Pointer Type Argument

[Brandon Smith]

I have used this exact code numerous times in the past weeks, and it has always worked flawlessly.

typedef struct _LLNode   
{
    int x;
    int y;
    int z;
    int dx;
    int dy;
    int dz;
    struct _LLNode *next;
} LLNode;

typedef struct _List     
{                       
    LLNode *head;       
} List;
 
void addToFront(List *list, int x, int y, int z, int dx, int dy, int dz);

#define FOREVER 1
int main() {
    List list = {NULL};
    addToFront(&list, 100, 60, 10, -1, 1, -1 );
    addToFront(&list, 200, 110, 5, -1, 1, 1);
    addToFront(&list, 30, 30, 1, 1, 1, 1);
    REG_DISPCTL = MODE4 | BG2_ENABLE;
    PALETTE[252] = BLACK;
    PALETTE[253] = BLUE;
    PALETTE[254] = YELLOW;
    PALETTE[255] = RED;
    while (FOREVER) {
        LLNode *hd;
        hd = list->head; // ERROR HERE
        for(int i = 0; i < 3; i++){
        hd->y = hd->y + hd->dy;
        hd->x = hd->x + hd->dx;
        hd->z = hd->z + hd->dz;
        if(hd->y < 0)
        {
            hd->y = 0;
            hd->dy = -hd->dy;
        }
        if(hd->y>159 - hd->z)
        {
            hd->y = 159 - hd->z;
            hd->dy = -hd->dy;
        }
        if(hd->x < 0)
        {
            hd->x = 0;
            hd->x = -hd->x;
        }
        if(hd->x>239 - hd->z)
        {
            hd->x = 239 - hd->z;
            hd->dx = -hd->dx;
        }
        if(hd->z < 0)
        {
            hd->z = 0;
            hd->dz = -hd->dz;
        }
        if(hd->z > 10)
        {
            hd->z = 10;
            hd->dz = -hd->dz;
        }
        hd = hd->next;
        }
}
}

I am getting my error with "hd = list->head;"
As you can see, I'm using this pointer to traverse through the linked list. Every iteration, it updates with hd=hd->next;

I am getting an error however. My compiler is telling me:
error: invalid type argument of `->'
However, the code seems valid to me, as I have used this method before. Any idea why I am getting an invalid type error?

---
In addition, my IDE is applying the appropriate colors to all items, as it is recognizing the links as valid.

[memcpy]

It's because "list" isn't a pointer type, so replace "->" with "."

[MK27]

That is not all the actual code. For starters, for NULL is from <stddef.h> or <stdio.h>.

[oogabooga]

I have used this exact code numerous times in the past weeks, and it has always worked flawlessly.

So you must have always been dynamically allocating the list head so -> would have worked. But memcpy's pointed out the problem.

[Brandon Smith]

I can't seem to spot the difference's in my code that was allowing -> to work before, and now the dot notation to work. Can you explain the difference?

[quzah]

To access the members of a structure, you use the dot operator. To access them via a pointer, you can use the arrow operator instead (you don't have to, but you can).

struct foo x, *p;
p = &x;

z = x.thing;
z = (*p).thing;
z = p->thing;

Take your pick.


Quzah.