Question

the code is:

     int main(int argc, const char* argv[] )
     {
        struct node * initialpointer=NULL;
        insert("asd", initialpointer, 1);
        if(initialpointer!=NULL)
            printf("isnotnull");
        if (initialpointer==NULL)
            printf("isnull");
     }
     insert(char* x,struct node * initialpointer, int numberofelements){
        struct node B;
        B.word = x;
        B.parent = NULL;
        B.leftchild = NULL;
        B.rightchild = NULL;
        printf("%d", 12);
        if ( initialpointer == NULL){
           initialpointer = &B;
           B.parent = NULL;
        }
     }

So in the end I want initialpointer to point where nde B is but in the main method it prints it is null. So how can I change the initialpointer in function insert permanently?

Answer 1

There are a couple problems with your code.

First is the problem you’re having. The pointer you’re passing to insert is passed by copy; the function gets a copy and main doesn’t see your change. The solution to this is to pass a pointer to the pointer. See the POSIX function strtol for a standard library example of how this looks.

The second problem is that you’re returning a pointer to a local, automatic (i.e. stack-allocated) variable. Since the stack is released on function exit, this pointer points to invalid memory when it is back in main. So instead you need to allocate B from the heap.

So, fixing things up:

 int main(int argc, const char* argv[] )
 {
    struct node * initialpointer=NULL;
    insert("asd", &initialpointer, 1);
    if(initialpointer!=NULL)
        printf("isnotnull");
    if (initialpointer==NULL)
        printf("isnull");
 }
 insert(char* x,struct node ** initialpointer, int numberofelements){
    struct node *B;
    B = (struct node*)malloc(sizeof(struct node));
    B->word = x;
    B->parent = NULL;
    B->leftchild = NULL;
    B->rightchild = NULL;
    printf("%d", 12);
    if ( *initialpointer == NULL){
       *initialpointer = B;
       B->parent = NULL;
    }
 }