Thread: Problem with double pointer

  1. #1
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    May 2008
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    Problem with double pointer

    I am having problem dereferencing a double pointer (headRef) and passing the address to struct pointer (temp). In the below deleteList function when i try to pass the address of headRef->next to temp I get the error..

    i.e
    temp = (headRef->next);

    Above statement give me the error:
    <--- gives error mentioning "Request for member 'next' in something is not a struct or union..


    When i try to comment out the error lines, step into deleteList function and the check the value (header->next) using gdb, i see the address of next node. But why cannot I assain the value to temp pointer. Please let me know.

    The program is for deleting the entire linked list. Have pasted the program below:



    Error Program:

    Code:
    deleteList(struct node **headRef)
    {
        struct node *temp; 
        temp = malloc(sizeof(struct node ) );    
            while (*headRef != NULL) {
                            /* Store the address of next node */
                temp = (headRef->next);   <--- gives error mentioning "Request for member 'next' in something is not a struct or union..  
                free(*headRef)
                *headRef = temp;
                
            }
        
        
    }
    Entire function

    Code:
    #include <stdlib.h>
    #include <string.h>
    #include <stdio.h>
    
    struct node *BuildOneTwoThree();
    void push(struct node** headRef, int newData);
    void deleteList(struct node **headRef);
    
    struct node
    {
        int data;
        struct node *next;
    };
    
    
    
    
    int main()
    { 
        struct node* myList = BuildOneTwoThree();    
        deleteList(&myList);
        
        return 0;
    
    }
    
    
    
    void
    deleteList(struct node **headRef)
    {
        struct node *temp; 
        
        temp = malloc(sizeof(struct node ) );    
            while (*headRef != NULL) {
                temp = (headRef->next);
                free(*headRef)
                *headRef = temp;
                
            }
        
        
    }
    
    void push(struct node** headRef, int newData)
    {
        struct node* newNode = malloc(sizeof(struct node));
        
        newNode->data = newData;
        newNode->next = (*headRef);
        *headRef = newNode;
        
    }
    
    
    /* Build and return the list */
    
    struct node *BuildOneTwoThree()
    {
        
        struct node* head = NULL;
        push(&head, 3);
        push(&head, 2);
        push(&head, 1);
        
        return(head);
    }

  2. #2
    and the hat of int overfl Salem's Avatar
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    Try (*headRef)->next
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by cjjoy1980 View Post
    Code:
    deleteList(struct node **headRef)
    {
        struct node *temp; 
        temp = malloc(sizeof(struct node ) );
    Don't call malloc from within a function whose sole purpose is to free a list's contents. All it does here is create a memory leak.
    Just delete the line of code with the malloc.
    My homepage
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  4. #4
    Registered User
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    May 2008
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    thanks Salem, iMalc.. yes (*headRef)->next worked.. Also removed redundant malloc statement.

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