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POINTER to POINTER Problem

Nosnibor's Avatar
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Join Date: Oct 2007
Posts: 10
#1: Oct 29 '07
Need some help with this pointer problem.
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  1.  
  2. void Function( ADT **a/*NULL*/,  ADT *b /*NOT NULL POINT TO SOME DATA*/)
  3. {
  4.  
  5.     ADT *c = NULL;
  6.  
  7.    if( a == NULL) 
  8.    {
  9.          *a  = b;                                              //ERROR READ OF ADDRESS VIOLATION
  10.  
  11. .
  12. .
  13. .
  14.  

Banfa's Avatar
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Join Date: Feb 2006
Location: South West UK
Posts: 6,169
#2: Oct 29 '07

re: POINTER to POINTER Problem

Quote:

Originally Posted by Nosnibor

Expand|Select|Wrap|Line Numbers
  1.    if( a == NULL) 
  2.    {
  3.          *a  = b;          //ERROR READ OF ADDRESS VIOLATION
  4.  
  5.  

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

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  1.    if( a != NULL) 
  2.    {
  3.          *a  = b;
  4.    }
  5.  
or perhaps you meant

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  1.    if( a != NULL && *a == NULL) 
  2.    {
  3.          *a  = b;
  4.    }
  5.  
Nosnibor's Avatar
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Join Date: Oct 2007
Posts: 10
#3: Oct 30 '07

re: POINTER to POINTER Problem

Think this, what I came up with...

Expand|Select|Wrap|Line Numbers
  1.  
  2. void SOMEFUNCTION( ADT **a, ADT **b USERS *c )
  3. {
  4.     ADT *p,
  5.            *t;
  6.  
  7.     if(b == (ADT **)NULL)                 
  8.         {
  9.               a->next = (ADT*)NULL;
  10.               b->prev = (ADT*)NULL;
  11.               *a = c ;     /////*******/////
  12.               *b= c;        ////*********/////
  13.         return;
  14.       }
  15.       t = (*at);
  16.       p = (ADT *)NULL;
  17.  
  18.       /*some other codes*/
  19.  
Banfa's Avatar
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Join Date: Feb 2006
Location: South West UK
Posts: 6,169
#4: Oct 30 '07

re: POINTER to POINTER Problem

Quote:

Originally Posted by Nosnibor

Think this, what I came up with...

Expand|Select|Wrap|Line Numbers
  1.  
  2. void SOMEFUNCTION( ADT **a, ADT **b USERS *c )
  3. {
  4.     ADT *p,
  5.            *t;
  6.  
  7.     if(b == (ADT **)NULL)                 
  8.         {
  9.               a->next = (ADT*)NULL;
  10.               b->prev = (ADT*)NULL;
  11.               *a = c ;     /////*******/////
  12.               *b= c;        ////*********/////
  13.         return;
  14.       }
  15.       t = (*at);
  16.       p = (ADT *)NULL;
  17.  
  18.       /*some other codes*/
  19.  

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(consistently) without crashing.
Nosnibor's Avatar
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Join Date: Oct 2007
Posts: 10
#5: Oct 30 '07

re: POINTER to POINTER Problem

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  1. /*Create a doubly linked*/
  2.  
  3. void store(  struct address *i  /* new element */, struct address **start /* first element in list */,  struct address **last /* last element in list */ )
  4. {
  5.   struct address *old, *p;
  6.  
  7.   if(*last==NULL) {  /* first element in list */
  8.     i->next = NULL;
  9.     i->prior = NULL;
  10.     *last = i;                  /*HERE WRITE OF ADDRESS ERROR*/
  11.     *start = i;                 /*HERE WRITE OF ADDRESS*/
  12.     return;
  13.   }
  14.   p = *start;                   /* start at top of list */
  15. /*CODE CONTINUES*/
  16.  
Banfa's Avatar
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Join Date: Feb 2006
Location: South West UK
Posts: 6,169
#6: Oct 30 '07

re: POINTER to POINTER Problem

Quote:

Originally Posted by Nosnibor

Expand|Select|Wrap|Line Numbers
  1. /*Create a doubly linked*/
  2.  
  3. void store(  struct address *i  /* new element */, struct address **start /* first element in list */,  struct address **last /* last element in list */ )
  4. {
  5.   struct address *old, *p;
  6.  
  7.   if(*last==NULL) {  /* first element in list */
  8.     i->next = NULL;
  9.     i->prior = NULL;
  10.     *last = i;                  /*HERE WRITE OF ADDRESS ERROR*/
  11.     *start = i;                 /*HERE WRITE OF ADDRESS*/
  12.     return;
  13.   }
  14.   p = *start;                   /* start at top of list */
  15. /*CODE CONTINUES*/
  16.  

