pointer to a struct becomes NULL at every function call

Question

The problem is that every time the function addNodePos is being called head pointer is NULL (saw that in debugger), and it just creates a list of one node, which points to itself as it is a circular doubly-linked list. And it displays "List is empty." because list is also NULL, when passing to a printList function. Have been trying to understand why but still there is no result.

Here is the code (removed excessive code according to SSCCE)

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

struct DoubleList
{
    int id;
    struct DoubleList *next;
    struct DoubleList *prev;
};

void addNodePos(struct DoubleList* head, int value, int position);
void printList (struct DoubleList* head);
//void clearList (struct DoubleList* head);


int main()
{
    int value, position;
    struct DoubleList *list = NULL;

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(list, value, position);

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(list, value, position);


    printList(list);
    //clearList(list);
    return 0;
}


void addNodePos(struct DoubleList* head, int value, int position)
{
    int i;
    struct DoubleList *node;

    if ( (node = malloc (sizeof(struct DoubleList))) != NULL ){
        node->id=value;
        if (head==NULL) {
            // points to itself as it is the only node in a list
            node->next=node;
            node->prev=node;
            head=node;
        } else {
            struct DoubleList *current=head;
            for (i = position; i > 1; i--)
                current=current->next;
            // reassign pointers -- relink nodes
            current->prev->next=node;
            node->prev=current->prev;
            node->next=current;
            current->prev=node;
        }
    }

    printf("Element has been added.\n\n");
}


void printList(struct DoubleList* head)
{
    if (head==NULL)
        printf("\nList is empty.\n\n");
    else {
        struct DoubleList *current=head;
        printf("\nThe list: ");
        do {
            printf("%d", current->id);
            current=current->next;
            if(current != head)
                printf("<->");
        } while(current!=head);
        printf("\n\n");
    }
}

http://stackoverflow.com/questions/373419/whats-the-difference-between-passing-by-reference-vs-passing-by-value — AnT stands with Russia, Dec 09 '14 at 23:00
http://stackoverflow.com/questions/10959694/why-does-call-by-value-example-not-modify-input-parameter — AnT stands with Russia, Dec 09 '14 at 23:02

score 0 · Answer 1 · answered Dec 09 '14 at 23:03

The address of head is passed by value, so your changes are only reflected in the function itself. You have to pass a pointer to the address of head so that you can change the value.

int main() { 
   ...
   addNodePos(&list, value, position);
   ...
}

void addNodePos(struct DoubleList** headPtr, int value, int position)
{
    struct DoubleList *head = *headPtr;
    int i;
    struct DoubleList *node;

    if ( (node = malloc (sizeof(struct DoubleList))) != NULL ){
        node->id=value;
        if (head==NULL) {
            // points to itself as it is the only node in a list
            node->next=node;
            node->prev=node;
            head=node;
        } else {
            struct DoubleList *current=head;
            for (i = position; i > 1; i--)
                current=current->next;
            // reassign pointers -- relink nodes
            current->prev->next=node;
            node->prev=current->prev;
            node->next=current;
            current->prev=node;
        }
    }

    printf("Element has been added.\n\n");
}

You never update `*headPtr` again once you have modified `head` — M.M, Dec 10 '14 at 09:52

yulian · Accepted Answer · 2014-12-12T13:40:44.477

With a great help of users (which shared some very useful links), I have managed solving it.

Solution: to modify caller memory, pass a pointer to that memory.

After a bit updating, I leave here correctly working code:

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

struct DoubleList
{
    int id;
    struct DoubleList *next;
    struct DoubleList *prev;
};

void addNodePos(struct DoubleList** headRef, int value, int position);
void printList (struct DoubleList** headRef);
//void clearList (struct DoubleList** headRef);


int main()
{
    int value, position;

    struct DoubleList *list = NULL;

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(&list, value, position);

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(&list, value, position);


    printList(&list);
    //clearList(head);
    return 0;
}




void addNodePos(struct DoubleList** headRef, int value, int position)
{
    int i;
    struct DoubleList *node;

    if ( (node = malloc (sizeof(struct DoubleList))) != NULL ){
        node->id=value;
        if ( (*headRef)==NULL) {
            // points to itself
            node->next=node;
            node->prev=node;
            (*headRef)=node;
        } else {
            struct DoubleList *current=(*headRef);
            for (i = position; i > 1; i--)
                current=current->next;
            // reassign pointers -- relink nodes
            current->prev->next=node;
            node->prev=current->prev;
            node->next=current;
            current->prev=node;
        }
    }
    printf("Element has been added.\n\n");
}



void printList(struct DoubleList** headRef)
{
    if ( (*headRef)==NULL)
        printf("\nList is empty.\n\n");
    else {
        struct DoubleList *current=(*headRef);
        printf("\nThe list: ");
        do {
            printf("%x", current->id);
            current=current->next;
            if(current != (*headRef))
                printf("<->");
        } while(current!=(*headRef));
        printf("\n\n");
    }
}

@MattMcNabb, good mention) In this way the code will look a bit more clear. Updated! — yulian, Dec 10 '14 at 12:13

pointer to a struct becomes NULL at every function call

2 Answers2