NULL pointer and zero value

**Richard Heathfield** · Nov 14 '05, 12:47 AM

Re: NULL pointer and zero value

vp wrote:
[color=blue]
> If I have a pointer char * p, is it correct to assign NULL to this
> pointer by:
>
> "memset( &p, 0, sizeof(p));" instead of "p = NULL;"[/color]

No. NULL is not necessarily all-bits-zero.

Your technique will, however, work by chance on quite a few platforms,
because quite a few platforms do use all-bits-zero for NULL. If you like
programming-by-casino, go ahead. :-)

<snip>

--
Richard Heathfield : binary@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

**Christopher Benson-Manica** · Nov 14 '05, 12:47 AM

Re: NULL pointer and zero value

Richard Heathfield <invalid@addres s.co.uk.invalid > spoke thus:
[color=blue][color=green]
>> (all-bits 0 as NULL post)[/color]
> If you like programming-by-casino, go ahead. :-)[/color]

So the house wins on NULL and double NULL? ;)

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

**Martin Dickopp** · Nov 14 '05, 12:47 AM

Re: NULL pointer and zero value

dt1649651@yahoo .com (vp) writes:
[color=blue]
> If I have a pointer char * p, is it correct to assign NULL to this
> pointer by:
>
> "memset( &p, 0, sizeof(p));" instead of "p = NULL;"[/color]

No. The former statement sets all bits of `p' to zero, which is not
necessarily a representation of the null pointer.
[color=blue]
> The reason I ask is I have an array of structure of N function
> variables, for example,
>
> typedef struct
> {
> int (* func1)();
> int (* func2)();
> void * (* func2)(int );
> } ModuleFunctions ;
>
> #define N 100
> ModuleFunction garMF[N];
>
> When initializing, instead of making a loop to assgin NULL to each
> function member of struct ModuleFunction for each member of array, is
> it correct to do something like
>
> memset( garMF, 0, sizeof(ModuleFu nctions)*N );[/color]

If `garMF' has static storage duration, it is automatically initialized to
zero if you don't initialize it explicitly. "Initialize d to zero" means
that each non-compound object is initialized as if you assigned `0' to it
(i.e. pointers are initialized to null pointers). For compound objects,
its members or elements are recursively initialized in this way.

Otherwise (if `garMF' doesn't have static storage duration) you'll have
to loop.

Martin

**Mark A. Odell** · Nov 14 '05, 12:47 AM

Re: NULL pointer and zero value

Richard Heathfield <invalid@addres s.co.uk.invalid > wrote in
news:3ff17d2f@n ews2.power.net. uk:
[color=blue]
> No. NULL is not necessarily all-bits-zero.
>
> Your technique will, however, work by chance on quite a few platforms,
> because quite a few platforms do use all-bits-zero for NULL. If you like
> programming-by-casino, go ahead. :-)[/color]

So are you saying that this isn't safe?

#include <stdlib.h>

int main(void)
{
int *pVar = malloc(1024);

if (pVar) /* <--- Not safe? */
{
free(pVar);
}

return EXIT_SUCCESS;
}

--
- Mark ->
--

**pete** · Nov 14 '05, 12:47 AM

Re: NULL pointer and zero value

Martin Dickopp wrote:[color=blue]
>
> dt1649651@yahoo .com (vp) writes:
>[color=green]
> > If I have a pointer char * p, is it correct to assign NULL to this
> > pointer by:
> >
> > "memset( &p, 0, sizeof(p));" instead of "p = NULL;"[/color]
>
> No. The former statement sets all bits of `p' to zero, which is not
> necessarily a representation of the null pointer.
>[color=green]
> > The reason I ask is I have an array of structure of N function
> > variables, for example,
> >
> > typedef struct
> > {
> > int (* func1)();
> > int (* func2)();
> > void * (* func2)(int );
> > } ModuleFunctions ;
> >
> > #define N 100
> > ModuleFunction garMF[N];
> >
> > When initializing, instead of making a loop to assgin NULL to each
> > function member of struct ModuleFunction
> > for each member of array, is it correct to do something like
> >
> > memset( garMF, 0, sizeof(ModuleFu nctions)*N );[/color][/color]
[color=blue]
> Otherwise (if `garMF' doesn't have static storage duration)
> you'll have to loop.[/color]

I disagree about the necessity of a loop.

