NULL pointer and zero value

[Jack Klein]

Re: NULL pointer and zero value

On Tue, 30 Dec 2003 14:07:21 GMT, pete <pfiland@mindsp ring.com> wrote
in comp.lang.c:
[color=blue]
> Martin Dickopp wrote:[color=green]
> >
> > dt1649651@yahoo .com (vp) writes:
> >[color=darkred]
> > > 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=darkred]
> > > 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=green]
> > 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};[/color]

I would much prefer {0} to {NULL} here.

--
Jack Klein
Home: http://JK-Technology.Com
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[Kevin Goodsell]

Re: NULL pointer and zero value

Mark A. Odell wrote:
[color=blue]
>
> 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]

Answers already given, but I wanted to mention that this is also valid:

int *p = 0;

regardless of the object-representation of the null pointer value. This
is somewhat obvious, because even on systems where the null-pointer
value is not represented by all-bits-zero, NULL must still be defined as
a constant expression with the value 0, possibly cast to void*, so this:

int *p = NULL;

is translated by the preprocessor to something equivalent to one of the
following:

int *p = 0;

or

int *p = (void *)0;

It follows somewhat logically that things like these:

if (p)
if (p == 0)

are actually testing p against the null pointer value, whatever its
representation is.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Kevin Goodsell]

Re: NULL pointer and zero value

Jack Klein wrote:
[color=blue]
> On Tue, 30 Dec 2003 14:07:21 GMT, pete <pfiland@mindsp ring.com> wrote
> in comp.lang.c:
>[color=green]
>>
>>ModuleFunctio n garMF[N] = {NULL};[/color]
>
>
> I would much prefer {0} to {NULL} here.
>[/color]

Is {NULL} even correct here if NULL happens to be #defined with a cast
to void*?

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Peter Nilsson]

Re: NULL pointer and zero value

"Kevin Goodsell" <usenet1.spamfr ee.fusion@never box.com> wrote in message
news:ZAmIb.2649 5$Pg1.22006@new sread1.news.pas .earthlink.net. ..[color=blue]
> Jack Klein wrote:
>[color=green]
> > On Tue, 30 Dec 2003 14:07:21 GMT, pete <pfiland@mindsp ring.com> wrote
> > in comp.lang.c:
> >[color=darkred]
> >>
> >>ModuleFunctio n garMF[N] = {NULL};[/color]
> >
> >
> > I would much prefer {0} to {NULL} here.
> >[/color]
>
> Is {NULL} even correct here if NULL happens to be #defined with a cast
> to void*?[/color]

Yes. Since NULL is a null pointer constant.

6.3.2.3p4: "Conversion of a null pointer to another pointer type yields a
null pointer of that type. ...". [There is no limitation with respect to
function pointers in this regard.]

6.7.8p11 Initialization: "...the same type constraints and conversions as
for simple assignment apply,..."

6.5.16.1p1 Simple Assignment - Constraints: "... - the left operand is a
pointer and the right is a null pointer constant; ..."

6.5.16.1p2 Simple Assignment - Semantics: "In simple assignment (=), the
value of the right operand is converted to the type of the assignment
expression and replaces the value stored in the object designated by the
left operand."

I think Jack prefers {0} because it's a more consistent (and well
understood) paradigm in general.

--
Peter

[pete]

Re: NULL pointer and zero value

Jack Klein wrote:[color=blue]
>
> On Tue, 30 Dec 2003 14:07:21 GMT,
> pete <pfiland@mindsp ring.com wrote in comp.lang.c:[/color]
[color=blue][color=green][color=darkred]
> > > > typedef struct
> > > > {
> > > > int (* func1)();
> > > > int (* func2)();
> > > > void * (* func2)(int );
> > > > } ModuleFunctions ;
> > > >
> > > > #define N 100
> > > > ModuleFunction garMF[N];[/color][/color][/color]
[color=blue][color=green]
> > ModuleFunction garMF[N] = {NULL};[/color]
>
> I would much prefer {0} to {NULL} here.[/color]

All of the members of all of the elements of the array
will be initialized to NULL. Why do you like {0} better ?

--
pete

[Jack Klein]

Re: NULL pointer and zero value

On Wed, 31 Dec 2003 12:46:37 GMT, pete <pfiland@mindsp ring.com> wrote
in comp.lang.c:
[color=blue]
> Jack Klein wrote:[color=green]
> >
> > On Tue, 30 Dec 2003 14:07:21 GMT,
> > pete <pfiland@mindsp ring.com wrote in comp.lang.c:[/color]
>[color=green][color=darkred]
> > > > > typedef struct
> > > > > {
> > > > > int (* func1)();
> > > > > int (* func2)();
> > > > > void * (* func2)(int );
> > > > > } ModuleFunctions ;
> > > > >
> > > > > #define N 100
> > > > > ModuleFunction garMF[N];[/color][/color]
>[color=green][color=darkred]
> > > ModuleFunction garMF[N] = {NULL};[/color]
> >
> > I would much prefer {0} to {NULL} here.[/color]
>
> All of the members of all of the elements of the array
> will be initialized to NULL. Why do you like {0} better ?[/color]

I should have elaborated. {0} works in all cases to initialize any
static or automatic scalar or aggregate type.

