pointer notation

pointer notation

[km]

 Hi all,
 how different is the notation 
 (p)->next (what do the parenthesis around the pointer signify ? )
 different from  
 p ( while p is a pointer)
 
 regards,
 KM

Re: pointer notation

[Thomas Steudten]

Hi

(p)->next and p->next are the same. I think you saw a sourcecode, where
maybe p is replaced by a macro like #define p ptr->a.
However the priority in c is "()" first, then "->".


>  Hi all,
>  how different is the notation 
>  (p)->next (what do the parenthesis around the pointer signify ? )
>  different from  
>  p ( while p is a pointer)

-- 
Tom

LINUX user since kernel 0.99.x 1994.
RPM Alpha packages at http://alpha.steudten.com/packages
Want to know what S.u.S.E 1995 cdrom-set contains?

Re: pointer notation

[Luciano Moreira - igLnx]

Try to think that p is a MACRO, like as (arbitrary macro):
"#define p  pBaseList+5*(x->element)+1"

When you do:
"p->next"
it ll be:
"pBaseList+5*(x->element)+1->next"

the code above seems to be unsafe, so, if you do:
"(p)->next"
or:
"#define p  (pBaseList+5*(x->element)+1)"
it ll free or code of  "precedence bugs".

Luciano

km wrote, On 28/10/2003 07:42:

> Hi all,
> how different is the notation 
> (p)->next (what do the parenthesis around the pointer signify ? )
> different from  
> p ( while p is a pointer)
> 
> regards,
> KM
>
>-
>To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>
>  
>

Re: pointer notation

[km]

Hi all,
i should have posted the snippet before itself to be clear, which i am doing now.

It is from a double-linked list snippet as follows : 
----------------CODE START---------------------------------------
#include<stdio.h>
#include<stdlib.h>
struct node
{
int content;
struct node *next;
struct node *previous;
};
/* prototype declaration */
void add(struct node **s,int c);
/*main program*/
int main()
{
struct node *p;
p = NULL;
add(&p,21);
return 0;
}

/* function */
void add(struct node **s,int c)
{
	struct node *r,*q=*s;

if(*s==NULL)
{
	*s = (struct node *)malloc(sizeof(struct node));
	(*s)->previous = NULL; 		/* (*s) ??? whats that ? */
	(*s)->next = NULL;
	(*s)->content = c
}
else
{
/*bla bla*/
}
}
----------------CODE END-------------------------------------
In the above code we can see the statement called "(*s)->previous = NULL;". I didnt get what exactly it means to be and how different is *s different from (*s) in this context. can it be written in any other form alternatively ? 
	thanks for the feedback,
regards,
KM


-----------------------------------------------------------------
On Tue, Oct 28, 2003 at 10:00:01AM -0200, Luciano Moreira - igLnx wrote:
> Try to think that p is a MACRO, like as (arbitrary macro):
> "#define p  pBaseList+5*(x->element)+1"
> 
> When you do:
> "p->next"
> it ll be:
> "pBaseList+5*(x->element)+1->next"
> 
> the code above seems to be unsafe, so, if you do:
> "(p)->next"
> or:
> "#define p  (pBaseList+5*(x->element)+1)"
> it ll free or code of  "precedence bugs".
> 
> Luciano
> 
> km wrote, On 28/10/2003 07:42:
> 
> >Hi all,
> >how different is the notation 
> >(p)->next (what do the parenthesis around the pointer signify ? )
> >different from  
> >p ( while p is a pointer)
> >
> >regards,
> >KM
> >
> >-
> >To unsubscribe from this list: send the line "unsubscribe 
> >linux-c-programming" in
> >the body of a message to majordomo@vger.kernel.org
> >More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >
> >
> > 
> >
> 
> -
> To unsubscribe from this list: send the line "unsubscribe 
> linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: pointer notation

[Mike Pastore]

Quite simply, s is a pointer to a pointer to a struct. You want to
dereference it once (with a prefix "*") before dereferencing it a second
time (with an infix "->"). Maybe the following pseudo-equivalent code
will help you understand:

    struct node **s, *t; /* introduce temporary "t" */
    t = *s; /* dereference s, store result in t */
    t->previous = NULL; /* set member of t (*s) to NULL */

The infix "->" binds tighter (has a higher precedence) than the prefix
"*". The incorrect statement "*s->previous = NULL;" is therefore
equivalent to "*(s->previous) = NULL;" and will attempt to get the
"previous" member of s and then dereference the result. But s has no
previous member! It is a pointer to a pointer to a struct, NOT a pointer
to a struct!