ModuleFunction garMF[N] = {NULL};

--
pete

**pete** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

Mark A. Odell wrote:[color=blue]
>
> Richard Heathfield <invalid@addres s.co.uk.invalid > wrote in
> news:3ff17d2f@n ews2.power.net. uk:
>[color=green]
> > No. NULL is not necessarily all-bits-zero.
> >
> > Your technique will, however,
> > work by chance on quite a few platforms,
> > because quite a few platforms do use all-bits-zero for NULL.
> > If you like programming-by-casino, go ahead. :-)[/color]
>
> So are you saying that this isn't safe?
>
> #include <stdlib.h>
>
> int main(void)
> {
> int *pVar = malloc(1024);
>
> if (pVar) /* <--- Not safe? */[/color]

Your code does not check to see if all bits are zero.

Your question and corresponding code example
are not relevant to what Richard Heathfield said.

--
pete

**Martin Dickopp** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

"Mark A. Odell" <nospam@embedde dfw.com> writes:
[color=blue]
> Richard Heathfield <invalid@addres s.co.uk.invalid > wrote in
> news:3ff17d2f@n ews2.power.net. uk:
>[color=green]
> > No. NULL is not necessarily all-bits-zero.
> >
> > Your technique will, however, work by chance on quite a few platforms,
> > because quite a few platforms do use all-bits-zero for NULL. If you like
> > programming-by-casino, go ahead. :-)[/color]
>
> So are you saying that this isn't safe?
>
> #include <stdlib.h>
>
> int main(void)
> {
> int *pVar = malloc(1024);
>
> if (pVar) /* <--- Not safe? */
> {
> free(pVar);
> }
>
> return EXIT_SUCCESS;
> }[/color]

The program is correct (i.e. strictly conforming). The `if' statement
doesn't look at the representation (i.e. the bits) of `pVar', it simply
compares `pVar' to `0'. A comparison between an expression of pointer
type and `0', which is a null pointer constant in such a comparison, is
valid.

Martin

**pete** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

pete wrote:[color=blue]
>
> Mark A. Odell wrote:[color=green]
> >
> > Richard Heathfield <invalid@addres s.co.uk.invalid > wrote in
> > news:3ff17d2f@n ews2.power.net. uk:
> >[color=darkred]
> > > No. NULL is not necessarily all-bits-zero.
> > >
> > > Your technique will, however,
> > > work by chance on quite a few platforms,
> > > because quite a few platforms do use all-bits-zero for NULL.
> > > If you like programming-by-casino, go ahead. :-)[/color]
> >
> > So are you saying that this isn't safe?
> >
> > #include <stdlib.h>
> >
> > int main(void)
> > {
> > int *pVar = malloc(1024);
> >
> > if (pVar) /* <--- Not safe? */[/color]
>
> Your code does not check to see if all bits are zero.
>
> Your question and corresponding code example
> are not relevant to what Richard Heathfield said.[/color]

The question is relevant, the answer is "no".

--
pete

**CBFalconer** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

vp wrote:[color=blue]
>
> If I have a pointer char * p, is it correct to assign NULL to
> this pointer by:
>
> "memset( &p, 0, sizeof(p));" instead of "p = NULL;"[/color]

NO.

While it will often "work" on a particular system, it is not
portable and can lead to very mysterious bugs.

