Struct as pointers

Greetings.

I seek assistance with gaining an understanding of structs in the case when they are passed as pointers to function. Below are code snippets to illustrate my points of contention:

Code:

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typedef struct stack_t Stack;

struct stack_t{
  int * data;
  int top;              //points to top of the *data
}

---

I define & initialize the struct as follows:

Code:

---

Stack stk;
Stack * S = &stk;
init(S, 3);

---

It is as this point when I initialize the struct that my understanding of the subject leads me to attempt the following method, which results in compiler errors. The definition of init() is as follows:

Code:

---

void init(Stack * S, int N){
  int * stk = (int*)malloc(N*sizeof(int));
  stk = &(S->data);                                  //Causes error
  S->top = -1;
}

---

Would greatly appreciate assistance with addressing the above concern.

Look forward to a prompt response.

Best regards,
wirefree101

In the future, please show the actual errors you get; they are much more helpful than simply saying you got an error.

This case, though, I can eyeball easily. stk is a pointer to int. &S->data is a pointer to pointer to int. These are not the same thing and cannot be used interchangeably. For this case, I see no particular reason why you are even using a temporary variable at all. Why not:

Code:

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S->data = malloc(N * sizeof *S->data);

---

This, I imagine, is what you're trying to do. In your code, I presume you have the assignment turned around; that is, you want to assign stk to S->data, not the other way around.

Quote:

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Originally Posted by wirefree101

Greetings.

I seek assistance with gaining an understanding of structs in the case when they are passed as pointers to function. Below are code snippets to illustrate my points of contention:

Code:

---

typedef struct stack_t Stack;

struct stack_t{
  int * data;
  int top;                //points to top of the stack data
}

---

---

Shouldn't the struct definition be the other way around as top is not a pointer it can't point to the top of *data ie

Code:

---

struct stack_t
{
  int data;
  int *top;
}

---

Given the above struct definition, top can now point to the top of the data stack.

Quote:

---

Originally Posted by wirefree101

I define & initialize the struct as follows:

Code:

---

Stack stk;
Stack * S = &stk;
init(S, 3);

---

It is as this point when I initialize the struct that my understanding of the subject leads me to attempt the following method, which results in compiler errors. The definition of init() is as follows:

Code:

---

void init(Stack * S, int N){
  int * stk = (int*)malloc(N*sizeof(int));
  stk = &(S->data);                                  //Causes error
  S->top = -1;
}

---

---

Once again stuff seems to be switched around, as in

Code:

---

  int * stk = (int*)malloc(N*sizeof(int));
  //stk = &(S->data);    // no can't do as cas noted besides you're initializing S not stk
  S->top = stk;
  S->data = -1;

---

Code:

---

stk = (S->data);

---

stk is pointing to s-> next

Appreciate the responses.

Best regards,
wirefree101

Greetings.

I seek assistance with understanding the usage of pointer to a pointer in cases where struct is employed. I state my point of contention with the help of the following illustrative code:

Code:

---

int main(){
  Node *ptr = NULL;      //pointer to struct Node
  initialize(&ptr);
}

void initiatize(** ptrN){
  if(*ptrN == NULL)
      ...                  //code to initialize struct elements
      ...
}

---

My query is the following:

Qs.: What has been achieved that differs from the case where I call initialize(ptr) (not initialize(&ptr)) and initialize() receives (*ptrN) (not (**ptrN))?

Would greatly appreciate a simply stated & prompt response.

Best regards,
wirefree101

Quote:

---

Originally Posted by wirefree101

Qs.: What has been achieved that differs from the case where I call initialize(ptr) (not initialize(&ptr)) and initialize() receives (*ptrN) (not (**ptrN))?

Would greatly appreciate a simply stated & prompt response.

Best regards,
wirefree101

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replace the Node* type with int and follow the code

Code:

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int a = 0;
initialize(&a);

...

void initialize(int* b)
{
  *b = 5;
}

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what will happen if instead of initialize(int* b) you do initialize(int)
and instead of &a you pass just a?

Then replace "int" with your real type, eg Node* and watch how it follows the very same rules ;)

Quote:

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Originally Posted by vart

replace the Node* type with int and follow the code

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Appreciate the responses.

As suggested, I submit the following code for your comments.

Synopsis: Two functions perform the addition operation - first receives a pointer to a pointer while the second receives simply a pointer. They both achieve the desired addition operation. The detail is for the sake of completion.

Code:

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void addI(int **j){
        printf("%d\n",j);        // pointer's address
        printf("%d\n",*j);        // i's address
        printf("%d\n",**j);        // i
        **j = **j + 1;                        //store at this address the content of this address added to 1
        printf("---%d\n",*j);       
}

void addX(int *j){
        printf("%d\n",j);        //x's address
        printf("%d\n",*j);  //x
        *j = *j + 1;                        //store at this address the content of this address added to 1
}


int main(){
        int i = 5;
        int * ptrI = &i;
        printf("%d\n",ptrI);        //address stored by pointer i.e. i's address
        printf("%d\n",&ptrI);//pointer's own address
        printf("%d\n",*ptrI);//value at address stored by pointer i.e. i
        addI(&ptrI);
        printf("i=%d\n",i);
       
        int x = 5;
        int * ptrX = &x;
        addX(ptrX);
        printf("x=%d\n",x);
       
        return 0;
}

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Would appreciate your comments (corrections) against each of my comments in the code. The same would assist my understanding.

Best regards,
wirefree101