NULL pointer and zero value

[vp]

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;"

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 );
Thanks for your help,

DT

[Richard Heathfield]

vp wrote:

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;"

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 : bi****@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]

Richard Heathfield <in*****@addres s.co.uk.invalid > spoke thus:

(all-bits 0 as NULL post)

If you like programming-by-casino, go ahead. :-)

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]

dt*******@yahoo .com (vp) writes:

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;"
No. The former statement sets all bits of `p' to zero, which is not
necessarily a representation of the null pointer.
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 );

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]

Richard Heathfield <in*****@addres s.co.uk.invalid > wrote in
news:3f******@n ews2.power.net. uk:

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. :-)

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]

Martin Dickopp wrote:

dt*******@yahoo .com (vp) writes:

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;"
No. The former statement sets all bits of `p' to zero, which is not
necessarily a representation of the null pointer.

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 );

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

I disagree about the necessity of a loop.

ModuleFunction garMF[N] = {NULL};

--
pete

[pete]

Mark A. Odell wrote:

Richard Heathfield <in*****@addres s.co.uk.invalid > wrote in
news:3f******@n ews2.power.net. uk:

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. :-)

So are you saying that this isn't safe?

#include <stdlib.h>

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

if (pVar) /* <--- Not safe? */

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]

"Mark A. Odell" <no****@embedde dfw.com> writes:

Richard Heathfield <in*****@addres s.co.uk.invalid > wrote in
news:3f******@n ews2.power.net. uk:

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. :-)

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;
}

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]

pete wrote:

Mark A. Odell wrote:

Richard Heathfield <in*****@addres s.co.uk.invalid > wrote in
news:3f******@n ews2.power.net. uk:

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. :-)

So are you saying that this isn't safe?

#include <stdlib.h>

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

if (pVar) /* <--- Not safe? */

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.

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

--
pete

[CBFalconer]

vp wrote:

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;"

NO.

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

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