--
Chuck F (cbfalconer@yah oo.com) (cbfalconer@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

**Mark A. Odell** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

Martin Dickopp <expires-2004-01-31@zero-based.org> wrote in
news:bss11a$kme $02$2@news.t-online.com:
[color=blue][color=green]
>> So are you saying that this isn't safe?
>>
>> #include <stdlib.h>
>>
>> int main(void)
>> {
>> int *pVar = malloc(1024);
>>
>> if (pVar) /* <--- Not safe? */
>> {
>> free(pVar);
>> }
>>
>> return EXIT_SUCCESS;
>> }[/color]
>
> The program is correct (i.e. strictly conforming). The `if' statement
> doesn't look at the representation (i.e. the bits) of `pVar', it simply
> compares `pVar' to `0'. A comparison between an expression of pointer
> type and `0', which is a null pointer constant in such a comparison, is
> valid.[/color]

Excellent. Thank you.

--
- Mark ->
--

vp · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

dt1649651@yahoo .com (vp) wrote in message news:<244097ab. 0312300503.48de ff52@posting.go ogle.com>...[color=blue]
> If I have a pointer char * p, is it correct to assign NULL to this
> pointer by:
>
> "memset( &p, 0, sizeof(p));" instead of "p = NULL;"
>[/color]
[...]

Thanks for all of your replies and explanation. I see that I just can
not be lazy to make that memset. I don't want to put Las Vegas into my
app :-)

DT

vp · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

"Mark A. Odell" <nospam@embedde dfw.com> wrote in message news:<Xns94615B 16B259BCopyrigh tMarkOdell@130. 133.1.4>...[color=blue]
> Richard Heathfield <invalid@addres s.co.uk.invalid > wrote in
> news:3ff17d2f@n ews2.power.net. uk:
>[color=green]
> > No. NULL is not necessarily all-bits-zero.
> >
> > Your technique will, however, work by chance on quite a few platforms,
> > because quite a few platforms do use all-bits-zero for NULL. If you like
> > programming-by-casino, go ahead. :-)[/color]
>
> So are you saying that this isn't safe?
>
> #include <stdlib.h>
>
> int main(void)
> {
> int *pVar = malloc(1024);
>
> if (pVar) /* <--- Not safe? */
> {
> free(pVar);
> }
>
> return EXIT_SUCCESS;
> }[/color]

I guess the compiler will do the necessary comparison in this case. I
often see this style of code "if ( pointer )", but personally I often
write it like "if ( p != NULL )" just to remind me or my co-workers
that p is of pointer-type.

DT

**Mark A. Odell** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

dt1649651@yahoo .com (vp) wrote in
news:244097ab.0 312301010.4362a 0db@posting.goo gle.com:
[color=blue][color=green]
>> if (pVar) /* <--- Not safe? */
>> {
>> free(pVar);
>> }
>>
>> return EXIT_SUCCESS;
>> }[/color]
>
> I guess the compiler will do the necessary comparison in this case. I
> often see this style of code "if ( pointer )", but personally I often
> write it like "if ( p != NULL )" just to remind me or my co-workers
> that p is of pointer-type.[/color]

I avoid explicit checks for boolean conditions and since all my pointer
vars have a lower-case p in front, e.g. pSomeName, this is never an issue
for me. Besides, if I want to know if 'foo' is a pointer then I just hover
over it with my mouse and CodeWright shows me the definition. I thought
all editors could do such simple things. ;-)

--
- Mark ->
--

**Keith Thompson** · Nov 14 '05, 12:48 AM

Re: NULL pointer and zero value

dt1649651@yahoo .com (vp) writes:[color=blue]
> If I have a pointer char * p, is it correct to assign NULL to this
> pointer by:
>
> "memset( &p, 0, sizeof(p));" instead of "p = NULL;"[/color]

No.

The C FAQ is at <http://www.eskimo.com/~scs/C-faq/top.html>.
Read section 5, "Null Pointers".

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://www.sdsc.edu/~kst>
Schroedinger does Shakespeare: "To be *and* not to be"
(Note new e-mail address)

NULL pointer and zero value

NULL pointer and zero value

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