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strange integer-pointer behaviour

I've ran into some fishy code that, at first glance, is buggy, but it
seems to work correctly
and none of the compilers I've tried (five so far, on various systems)
gives any warnings.
The code:
=============== =============
#include <stdio.h>

void
fcn (char *str)
{
if (str == '\0')
{
printf ("str!\n");
}
}

int
main (void)
{
fcn ('\0');
return 0;
}
=============== =============
My understanding so far: '\0' is passed to fcn(), it equals integer 0,
which in turn "equals"
a NULL pointer in terms of zero-ness. The 0 in fcn() is then compared
to '\0' which is also a 0.
No warnings, no errors, code works (or so it seems).
But - if I pass e.g. '\1' to fcn() than all the compilers complain
with similar warnings,
i.e. making a pointer from integer without a cast.

My question is, why don't the compilers complain when '\0' is passed?
Shouldn't they give
the same warning, as it's also an integer (albeit 0 in value)?

--
WYCIWYG - what you C is what you get

Aug 13 '07
17 1873
CBFalconer <cb********@yah oo.comwrote:
Eric Sosman wrote:
Well, no, not exactly. There is no need to type-check
a NULL, because NULL is a perfectly good value for any kind
of pointer at all. The confusion (what remains of it) comes
from the fact that there are many ways to spell NULL.

Gertrude Stein never said "A NULL is a '\0' is a 0,"
but she could have.

I don't agree that '\0' is a spelling of NULL.
The only spelling of NULL, which is the name of a macro, is NULL. '\0'
is, however, a spelling of the null pointer constant.
It is a spelling of the int value zero.
No, it's an integer _constant_ of type int and value zero. Big
difference. An integer constant with value zero is a null pointer
constant; an int with value zero is not, and is not guaranteed (though
highly likely) to compare equal to a null pointer object.

Yes, I do agree that writing NULL when you mean "a null pointer
constant" is a bad idea, because it adds to the confusion many newbies
have concerning null pointers.
--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com
Fix your .sig.

Richard
Aug 14 '07 #11
CBFalconer wrote On 08/13/07 16:20,:
matevzb wrote:
>>I've ran into some fishy code that, at first glance, is buggy, but
it seems to work correctly and none of the compilers I've tried
(five so far, on various systems) gives any warnings. The code:

#include <stdio.h>

void fcn (char *str) {


This wants a pointer to char as the parameter.
Right.
> if (str == '\0') {


This tests the parameter against a fixed char.
No, t tests the parameter against a null pointer
constant.
> printf ("str!\n");
}
}

int main (void) {
fcn ('\0');


This passes an integer (char representation) to something that
wants a char. pointer.
No, it passes a NULL-valued `char*' to fcn(). '\0' is
a zero-valued integer constant expression, hence it is a
null pointer constant. It is used in a pointer context
(thanks to the prototype), so the N.P.C. becomes a null
pointer. The N.P. is converted to type `char*' (again,
thanks to the prototype) and that value is passed to the
function.
>
> return 0;
}
--
Er*********@sun .com

Aug 14 '07 #12
CBFalconer wrote:
matevzb wrote:
>>
I've ran into some fishy code that, at first glance, is buggy, but
it seems to work correctly and none of the compilers I've tried
(five so far, on various systems) gives any warnings. The code:

#include <stdio.h>

void fcn (char *str) {

This wants a pointer to char as the parameter.
> if (str == '\0') {

This tests the parameter against a fixed char.
As others have pointed out, '\0' is an allowable null pointer constant, but
also, even in other contexts, '\0' isn't a char. It's an int. You might be
thinking of C++, where it would be a char.
> printf ("str!\n");
}
}

int main (void) {
fcn ('\0');

This passes an integer (char representation) to something that
wants a char. pointer.
As above, it passes a null pointer constant to a function expecting a
pointer argument.
> return 0;
}
Aug 14 '07 #13
On Mon, 13 Aug 2007 20:04:50 -0400, CBFalconer wrote:
Eric Sosman wrote:
>>
... snip ...
>>
Well, no, not exactly. There is no need to type-check
a NULL, because NULL is a perfectly good value for any kind
of pointer at all. The confusion (what remains of it) comes
from the fact that there are many ways to spell NULL.

