPE *p)
{
const char *r = (const char *)p;
r += offsetof(MY_TYPE, name);
return r;
}

and your compiler is almost certainly producing the equivalent code from
your definition.

You will note that this does not dereference p at all. I'd be surprised
if it dereferenced p on any platform.

Phil
--
comp.lang.c.moderated - moderation address: [email]clcm@plethora.net[/email] -- you must
have an appropriate newsgroups line in your header for your mail to be seen,
or the newsgroup name in square brackets in the subject line. Sorry.