NULL to strlen function

**mark_bluemel@pobox.com** · Jan 11 '07, 01:05 PM

Re: NULL to strlen function

sam_...@yahoo.c o.in wrote:

Is it that strlen function can't handle NULL pointers?

Strlen expects a string as an argument. Is a NULL pointer a string?
(Hint: this is a rhetorical question).

**Richard Heathfield** · Jan 11 '07, 01:15 PM

Re: NULL to strlen function

sam_cit@yahoo.c o.in said:

Hi Everyone,
>
I tried the following program unit in Microsoft Visual c++ 6.0 and the
program caused unexpected behavior,

Undefined behaviour.

#include <stdio.h>
#include <string.h>
>
int main()
{
char *p;
p = NULL;
printf("%d\n",s trlen(p));
}
>
Is it that strlen function can't handle NULL pointers?

Or is it that strlen returns size_t and your format specifier requires int?

It is impossible to tell which of these two produces the undefined behaviour
you have noticed.

Any good C book will explain the semantics of strlen.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at the above domain, - www.

**Barry** · Jan 11 '07, 01:25 PM

Re: NULL to strlen function

<sam_cit@yahoo. co.inwrote in message
news:1168519098 .421364.11980@o 58g2000hsb.goog legroups.com...

Hi Everyone,
>
I tried the following program unit in Microsoft Visual c++ 6.0 and the
program caused unexpected behavior,
>
#include <stdio.h>
#include <string.h>
>
int main()
{
char *p;
p = NULL;
printf("%d\n",s trlen(p));
}
>
Is it that strlen function can't handle NULL pointers?
>

My interpretation would be by definition strlen cannot
properly handle a NULL pointer.

Returns

The strlen function returns the number of characters that precede the
terminating null
character.

**Francois Grieu** · Jan 11 '07, 01:25 PM

Re: NULL to strlen function

In article <1168519098.421 364.11980@o58g2 000hsb.googlegr oups.com>,
sam_cit@yahoo.c o.in wrote:

I tried the following program unit in Microsoft Visual c++ 6.0
and the program caused unexpected behavior,

#include <stdio.h>
#include <string.h>

int main()
{
char *p;
p = NULL;
printf("%d\n",s trlen(p));
}

Is it that strlen function can't handle NULL pointers?

There is no requirement that it does.
NULL is not a valid argument for a char* or const char*
in most functions of <string.h>.
This is hinted in by the lack of explict allowance for
NULL in 7.21.1 (String functions conventions) of
ISO/IEC 9899:1999

For an empty string, try
p = "";

Francois Grieu

**Barry** · Jan 11 '07, 01:45 PM

Re: NULL to strlen function

"Francois Grieu" <fgrieu@gmail.c omwrote in message
news:fgrieu-6AD040.14185011 012007@news-3.proxad.net...

In article <1168519098.421 364.11980@o58g2 000hsb.googlegr oups.com>,
sam_cit@yahoo.c o.in wrote:
>

I tried the following program unit in Microsoft Visual c++ 6.0
and the program caused unexpected behavior,

>
#include <stdio.h>
#include <string.h>
>
int main()
{
char *p;
p = NULL;
printf("%d\n",s trlen(p));
}
>

Is it that strlen function can't handle NULL pointers?

>
There is no requirement that it does.
NULL is not a valid argument for a char* or const char*
in most functions of <string.h>.
This is hinted in by the lack of explict allowance for
NULL in 7.21.1 (String functions conventions) of
ISO/IEC 9899:1999
>
For an empty string, try
p = "";
>
>
Francois Grieu

This doesn't need to be hinted by anything.
strlen() returns a size_t i.e. an unsigned integer.

**Chris Dollin** · Jan 11 '07, 02:05 PM

Re: NULL to strlen function

Barry wrote:

"Francois Grieu" <fgrieu@gmail.c omwrote in message
news:fgrieu-6AD040.14185011 012007@news-3.proxad.net...

>In article <1168519098.421 364.11980@o58g2 000hsb.googlegr oups.com>,
> sam_cit@yahoo.c o.in wrote:
>>

I tried the following program unit in Microsoft Visual c++ 6.0
and the program caused unexpected behavior,

>>
>#include <stdio.h>
>#include <string.h>
>>
>int main()
>{
> char *p;
> p = NULL;
> printf("%d\n",s trlen(p));
>}
>>

Is it that strlen function can't handle NULL pointers?

