C passing by reference-CodePudding

I want this function to add a point to the beginning of the linked list:

void addPoint(Point *head, int x, int y, SDL_bool dir) {
    Point *p = malloc(sizeof *p);
    p->x = x;
    p->y = y;
    p->dir = dir;
    p->next = head;
    head = p;
}

The head is initialized earlier like so:

Point *down = NULL;

Afterwards I call the function like so:

addPoint(&down, x * grid_cell_width, (y - 1) * grid_cell_height, SDL_FALSE);

Unfortunately this does not work as After after the call the head is still NULL.

CodePudding user response：

You are passing a pointer to head, meaning you are passing a copy of the address of head to function addPoint. In the line where you are changing the value of head you are actually modifying the local pointer. To change that, you need to pass a pointer to a pointer, like so:

void addPoint(Point **head, int x, int y, SDL_bool dir) {
    Point *p = malloc(sizeof(Point));
    p->x = x;
    p->y = y;
    p->dir = dir;
    p->next = head;
    *head = p;
}

CodePudding user response：

Try something like:

void addPoint(Point **head, int x, int y, SDL_bool dir) {
    Point p = malloc(sizeof(*p));
    p->x = x;
    p->y = y;
    p->dir = dir;
    p->next = head;
    *head = p;
}

In C there is no pass-by-reference mechanism per se, you just pass pointers to arguments instead (in this case, a pointer to a pointer). But those pointers are still just values, so a change to them is not visible outside of the function. What will be visible is a change to the value that the pointer points to. That's why you need to assign your p pointer to the location that the pointer head points to, hence you need to dereference it with (*) operator first.

CodePudding user response：

You got this part right:

addPoint(&down,...);

but the type of the expression &down is Point **, not Point *, so you need to change the definition of addPoint to

void addPoint(Point **head, int x, int y, SDL_bool dir) {
    ...
    p->next = *head;
    *head = p;
}

With this code, you have the following relationships:

 head == &down // Point ** == Point **
*head ==  down // Point * == Point *

So in the addPoint function, writing to *head is the same as writing to down.