The C standard allows two definitions for the macro NULL. For
simplicity's sake, they are "0" and "(void *)0", or equivalent. My
experience is that most (but definitely not all) C compilers use the
cast to (void *) rather than the raw numeric constant.

In this case, since the first element of the structure has pointer
type, either definition of NULL will work. If the structure had a
non-pointer first member, it would be correct on implementations that
used a raw 0 for NULL, but be a constraint violation on those that
defined NULL as "(void *)0".

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
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alt.comp.lang.l earn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq

[Jack Klein]

Re: NULL pointer and zero value

On Tue, 30 Dec 2003 22:01:29 GMT, Kevin Goodsell
<usenet1.spamfr ee.fusion@never box.com> wrote in comp.lang.c:
[color=blue]
> Jack Klein wrote:
>[color=green]
> > On Tue, 30 Dec 2003 14:07:21 GMT, pete <pfiland@mindsp ring.com> wrote
> > in comp.lang.c:
> >[color=darkred]
> >>
> >>ModuleFunctio n garMF[N] = {NULL};[/color]
> >
> >
> > I would much prefer {0} to {NULL} here.
> >[/color]
>
> Is {NULL} even correct here if NULL happens to be #defined with a cast
> to void*?[/color]

I should have elaborated. {0} works in all cases to initialize any
static or automatic scalar or aggregate type.

The C standard allows two definitions for the macro NULL. For
simplicity's sake, they are "0" and "(void *)0", or equivalent. My
experience is that most (but definitely not all) C compilers use the
cast to (void *) rather than the raw numeric constant.

In this case, since the first element of the structure has pointer
type, either definition of NULL will work. If the structure had a
non-pointer first member, it would be correct on implementations that
used a raw 0 for NULL, but be a constraint violation on those that
defined NULL as "(void *)0".

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq

[stau]

Re: NULL pointer and zero value

On Wed, 31 Dec 2003 15:37:42 +1100, Peter Nilsson wrote:
[color=blue]
> I think Jack prefers {0} because it's a more consistent (and well
> understood) paradigm in general.[/color]

No, he explained why,and it makes logic.

[Peter Nilsson]

Re: NULL pointer and zero value

"stau" <stau@pretogal. pt> wrote in message
news:pan.2004.0 1.01.01.09.00.1 02931@pretogal. pt...[color=blue]
> On Wed, 31 Dec 2003 15:37:42 +1100, Peter Nilsson wrote:
>[color=green]
> > I think Jack prefers {0} because it's a more consistent (and well
> > understood) paradigm in general.[/color]
>
> No, he explained why,and it makes logic.[/color]

How is "{0} works in all cases..." significantly different to what I said?
It's more succinct and better phrased, I grant you, but I don't see how my
sentence runs contrary to it.

P.S. Jack's replies were not available on my news server at the time I made
the post.

--
Peter

[pete]

Re: NULL pointer and zero value

Jack Klein wrote:[color=blue]
>
> On Wed, 31 Dec 2003 12:46:37 GMT, pete <pfiland@mindsp ring.com> wrote
> in comp.lang.c:
>[color=green]
> > Jack Klein wrote:[color=darkred]
> > >
> > > On Tue, 30 Dec 2003 14:07:21 GMT,
> > > pete <pfiland@mindsp ring.com wrote in comp.lang.c:[/color]
> >[color=darkred]
> > > > > > typedef struct
> > > > > > {
> > > > > > int (* func1)();
> > > > > > int (* func2)();
> > > > > > void * (* func2)(int );
> > > > > > } ModuleFunctions ;
> > > > > >
> > > > > > #define N 100
> > > > > > ModuleFunction garMF[N];[/color]
> >[color=darkred]
> > > > ModuleFunction garMF[N] = {NULL};
> > >
> > > I would much prefer {0} to {NULL} here.[/color]
> >
> > All of the members of all of the elements of the array
> > will be initialized to NULL. Why do you like {0} better ?[/color]
>
> I should have elaborated. {0} works in all cases to initialize any
> static or automatic scalar or aggregate type.[/color]

What you said, could also be used to explain a preference for
char *pointer = 0;
over
char *pointer = NULL;

Is that your preference also ?