Gertrude Stein never said "A NULL is a '\0' is a 0,"
but she could have.

I don't agree that '\0' is a spelling of NULL. It is a spelling of
the int value zero. The primes make it precisely an int.
Nothing in the standard forbids <stddef.hor other standard
headers to #define NULL '\0'.
--
Army1987 (Replace "NOSPAM" with "email")
No-one ever won a game by resigning. -- S. Tartakower

Aug 14 '07 #14
rl*@hoekstra-uitgeverij.nl (Richard Bos) writes:
CBFalconer <cb********@yah oo.comwrote:
[...]
>I don't agree that '\0' is a spelling of NULL.

The only spelling of NULL, which is the name of a macro, is NULL. '\0'
is, however, a spelling of the null pointer constant.
Of *a* null pointer constant.
>It is a spelling of the int value zero.

No, it's an integer _constant_ of type int and value zero. Big
difference. An integer constant with value zero is a null pointer
constant;
Right. More generally, so is an integer constant expression with
value zero.
an int with value zero is not, and is not guaranteed (though
highly likely) to compare equal to a null pointer object.
An "int with value zero" presumably refers to an object that exists
during execution time. A null pointer constant exists only in source
code; there's no such thing during execution.

An int with value zero cannot compare equal (or unequal) to a pointer
whose value is a null pointer. An attempt to compare them is a
constraint violation, because the types are incompatible.

It's likely, but not guaranteed, that *converting* a (non-constant)
int with value zero to a pointer type will yield a null pointer value.
Since integer-to-pointer conversion is implementation-defined, this is
actually independent of the question of whether a null pointer value
is represented as all-bits-zero.

[...]

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Aug 14 '07 #15
Keith Thompson <ks***@mib.orgw rote:
rl*@hoekstra-uitgeverij.nl (Richard Bos) writes:
CBFalconer <cb********@yah oo.comwrote:
[...]
I don't agree that '\0' is a spelling of NULL.
The only spelling of NULL, which is the name of a macro, is NULL. '\0'
is, however, a spelling of the null pointer constant.

Of *a* null pointer constant.
_The_ spelling of _a_ null pointer constant, or _a_ spelling of _the_
null pointer constant. Take your pick; IMO, the difference is of an
angels-on-needlepoints value.

Richard
Aug 15 '07 #16
rl*@hoekstra-uitgeverij.nl (Richard Bos) writes:
Keith Thompson <ks***@mib.orgw rote:
>rl*@hoekstra-uitgeverij.nl (Richard Bos) writes:
CBFalconer <cb********@yah oo.comwrote:
[...]
>I don't agree that '\0' is a spelling of NULL.

The only spelling of NULL, which is the name of a macro, is NULL. '\0'
is, however, a spelling of the null pointer constant.

Of *a* null pointer constant.

_The_ spelling of _a_ null pointer constant, or _a_ spelling of _the_
null pointer constant. Take your pick; IMO, the difference is of an
angels-on-needlepoints value.
It's not a big deal, but there isn't a single null pointer constant;
there are arbitrarily many of them.

An integer constant expression with the value 0, or such an
expression cast to type void *, is called a _null pointer
constant_.

Referring to "the null pointer constant" makes no more sense than
referring to "the integer constant". (A difference is that all null
pointer constants refer to the same value, but they're still distinct
constants.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Aug 15 '07 #17
Eric Sosman wrote:
>
matevzb wrote On 08/13/07 14:30,:
[...]
Yes, sorry about even posting this. Moments after I posted, I realized
that all bets are off with 0 with regards to (pointer) type
checking...

Well, no, not exactly. There is no need to type-check
a NULL, because NULL is a perfectly good value for any kind
of pointer at all. The confusion (what remains of it) comes
from the fact that there are many ways to spell NULL.

Gertrude Stein never said "A NULL is a '\0' is a 0,"
but she could have.
A NULL by any other name would smell as sweet to those nasal demons.

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>
Aug 16 '07 #18

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