The use of parenthesis in "(*s)->previous = NULL;" establishes a clear
order of operations (dereference and THEN get member), which a couple
folks have already mentioned.

--
Mike Pastore
mike@oobak.org

On Tue, 2003-10-28 at 17:46, km wrote:
> Hi all,
> i should have posted the snippet before itself to be clear, which i am doing now.
> 
> It is from a double-linked list snippet as follows : 
> ----------------CODE START---------------------------------------
> #include<stdio.h>
> #include<stdlib.h>
> struct node
> {
> int content;
> struct node *next;
> struct node *previous;
> };
> /* prototype declaration */
> void add(struct node **s,int c);
> /*main program*/
> int main()
> {
> struct node *p;
> p = NULL;
> add(&p,21);
> return 0;
> }
> 
> /* function */
> void add(struct node **s,int c)
> {
> 	struct node *r,*q=*s;
> 
> if(*s==NULL)
> {
> 	*s = (struct node *)malloc(sizeof(struct node));
> 	(*s)->previous = NULL; 		/* (*s) ??? whats that ? */
> 	(*s)->next = NULL;
> 	(*s)->content = c
> }
> else
> {
> /*bla bla*/
> }
> }
> ----------------CODE END-------------------------------------
> In the above code we can see the statement called "(*s)->previous = NULL;". I didnt get what exactly it means to be and how different is *s different from (*s) in this context. can it be written in any other form alternatively ? 
> 	thanks for the feedback,
> regards,
> KM
> 
> 
> -----------------------------------------------------------------
> On Tue, Oct 28, 2003 at 10:00:01AM -0200, Luciano Moreira - igLnx wrote:
> > Try to think that p is a MACRO, like as (arbitrary macro):
> > "#define p  pBaseList+5*(x->element)+1"
> > 
> > When you do:
> > "p->next"
> > it ll be:
> > "pBaseList+5*(x->element)+1->next"
> > 
> > the code above seems to be unsafe, so, if you do:
> > "(p)->next"
> > or:
> > "#define p  (pBaseList+5*(x->element)+1)"
> > it ll free or code of  "precedence bugs".
> > 
> > Luciano
> > 
> > km wrote, On 28/10/2003 07:42:
> > 
> > >Hi all,
> > >how different is the notation 
> > >(p)->next (what do the parenthesis around the pointer signify ? )
> > >different from  
> > >p ( while p is a pointer)
> > >
> > >regards,
> > >KM
> > >
> > >-
> > >To unsubscribe from this list: send the line "unsubscribe 
> > >linux-c-programming" in
> > >the body of a message to majordomo@vger.kernel.org
> > >More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > >
> > >
> > > 
> > >
> > 
> > -
> > To unsubscribe from this list: send the line "unsubscribe 
> > linux-c-programming" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: pointer notation

[Jeff Woods]

At 10/28/2003 11:16 PM +0530, km wrote:
>In the above code we can see the statement called "(*s)->previous = 
>NULL;". I didnt get what exactly it means to be and how different is *s 
>different from (*s) in this context. can it be written in any other form 
>alternatively ?

"*s" and "(*s)" are exactly the same.  However, "*s->next" and "(*s)->next" 
are NOT the same because without the parentheses field selection through 
the pointer ("->") wouild be done before indirection ("*") of the "handle" 
(pointer to a pointer) "s".  IOW, the parentheses are only there (as is 
usually the case in a C expression) to control precedence of binding.

--
Jeff Woods <kazrak+kernel@cesmail.net>