Well this looks correct, so if you are getting errors it may be how you are calling it.
Nosnibor's Avatar
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Join Date: Oct 2007
Posts: 10
#7: Oct 31 '07

re: POINTER to POINTER Problem

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

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  1. void main()
  2. {
  3.  
  4.   ADT     *Head = NULL,*End = NULL;
  5.  
  6.  
  7.  ANYFUNCTION(  Head, End );
  8. .
  9. .
  10. .
  11.  
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  1. void ANYFUNCTION( ADT  *A, ADT *B )
  2. {
  3.  /*Performs some operations between ere*/
  4. .
  5. .
  6. .
  7. /*here*/
  8.  
  9. SOMEOTHERFUNCTION( &A , &B  );
  10.  
  11. }
  12.  
Expand|Select|Wrap|Line Numbers
  1.  
  2. void SOMEOTHERFUNCTION( **C , **D )
  3.  /*Performs some operations between ere*/
  4. .
  5. .
  6. .
  7. /*here*/
  8. return;
  9. }
  10.  
My question is how do I get Head and End to reflect the changes made to A and B?
With using globals.
Ganon11's Avatar
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Join Date: Oct 2006
Location: New York, United States of America
Posts: 3,428
#8: Oct 31 '07

re: POINTER to POINTER Problem

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 SOMEOTHERFUNCTION), and work around dereferencing twice (or three times) in your function.
Nosnibor's Avatar
Newbie
  
Join Date: Oct 2007
Posts: 10
#9: Nov 2 '07

re: POINTER to POINTER Problem

Quote:

Originally Posted by Nosnibor

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

Expand|Select|Wrap|Line Numbers
  1. void main()
  2. {
  3.  
  4.   ADT     *Head = NULL,*End = NULL;
  5.  
  6.  
  7.  ANYFUNCTION(  Head, End );
  8. .
  9. .
  10. .
  11.  
Expand|Select|Wrap|Line Numbers
  1. void ANYFUNCTION( ADT  *A, ADT *B )
  2. {
  3.  /*Performs some operations between ere*/
  4. .
  5. .
  6. .
  7. /*here*/
  8.  
  9. SOMEOTHERFUNCTION( &A , &B  );
  10.  
  11. }
  12.  
Expand|Select|Wrap|Line Numbers
  1.  
  2. void SOMEOTHERFUNCTION( **C , **D )
  3.  /*Performs some operations between ere*/
  4. .
  5. .
  6. .
  7. /*here*/
  8. return;
  9. }
  10.  
My question is how do I get Head and End to reflect the changes made to A and B?
With using globals.


Expand|Select|Wrap|Line Numbers
  1. void main()
  2. {
  3.  
  4.   ADT     *Head = NULL,*End = NULL;
  5.  
  6.  
  7.  ANYFUNCTION(  &Head, &End );
  8. .
  9. .
  10. .
  11.  
Expand|Select|Wrap|Line Numbers
  1. void ANYFUNCTION( ADT  **A, ADT **B )
  2. {
  3.  /*Performs some operations between ere*/
  4. .
  5. .
  6. .
  7. /*here*/
  8.  
  9. SOMEOTHERFUNCTION( A , B  );
  10.  
  11. }
  12.  
Expand|Select|Wrap|Line Numbers
  1.  
  2. void SOMEOTHERFUNCTION( **C , **D )
  3.  /*Performs some operations between ere*/
  4. .
  5. .
  6. .
  7. /*here*/
  8. return;
  9. }
  10.  
I made the following changes and they seem work seemly. Thanks
Banfa's Avatar
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Join Date: Feb 2006
Location: South West UK
Posts: 6,169
#10: Nov 2 '07

re: POINTER to POINTER Problem

Note

Expand|Select|Wrap|Line Numbers
  1. void main()
  2. {
  3. <code here>
  4. }
  5.  
is quite wrong and could cause you program to function incorrectly. main always returns int

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  1. int main()
  2. {
  3. <code here>
  4.  
  5.     return 0;
  6. }
  7.  
Nosnibor's Avatar
Newbie
  
Join Date: Oct 2007
Posts: 10
#11: Nov 2 '07

re: POINTER to POINTER Problem

Quote:

Originally Posted by Banfa

Note

Expand|Select|Wrap|Line Numbers
  1. void main()
  2. {
  3. <code here>
  4. }
  5.  
is quite wrong and could cause you program to function incorrectly. main always returns int

Expand|Select|Wrap|Line Numbers
  1. int main()
  2. {
  3. <code here>
  4.  
  5.     return 0;
  6. }
  7.  

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's Avatar
AdministratorVoR
  
Join Date: Feb 2006
Location: South West UK
Posts: 6,169
#12: Nov 2 '07

re: POINTER to POINTER Problem

Quote:

Originally Posted by Nosnibor

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.
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