How do i change pointer in function?

Question

I have a recursive function in C and i want the returned struct pointer to become the new struct in the function. Im having a problem with the returned struct because it doesnt change.this is the structure of my code:

struct location_t{
   int x,y;
   location_t * next;
   int dir;
}

location_t * recursive_foo(location_t * loc, maze_t * m){

    int x = loc->x;
    int y = loc->y;
    int dir = loc->dir;

    loc->next = malloc(sizeof(location_t));
    location_t * temp = loc->next;

    if(m->map[--x][y] != '#' && dir != 0){
        temp->x = x;
        temp->y = y;
        temp->dir = 2;
        loc = recursive_foo(temp);
    }
    if(m->map[--x][y] != '#' && dir != 1){
        temp->x = x;
        temp->y = y;
        temp->dir = 3;
        loc = recursive_foo(temp);
    }
    if(m->map[--x][y] != '#' && dir != 2){
        temp->x = x;
        temp->y = y;
        temp->dir = 0;
        loc = recursive_foo(temp);
    }
    if(m->map[--x][y] != '#' && dir != 3){
        temp->x = x;
        temp->y = y;
        temp->dir = 1;
        loc = recursive_foo(temp);
    }

    return loc;

}

Im having a problem with the returned struct because it doesnt change.

It is meant to stack these structs by referring to each other.

Can you post a minimal working program? What you've posted has typos that will fail to compile, and not enough logic to find the bug. — John Zwinck
– John Zwinck, Commented Mar 24, 2013 at 11:03
Make sure x doesn't become negative! You really wouldn't want m->map[-1].... — autistic
– autistic, Commented Mar 24, 2013 at 11:30

Alex · Accepted Answer · 2013-03-24 11:31:01Z

2

mystruct is a stack variable. In other words, you are passing the pointer by value, instead of passing it by reference.

What have you done at the moment is essentially the same as:

int f(int i) {
   ...
   i = <any value>;
   ...
}

In this case you are modifying only a copy of the value.

In your program, you are also modifying a copy of the pointer. Outside of the function the pointer stays not modified.

If you want to modify it, you need to pass a pointer to it:

location_t * recursive_foo(location_t** loc, maze_t * m){
    int x = (*loc)->x;
    int y = (*loc)->y;
    int dir = (*loc)->dir;
    ...
    *loc = recursive_foo(&temp);
    ...
    return *loc;
}

edited Mar 24, 2013 at 11:31

answered Mar 24, 2013 at 11:05

Alex

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1 Comment

Funonly Over a year ago

But if i make a ** out of temp how do i assign the vars x, y and dir?

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