POINTER to POINTER Problem

[Nosnibor]

Need some help with this pointer problem.

void Function( ADT **a/*NULL*/,  ADT *b /*NOT NULL POINT TO SOME DATA*/)
{
	
    ADT *c = NULL;
   
   if( a == NULL) 
   {
         *a  = b;                                              //ERROR READ OF ADDRESS VIOLATION

.
.
.

[Banfa]

[code=c]
if( a == NULL)
{
*a = b; //ERROR READ OF ADDRESS VIOLATION

[/code]

You just checked to see if a was NULL and if it was assign to were it was pointing. You have deferenced the NULL pointer always a bad idea.

Perhaps you meant

[code=c]
if( a != NULL)
{
*a = b;
}
[/code]

or perhaps you meant

[code=c]
if( a != NULL && *a == NULL)
{
*a = b;
}
[/code]

[Nosnibor]

Think this, what I came up with...

void SOMEFUNCTION( ADT **a, ADT **b USERS *c )
{
	ADT *p,
   		*t;

	if(b == (ADT **)NULL) 				
        {
              a->next = (ADT*)NULL;
    	      b->prev = (ADT*)NULL;
              *a = c ; 	/////*******/////
              *b= c;		////*********/////
    	return;
  	}
      t = (*at);
      p = (ADT *)NULL;

      /*some other codes*/

[Banfa]

Think this, what I came up with...

void SOMEFUNCTION( ADT **a, ADT **b USERS *c )
{
	ADT *p,
   		*t;

	if(b == (ADT **)NULL) 				
        {
              a->next = (ADT*)NULL;
    	      b->prev = (ADT*)NULL;
              *a = c ; 	/////*******/////
              *b= c;		////*********/////
    	return;
  	}
      t = (*at);
      p = (ADT *)NULL;

      /*some other codes*/

I would be really surprised it this compiled with errors and warnings so perhaps you'd better post what you get.

If it did compile I would be surprised if it ran(consistentl y) without crashing.

[Nosnibor]

/*Create a doubly linked*/

void store(  struct address *i  /* new element */, struct address **start /* first element in list */,  struct address **last /* last element in list */ )
{
  struct address *old, *p;

  if(*last==NULL) {  /* first element in list */
    i->next = NULL;
    i->prior = NULL;
    *last = i;                  /*HERE WRITE OF ADDRESS ERROR*/
    *start = i;                 /*HERE WRITE OF ADDRESS*/
    return;
  }
  p = *start;                   /* start at top of list */
/*CODE CONTINUES*/

[Banfa]

/*Create a doubly linked*/

void store(  struct address *i  /* new element */, struct address **start /* first element in list */,  struct address **last /* last element in list */ )
{
  struct address *old, *p;

  if(*last==NULL) {  /* first element in list */
    i->next = NULL;
    i->prior = NULL;
    *last = i;                  /*HERE WRITE OF ADDRESS ERROR*/
    *start = i;                 /*HERE WRITE OF ADDRESS*/
    return;
  }
  p = *start;                   /* start at top of list */
/*CODE CONTINUES*/

Well this looks correct, so if you are getting errors it may be how you are calling it.

[Nosnibor]

Ok Thanks. But Still lost in pointers. So lets say

void main()
{

  ADT 	*Head = NULL,*End = NULL;


 ANYFUNCTION(  Head, End );
.
.
.

void ANYFUNCTION( ADT  *A, ADT *B )
{
 /*Performs some operations between ere*/
.
.
.
/*here*/
          
SOMEOTHERFUNCTION( &A , &B  );

}

 
void SOMEOTHERFUNCTION( **C , **D )
 /*Performs some operations between ere*/
.
.
.
/*here*/
return;
}

My question is how do I get Head and End to reflect the changes made to A and B?
With using globals.

[Ganon11]

You could pass those pointers 'by reference', or you could pass the address of the pointers in each case. If you choose the former, it's very easy - just put the & symbol after the type name in your function headers. If you choose the latter, you have to change your function headers to accept double pointers (and triple pointers, for SOMEOTHERFUNCTI ON), and work around dereferencing twice (or three times) in your function.

[Nosnibor]

Ok Thanks. But Still lost in pointers. So lets say

void main()
{

  ADT 	*Head = NULL,*End = NULL;


 ANYFUNCTION(  Head, End );
.
.
.

void ANYFUNCTION( ADT  *A, ADT *B )
{
 /*Performs some operations between ere*/
.
.
.
/*here*/
          
SOMEOTHERFUNCTION( &A , &B  );

}

 
void SOMEOTHERFUNCTION( **C , **D )
 /*Performs some operations between ere*/
.
.
.
/*here*/
return;
}

My question is how do I get Head and End to reflect the changes made to A and B?
With using globals.

void main()
{

  ADT 	*Head = NULL,*End = NULL;


 ANYFUNCTION(  &Head, &End );
.
.
.

void ANYFUNCTION( ADT  **A, ADT **B )
{
 /*Performs some operations between ere*/
.
.
.
/*here*/
          
SOMEOTHERFUNCTION( A , B  );

}

 
void SOMEOTHERFUNCTION( **C , **D )
 /*Performs some operations between ere*/
.
.
.
/*here*/
return;
}

I made the following changes and they seem work seemly. Thanks

[Banfa]

Note

[code=c]void main()
{
<code here>
}
[/code]

is quite wrong and could cause you program to function incorrectly. main always returns int

[code=c]int main()
{
<code here>

return 0;
}
[/code]

[Nosnibor]

Note

[code=c]void main()
{
<code here>
}
[/code]

is quite wrong and could cause you program to function incorrectly. main always returns int

[code=c]int main()
{
<code here>

return 0;
}
[/code]

Do not understand what you are saying in post could you be a bit more vivid?
Are you saying a void main would clause this not operate correctly?
And if so why?
What is the difference with a void main with no return, and a int main with a return? What does main actually returns to?

[Banfa]

Do not understand what you are saying in post could you be a bit more vivid?
Are you saying a void main would clause this not operate correctly?
And if so why?
What is the difference with a void main with no return, and a int main with a return? What does main actually returns to?

Oops sorry should have said

Yes using void main could cause your code to work incorrectly, it is wrong.

Using void main invokes undefined behaviour (because the C/C++ standard says so). Undefined behaviour is bad because the program and quite literally do anything (although the most common results are incorrect calculations, program crashes and appearing to work without a problem).

main returns to the c start-up code and the difference is that this start-up code is expecting main to return int not return void.