double astrix pointer question..

**laserlight** · 01-18-2009

Originally Posted by MK27

which I imagine is not the same thing, but then (*p)->next causes a segfault.

You know the mantra

Post the smallest and simplest compilable program that demonstrates the problem.

Originally Posted by transgalactic2

so you agree with me that if we define
Node **p;
then we have two pointers in this definition
in which
p points *p

correct?

No, there is only one pointer, namely, p. p does not point to *p yet because p is left uninitialised.

**Elysia** · 01-18-2009

I must add, once again, that it is only a VARIABLE. It creates ONE VARIABLE, with a certain type. That is all. It is uninitialized, filled with random garbage just like ANY OTHER VARIABLE.

**~~transgalactic2~~** · 01-18-2009

can you give me an example with function calls of actual addresses to understand Node**p
for every address i could see to what node/pointer it corresponds

because i am really confused you said that
*p does point to a node
but to a pointer node

but when i am saying that p points to *p
you say its wrong

**MK27** · 01-18-2009

Originally Posted by laserlight

You know the mantra

Post the smallest and simplest compilable program that demonstrates the problem.

Hopefully that will be this:

Code:

#include <stdlib.h>

typedef struct _node {
	struct _node *next;
} node;

int main () {
	node *test=malloc(sizeof(node)), **p;
	*p=test;
	return 0;
}

gcc -Wall test.c
./a.out
Segmentation fault

**Elysia** · 01-18-2009

Try compiling this code, please.
Compile ALL the code you post before you ask.
Go ahead and do it right now. We can wait.

**Elysia** · 01-18-2009

Originally Posted by transgalactic2

can you give me an example with actual addreses to understand Node**p

because i am really confused you said that
*p does point to a node
but to a pointer node

but when i am saying that p points to *p
you say its wrong

Code:

#include <stdio.h>

int main()
{
	int Temp = 50;
	int* p1 = &Temp;
	int** p_to_p = &p1;

	printf("Integer:\t%i\n", Temp);
	printf("p1:\t\t%p\n", p1);
	printf("Integer:\t%i\n", *p1);
	printf("p-to-p:\t\t%p\n", p_to_p);
	printf("p1:\t\t%p\n", *p_to_p);
	printf("Integer:\t%i\n", **p_to_p);
}

**laserlight** · 01-18-2009

Originally Posted by MK27

Hopefully that will be this:

Eh, but that improves on nothing from what transgalactic2 posted and does not use the (*p)->next syntax. p is still left uninitialised. I am talking about something like this:

Code:

#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>

typedef struct node
{
    struct node *next;
} node;

int main()
{
    node node_obj;
    node *node_ptr = &node_obj;
    node **p = malloc(sizeof(*p));
    node_obj.next = NULL;
    *p = node_ptr;
    (*p)->next = malloc(sizeof(node));
    printf("%p\n", (void*)node_obj.next);
    free(p);
    free(node_obj.next);
    return 0;
}

Note that I am assuming that malloc() succeeds in allocating memory.

**MK27** · 01-18-2009

Looks like I was wrong about "The other one":

Code:

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

typedef struct _node {
	char content[64];
	struct _node *next;
} node;

int main () {
	node *test=malloc(sizeof(node)), **p;
	strcpy(test->content,"hello world");
	p=&test;
	printf("%s\n",(*p)->content);	
	return 0;
}

Works fine, sorry...

**Elysia** · 01-18-2009

Yeah, and don't forget to free.

**MK27** · 01-18-2009

Ah, so with:

Code:

*p=node_ptr;

**p must first be allocated space since *p is what the address in **p would point to?
Whereas:

Code:

p=&node_ptr;

writes the address of node_ptr into **p?
By using the dereference (*p), aren't we just compensating for the fact that p is a pointer to a pointer? Meaning why not use a pointer in the first place?

**laserlight** · 01-18-2009

Originally Posted by MK27

By using the dereference (*p), aren't we just compensating for the fact that p is a pointer to a pointer? Meaning why not use a pointer in the first place?

Because you might want to change the pointer itself, rather than merely change what the pointer points to.

**~~transgalactic2~~** · 01-18-2009

you posted many codes and argued regarding them
which one do i need to compile?

**Elysia** · 01-18-2009

You need to compile your own code that post. The last one you posted, which you edited out would have worked horribly wrong.

**~~transgalactic2~~** · 01-18-2009

okkk

**~~transgalactic2~~** · 01-18-2009

i tried to run the function like this
its not running

Code:

#include <stdio.h>
void what1(Node ** p,int num);
 
typedef struct node {
    int value;
    struct node *next;
}Node;

int main()
{
	Node **ptr;
	ptr=(**Node)malloc(sizeof(Node));
    void what1(ptr,5);
    return 0;
}

void what1(Node ** p,int num){
   Node *elt;
   elt=(Node*)malloc(sizeof(Node));
   elt->next=*p;
   elt->value=num;
   *p=elt;
}

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