>>
>There is no requirement that it does.
>NULL is not a valid argument for a char* or const char*
>in most functions of <string.h>.
>This is hinted in by the lack of explict allowance for
>NULL in 7.21.1 (String functions conventions) of
>ISO/IEC 9899:1999
>>
>For an empty string, try
> p = "";
>>
> Francois Grieu

>
This doesn't need to be hinted by anything.
strlen() returns a size_t i.e. an unsigned integer.

Even if that is fixed, it doesn't matter. `strlen` is a
Standard library function, and hence, unless otherwise
specified, is undefined when a pointer argument is null.

As someone else said: NULL isn't a string. So asking for
its string-length is like asking custard for its display
resolution.

--
Chris "first on the Underground!" Dollin
A rock is not a fact. A rock is a rock.

**Clark S. Cox III** · Jan 11 '07, 02:45 PM

Re: NULL to strlen function

Barry wrote:

I thought (perhaps mistakenly) the question was trying to be more
thoughtful and the OP meant to ask something like:
>
Why doesn't strlen check for a NULL pointer and return.

If I pass NULL to strlen, then it was a mistake; simply returning would
only server to mask that error, and potentially hide it until my product
was already released. If strlen attempts to dereference NULL then it
will interrupt the program (on every implementation that I use, and yes,
I test this assumption before relying on it on every platform for which
I code).

I prefer my errors to be caught *before* release as much as possible.

--
Clark S. Cox III
clarkcox3@gmail .com

**Barry** · Jan 11 '07, 03:05 PM

Re: NULL to strlen function

"Chris Dollin" <chris.dollin@h p.comwrote in message
news:eo5fj9$o3d $1@murdoch.hpl. hp.com...

Barry wrote:
>

"Francois Grieu" <fgrieu@gmail.c omwrote in message
news:fgrieu-6AD040.14185011 012007@news-3.proxad.net...

In article <1168519098.421 364.11980@o58g2 000hsb.googlegr oups.com>,
sam_cit@yahoo.c o.in wrote:
>
I tried the following program unit in Microsoft Visual c++ 6.0
and the program caused unexpected behavior,
>
#include <stdio.h>
#include <string.h>
>
int main()
{
char *p;
p = NULL;
printf("%d\n",s trlen(p));
}
>
Is it that strlen function can't handle NULL pointers?
>
There is no requirement that it does.
NULL is not a valid argument for a char* or const char*
in most functions of <string.h>.
This is hinted in by the lack of explict allowance for
NULL in 7.21.1 (String functions conventions) of
ISO/IEC 9899:1999
>
For an empty string, try
p = "";
>
Francois Grieu

This doesn't need to be hinted by anything.
strlen() returns a size_t i.e. an unsigned integer.

>
Even if that is fixed, it doesn't matter. `strlen` is a
Standard library function, and hence, unless otherwise
specified, is undefined when a pointer argument is null.
>
As someone else said: NULL isn't a string. So asking for
its string-length is like asking custard for its display
resolution.

The OP often posts questions that demonstrate he/she
hasn't applied what he/she should have taken away from
previous responses.

I thought (perhaps mistakenly) the question was trying to
be more thoughtful and the OP meant to ask something like:

Why doesn't strlen check for a NULL pointer and return.

**David T. Ashley** · Jan 11 '07, 03:05 PM

Re: NULL to strlen function

<sam_cit@yahoo. co.inwrote in message
news:1168519098 .421364.11980@o 58g2000hsb.goog legroups.com...

Hi Everyone,
>
I tried the following program unit in Microsoft Visual c++ 6.0 and the
program caused unexpected behavior,
>
#include <stdio.h>
#include <string.h>
>
int main()
{
char *p;
p = NULL;
printf("%d\n",s trlen(p));
}
>
Is it that strlen function can't handle NULL pointers?

You're apparently a new programmer, so let me add this:

A NULL pointer and a string of length zero are not the same thing.

A NULL pointer traditionally has the value of 0, which is usually an
unsuitable pointer; but in any case it is a reserved value defined to be a
non-functional pointer.

A string of length 0 has a non-NULL pointer, but the area of memory that is
pointed to has its first byte as 0.

The two are very different.

**Christopher Benson-Manica** · Jan 11 '07, 03:15 PM

Re: NULL to strlen function

Clark S. Cox III <clarkcox3@gmai l.comwrote:

If I pass NULL to strlen, then it was a mistake

On the contrary, I think it's certainly possible to imagine code that
could profitably take advantage of the hypothetical ability of
strlen() to deal reasonably with a null pointer. Given how the
language is defined, of course, it's a mistake.

only server to mask that error, and potentially hide it until my product
was already released. If strlen attempts to dereference NULL then it
will interrupt the program (on every implementation that I use, and yes,
I test this assumption before relying on it on every platform for which
I code).

