printf("%p\n", (void *)0); /* UB, or not? Please explain your answer. */
Nov 14 '05
188  17428 
Antoine Leca wrote:
.... snip ...
I was pointing out that specifying a behaviour for fprintf("%p",
(void*)0) would be useless if (because of the symmetry) you did
not specify that fscanf("%p", &p) could make a null pointer to be
stored in p. And I guess while a number of /implementations/ are
in fact ready to digest the first form (as you pointed out),
there is also a number of /programs/ that might *not* like
receiving a null pointer.
From N869. I see nothing to prevent bandying NULL about.
p Matches an implementation-defined set of sequences,
which should be the same as the set of sequences
that may be produced by the %p conversion of the
fprintf function. The corresponding argument shall
be a pointer to a pointer to void. The input item
is converted to a pointer value in an
implementation-defined manner. If the input item is
a value converted earlier during the same program
execution, the pointer that results shall compare
equal to that value; otherwise the behavior of the
%p conversion is undefined.
Color me dense, but I see no real use for the facility.
--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
[fscanf which could create null pointers if the standard is revised, while
now it can't.]
En 42************* **@yahoo.com, CBFalconer va escriure: I see nothing to prevent bandying NULL about.
If the input item is
a value converted earlier during the same program
execution, the pointer that results shall compare
equal to that value; otherwise the behavior of the
%p conversion is undefined.
Since previous use of printf cannot convert a null pointer, you presently
fear no risk to actually receive an input item that could be converted into
a null pointer, since such a thing (that input item) is not reputed to exist
at all.
Now if the text for printf is updated, _automatically_ (no change of words)
fscanf can now store null pointers for the %p conversions. Which might upset
people which are writing e.g. portable libraries, because that now mean they
should check for these new null pointers, causing code bloat.
I see no real use for the facility.
Which one? The %p conversion? fscanf? ;-)
Antoine
aegis wrote: p The argument shall be a pointer to void. The value of the
pointer is converted to a sequence of printing characters, in
an implementation-defined manner.
(void*)0 is a valid value of the type "pointer to void".
Thus printf is required to accept it in this context.
pete wrote: How about printing the value of the one past pointer,
do you think that is undefined?
printf("&a + 1 is %p.\n", (void *)(&a + 1));
There's another case of a valid value for a pointer of
the type "pointer to void". Thus printf is required to
accept it and do the right thing.
Antoine Leca wrote: If the input item is
a value converted earlier during the same program
execution, the pointer that results shall compare
equal to that value; otherwise the behavior of the
%p conversion is undefined. Since previous use of printf cannot convert a null pointer, you presently
fear no risk to actually receive an input item that could be converted into
a null pointer, since such a thing (that input item) is not reputed to exist
at all.
No, that's all wrong. Not only can fprintf convert a null
pointer value of type void* using the %p format specifier,
but there is also no problem in fscanf converting that
text sequence back to a null pointer value, which of course
will compare equal to the original value.
Now if the text for printf is updated, _automatically_ (no change of words)
fscanf can now store null pointers for the %p conversions. Which might upset
people which are writing e.g. portable libraries, because that now mean they
should check for these new null pointers, causing code bloat.
? Of course pointers need to be used properly. This is a
non-problem.
"Antoine Leca" <ro**@localhost .invalid> writes: [fscanf which could create null pointers if the standard is revised, while
now it can't.]
En 42************* **@yahoo.com, CBFalconer va escriure: I see nothing to prevent bandying NULL about.
If the input item is
a value converted earlier during the same program
execution, the pointer that results shall compare
equal to that value; otherwise the behavior of the
%p conversion is undefined.
Since previous use of printf cannot convert a null pointer, you presently
fear no risk to actually receive an input item that could be converted into
a null pointer, since such a thing (that input item) is not reputed to exist
at all.
Are you assuming that printf("%p\n", (void*)0) invokes undefined
behavior? Since that's the point under dispute here, you might want
to make that assumption explicit.
--
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.
"Douglas A. Gwyn" <DA****@null.ne t> writes: aegis wrote: > p The argument shall be a pointer to void. The value of the
> pointer is converted to a sequence of printing characters, in
> an implementation-defined manner.
(void*)0 is a valid value of the type "pointer to void".
Thus printf is required to accept it in this context.
But part of the standard can be read to imply that (void*)0 is not a
valid value of type "pointer to void".
7.1.4p1 says:
Each of the following statements applies unless explicitly stated
otherwise in the detailed descriptions that follow: If an argument
to a function has an invalid value (such as [...], or a null
pointer, [...]) or [...], the behavior is undefined.
This doesn't *quite* state that a null pointer is invalid unless
explicitly stated otherwise, but it's easy to interpret it that way.
