changing struct/variable content in C

Question

I would like to get some help with the following question. I have a struct Node and I want to change it's insides using some method. and I want to print the content of the changed struct inside my main method. how do I get the struct changed and see the changed value in my main without returning the node as a return value. I guess it might be solved with passing the struct Node as a pointer and then editing it. what is the right way to do so?

for example:

typedef struct Node{
   struct Node * right;
   struct Node * left;
   void * data;
}Node;

void insert(void * element, Node* root){
   if(root==NULL){
       root=(Node*)malloc(sizeof(Node));
       root->data=element;
   }
}


int main(){
    Node a;
    int b=8;
    insert(&b,&a);
    printf("%d",*(int*)a.data);
    return 0;   
}

printf doesn't print 8 it prints 1 (i guess some garbage)

Solution

It sounds like you are trying to do the following

• Create a struct in one method, say main
• Pass it to a second method, say example
• Have example modify the struct and have the results visible in main

If so then the way to do this in C is by passing the struct as a pointer to example.

struct Node {
  int data;
  struct Node* pNext;
};

void example(struct Node* pNode) {
  pNode->data = 42;
  pNode->pNext = NULL;
}

int main() {
  struct Node n;
  example(&n);
  printf("%d\n", n.data);
}

EDIT

Responding to the updated question.

To see the result of a modification of a Node you must pass a Node*. And accordingly to see the result of a Node* modification you need to pass a Node**. Essentially you need to pass one more level of indirection than the value you want to mutate / return.

void insert(void* element, Node** ppRoot){
  if (NULL == *ppRoot) {
    Node* pTemp = malloc(sizeof(Node));
    pTemp->data = element;
    *ppRoot = pTemp;
  }
}

Comments

[Juan]

If this is for homework, please tag it as such. And yes, pointers are the way to go for that.

[mary]

when I do as you say it comes out correctly but what if I want to allocate memory using malloc as i did. why is it not working then?

[JaredPar]

@mary the reason why is everything in C is pass by value. Essentially a copy of the local is passed to the function. So with a single pointer you just assign memory to a copy which is not visible from main. With a double pointer though you pass a copy of the double pointer but it still points to the same pointer as the value in main. So when you change the pointed to value it's visible to both methods

[mary]

I have another question. if I use if(root==NULL) is the only way to change the fact that root is null is by mallocing some memory to it?

[Gray Programmerz]

Answer was life saver. Thanks alot.