If you find a platform for which your assumption does not hold, what
are your plans for testing your code?

--
C. Benson Manica | I *should* know what I'm talking about - if I
cbmanica(at)gma il.com | don't, I need to know. Flames welcome.

**David T. Ashley** · Jan 11 '07, 03:25 PM

Re: NULL to strlen function

"Christophe r Benson-Manica" <ataru@otaku.fr eeshell.orgwrot e in message
news:eo5jps$p6m $1@chessie.cirr .com...

Clark S. Cox III <clarkcox3@gmai l.comwrote:
>

>If I pass NULL to strlen, then it was a mistake

>
On the contrary, I think it's certainly possible to imagine code that
could profitably take advantage of the hypothetical ability of
strlen() to deal reasonably with a null pointer. Given how the
language is defined, of course, it's a mistake.
>

>only server to mask that error, and potentially hide it until my product
>was already released. If strlen attempts to dereference NULL then it
>will interrupt the program (on every implementation that I use, and yes,
>I test this assumption before relying on it on every platform for which
>I code).

>
If you find a platform for which your assumption does not hold, what
are your plans for testing your code?

I think Clark's comments are reasonable. It is no different than generously
using assert(), except he is relying on the [hardware/OS] platform to check
the assertions.

**Barry** · Jan 11 '07, 03:35 PM

Re: NULL to strlen function

"Clark S. Cox III" <clarkcox3@gmai l.comwrote in message
news:12qcj50d7f cii90@corp.supe rnews.com...

Barry wrote:

I thought (perhaps mistakenly) the question was trying to be more
thoughtful and the OP meant to ask something like:

Why doesn't strlen check for a NULL pointer and return.

>
If I pass NULL to strlen, then it was a mistake; simply returning would
only server to mask that error, and potentially hide it until my product
was already released. If strlen attempts to dereference NULL then it
will interrupt the program (on every implementation that I use, and yes,
I test this assumption before relying on it on every platform for which
I code).
>
I prefer my errors to be caught *before* release as much as possible.

Your inability to read an entire message or thread and reply is
laughable.

**Barry** · Jan 11 '07, 03:35 PM

Re: NULL to strlen function

"Clark S. Cox III" <clarkcox3@gmai l.comwrote in message
news:12qcj50d7f cii90@corp.supe rnews.com...

Barry wrote:

I thought (perhaps mistakenly) the question was trying to be more
thoughtful and the OP meant to ask something like:

Why doesn't strlen check for a NULL pointer and return.

>
If I pass NULL to strlen, then it was a mistake; simply returning would
only server to mask that error, and potentially hide it until my product
was already released. If strlen attempts to dereference NULL then it
will interrupt the program (on every implementation that I use, and yes,
I test this assumption before relying on it on every platform for which
I code).
>
I prefer my errors to be caught *before* release as much as possible.
>
--
Clark S. Cox III
clarkcox3@gmail .com

The NEXT time you choose to pick a sentence out of context you
should properly apologize in the same forum.

**Clark S. Cox III** · Jan 11 '07, 04:05 PM

Re: NULL to strlen function

Christopher Benson-Manica wrote:

Clark S. Cox III <clarkcox3@gmai l.comwrote:
>

>If I pass NULL to strlen, then it was a mistake

>
On the contrary, I think it's certainly possible to imagine code that
could profitably take advantage of the hypothetical ability of
strlen() to deal reasonably with a null pointer. Given how the
language is defined, of course, it's a mistake.

Other than assert'ing that the parameter is non-NULL, what would you
consider a reasonable way for strlen to deal with NULL?

only server to mask that error, and potentially hide it until my product
was already released. If strlen attempts to dereference NULL then it
will interrupt the program (on every implementation that I use, and yes,
I test this assumption before relying on it on every platform for which
I code).

>
If you find a platform for which your assumption does not hold, what
are your plans for testing your code?

If I find a platform where that assumption doesn't hold, then, as I said
above, I won't rely on it. The last platform on which I did any
extensive coding that didn't, upon a read from NULL, cause programs to
immediately crash, interrupt, drop into a debugger, etc. was the old
(i.e. pre MacOSX, pre-VM) MacOS. However, even there, it was a simple
matter to configure the debugger to watch for access to that memory
location.

--
Clark S. Cox III
clarkcox3@gmail .com

NULL to strlen function

NULL to strlen function

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