The description of free() explicitly states that a null pointer is
allowed. The description of strlen() doesn't say anything about null
pointers, but we can infer from the above *and* from the description
of the argument ("the string pointed to by s") that a call invokes
undefined behavior if s doesn't point to a string.
On the other hand, the description of the 'p' specifier doesn't say
anything about null pointers. Since 7.1.4p1 weakly implies that null
pointers are invalid, it's too easy to infer that
printf("%p\n", (void*)0) invokes undefined behavior.
Common sense tells me that it *shouldn't* invoke undefined behavior,
but rather it should produce some implementation-defined textual
representation of a null pointer value, and I'm certain that that was
the intent. More specifically, I believe that any pointer value that
can be evaluated without invoking undefined behavior should not invoke
undefined behavior when passed to printf("%p", ...).
But I'm not comfortable depending on "common sense" to determine what
the standard means.
I can imagine a stubborn implementer creating a version of printf()
that depends on the pointer value being non-null (perhaps it shows it
as a base address and offset, or perhaps it attempts to show a few
bytes of what it points to). If I filed a bug report, I'm not sure
what chapter and verse I could cite to convince the implementer that
this is a bug. This probably isn't a realistic situation (I doubt
that any actual printf() implementation blows up on null pointers),
but it's still the kind of thing the standard should address.
--
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.
On Tue, 08 Mar 2005 20:18:22 GMT in comp.std.c, Keith Thompson
<ks***@mib.or g> wrote: "Douglas A. Gwyn" <DA****@null.ne t> writes:
aegis wrote:
> p The argument shall be a pointer to void. The value of the
> pointer is converted to a sequence of printing characters, in
> an implementation-defined manner.
(void*)0 is a valid value of the type "pointer to void".
Thus printf is required to accept it in this context.
But part of the standard can be read to imply that (void*)0 is not a
valid value of type "pointer to void".
But I'm not comfortable depending on "common sense" to determine what
the standard means.
I can imagine a stubborn implementer creating a version of printf()
that depends on the pointer value being non-null (perhaps it shows it
as a base address and offset, or perhaps it attempts to show a few
bytes of what it points to). If I filed a bug report, I'm not sure
what chapter and verse I could cite to convince the implementer that
this is a bug. This probably isn't a realistic situation (I doubt
that any actual printf() implementation blows up on null pointers),
but it's still the kind of thing the standard should address.
Isn't undefined behaviour any behaviour not explicitly defined in the
standard for the abstract machine or the library implementation?
So is the above (undefined) case not then undefined behaviour, as is
the case with strlen()?
The behaviour of printf() with format specifier "%p" is implementation
defined, but ISTM that leaves it open to each implementation whether
they define that part of the implementation to work will null
pointers.
If it could be undefined behaviour, we have no option but to treat it
as undefined behaviour in portable code.
--
Thanks. Take care, Brian Inglis Calgary, Alberta, Canada Br**********@CS i.com (Brian[dot]Inglis{at}Syste maticSW[dot]ab[dot]ca)
fake address use address above to reply
Keith Thompson wrote: But part of the standard can be read to imply that (void*)0 is not a
valid value of type "pointer to void".
7.1.4p1 says:
That's a misreading. The parenthetical phrase "(such as ...)"
is intended to clarify what is meant by "an invalid value",
not to completely characterize the set of invalid values.
In many function-call contexts a null pointer argument would
indeed be invalid, e.g. when a pointer to an array is specified,
but not in the case of printf("%p",vp) , where any well-defined
value is permitted. (A null pointer has a "valid" value.)
On the other hand, the description of the 'p' specifier doesn't say
anything about null pointers.
It doesn't have to. A null pointer value with type "pointer to
void" is just one case covered by the fprintf spec, and in this
context it does not have any special properties.
In article <ln************ @nuthaus.mib.or g>, Keith Thompson <ks***@mib.or g> writes: "Peter Nilsson" <ai***@acay.com .au> writes:
[...] Consider a cpu with 24-bit address space, but 32-bit address/data
registers. Such a machine may have 255 or more null pointer
representations , none of which point to a genuine addressable
memory location.
Actually it could have 4278190080 such pointer representations
(2**32 - 2**24). (Whether they all be treated as C null pointers is
another question.)
For a real-world example, let me invoke once again the mighty AS/400:
for its (conforming C90) implementations , the null pointer is
all-bits-zero, but all valid non-null pointer values have bit 127
set. Thus the '400 has 2**127 invalid pointer representations , but
only one is a null pointer.
(To approach the ostensible topic of this thread a bit, awhile back I
posted the output of %p formatting for some AS/400 pointers, I
believe. They're ... distinctive.)
--
Michael Wojcik mi************@ microfocus.com
She felt increasingly (vision or nightmare?) that, though people are
important, the relations between them are not, and that in particular
too much fuss has been made over marriage; centuries of carnal
embracement, yet man is no nearer to understanding man. -- E M Forster This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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