--
pete

[Peter Pichler]

Re: NULL pointer and zero value

"pete" <pfiland@mindsp ring.com> wrote...[color=blue]
> Jack Klein wrote:[color=green]
> >
> > I should have elaborated. {0} works in all cases to initialize any
> > static or automatic scalar or aggregate type.[/color]
>
> What you said, could also be used to explain a preference for
> char *pointer = 0;
> over
> char *pointer = NULL;[/color]

Not quite. Since a void* is implicitly converted to any type*, the two
examples are exactly equivalent regardless of any of the two standard
definitions of NULL.
[color=blue]
> Is that your preference also ?[/color]

My preference is the latter, but it is only a style issue, nothing else.

[pete]

Re: NULL pointer and zero value

Peter Pichler wrote:[color=blue]
>
> "pete" <pfiland@mindsp ring.com> wrote...[color=green]
> > Jack Klein wrote:[color=darkred]
> > >
> > > I should have elaborated.
> > > {0} works in all cases to initialize any
> > > static or automatic scalar or aggregate type.[/color]
> >
> > What you said, could also be used to explain a preference for
> > char *pointer = 0;
> > over
> > char *pointer = NULL;[/color]
>
> Not quite.
> Since a void* is implicitly converted to any type*, the two
> examples are exactly equivalent regardless of any of the two standard
> definitions of NULL.[/color]

Exactly quite.

ModuleFunction garMF[N] = {NULL};
and
ModuleFunction garMF[N] = {0};

are exactly equivalent regardless of any of the two standard
definitions of NULL.

[color=blue][color=green]
> > Is that your preference also ?[/color]
>
> My preference is the latter,
> but it is only a style issue, nothing else.[/color]

Style issues are on topic.
Reasons for prefering one style over another
which involve legibility and maintainability ,
are the more interesting ones.

--
pete

[Peter Pichler]

Re: NULL pointer and zero value

"pete" <pfiland@mindsp ring.com> wrote...[color=blue]
> Jack Klein wrote:[color=green]
> >
> > I should have elaborated. {0} works in all cases to initialize any
> > static or automatic scalar or aggregate type.[/color]
>
> What you said, could also be used to explain a preference for
> char *pointer = 0;
> over
> char *pointer = NULL;[/color]

Not quite. Since a void* is implicitly converted to any type*, the two
examples are exactly equivalent regardless of any of the two standard
definitions of NULL.
[color=blue]
> Is that your preference also ?[/color]

My preference is the latter, but it is only a style issue, nothing else.

[Peter Pichler]

Re: NULL pointer and zero value

"pete" <pfiland@mindsp ring.com> wrote...[color=blue]
> Jack Klein wrote:[color=green]
> >
> > I should have elaborated. {0} works in all cases to initialize any
> > static or automatic scalar or aggregate type.[/color]
>
> What you said, could also be used to explain a preference for
> char *pointer = 0;
> over
> char *pointer = NULL;[/color]

Not quite. Since a void* is implicitly converted to any type*, the two
examples are exactly equivalent regardless of any of the two standard
definitions of NULL.
[color=blue]
> Is that your preference also ?[/color]

My preference is the latter, but it is only a style issue, nothing else.

[Peter Pichler]

Re: NULL pointer and zero value

"pete" <pfiland@mindsp ring.com> wrote...[color=blue]
> Peter Pichler wrote:[color=green]
> > "pete" <pfiland@mindsp ring.com> wrote...[color=darkred]
> > > Jack Klein wrote:
> > >
> > > What you said, could also be used to explain a preference for
> > > char *pointer = 0;
> > > over
> > > char *pointer = NULL;[/color]
> >
> > Not quite.
> > Since a void* is implicitly converted to any type*, the two
> > examples are exactly equivalent regardless of any of the two standard
> > definitions of NULL.[/color]
>
> Exactly quite.
> ModuleFunction garMF[N] = {NULL};
> and
> ModuleFunction garMF[N] = {0};
> are exactly equivalent regardless of any of the two standard
> definitions of NULL.[/color]

Only if garMF[N] is a pointer.

If I remember correctly, the thread started with explicit initialization
of struct members with memset, {NULL} or {0}. Of those three, memset can
only be portably used if the struct is a wrapper for an usigned char array,
anything else may lead to a UB. {NULL} can only be used if the first member
of such struct is a pointer. {0}, however, can be used in any context.
[color=blue][color=green][color=darkred]
> > > Is that your preference also ?[/color]
> >
> > My preference is the latter,
> > but it is only a style issue, nothing else.[/color]
>
> Style issues are on topic.
> Reasons for prefering one style over another
> which involve legibility and maintainability ,
> are the more interesting ones.[/color]

I didn't say it was not on-topic. Whether or not it is a /good/ topic
is quite a different question though ;-)

Peter