Thread: NULL returns every time?

Hi,

I have a code which takes some arrays and pointers for a linked list but returns every time NULL for head. I couldn't find a solution for it.

Can you help me about this?

Thanks.

(I added my function)

Code:

int read_score(struct all_answers *curr, struct all_answers *head_t, struct all_answers *tail, int pick[], int guess[]){
  
  pm_unit pm_unit_0;
      
  pm_unit_0 = score(pick, guess);
  
  count++;
  printf("%d %d %d %d %d %d => +%d -%d\n", count, guess[0], guess[1], guess[2], guess[3], guess[4], pm_unit_0.plus, pm_unit_0.minus);
  
  answer[5] = pm_unit_0.plus; answer[6] = pm_unit_0.minus;
  answer[0] = guess[0]; answer[1] = guess[1]; answer[2] = guess[2]; answer[3] = guess[3]; answer[4] = guess[4];
  
  curr = (struct all_answers *)malloc(sizeof(struct all_answers));
  
  curr->answer_list[6] = answer[6];
  curr->answer_list[5] = answer[5];
  curr->answer_list[4] = answer[4];
  curr->answer_list[3] = answer[3];
  curr->answer_list[2] = answer[2];
  curr->answer_list[1] = answer[1];
  curr->answer_list[0] = answer[0];
  
  printf("curr->answer_list[4]: %d\n", curr->answer_list[4]);
  
  if(head_t == NULL){
    printf("NULL returned inside read_score for head\n");
    head_t = curr;
    curr->prev = NULL;
    if(head_t == NULL)
    printf("head returned NULL after assignment\n");
  }
  else{
    tail->next = curr;
    curr->prev = tail;
  }
  tail = curr;
  curr->next = NULL;
  

  
  printf("tail->answer_list[4]: %d\n", tail->answer_list[4]);
  
  return pm_unit_0.plus;
  
}

You cannot pass anything to a function and expect to change that thing's value. You have to use a pointer to that type. So if you want your function to fill up the head pointer if it's NULL, then you need a pointer to that type:

Code:

void fill( struct foo **x )
{
    if( x )
        *x = malloc( ... );
}
...
struct foo *head = NULL;
foo( &head );

Quzah.

If you want the head to change (and likewise the tail), you need to pass a pointer to a pointer.

Code:

void foo ( int **p, int *q ) {
  *p = q;
}
int main ( ) {
  int myInt;
  int *p = NULL;
  foo( &p, &myInt );
  return 0;
}

Passing head (by value, as you are at the moment) means your local changes inside the function do NOT affect the caller.


Edit:
What he said ^^^