Adding coordinates of clicks with glut to a linked list of vectors

Question

I want to create a linked list of vectors and with the help of GLUT library get the positions of the clicks and append them to the linked list.

These are the structs i wrote.

typedef struct vector{int x;int y;}Vector;
typedef struct VectorList{Vector X; struct VectorList*next; }node_v;

I globally defined a P vector and linked list of vectors prev.

Vector P;
node_v * prev=NULL;

Inside the mouse callback function _mouse_CB everytime left mouse button is clicked, i want to update the P vector with the current x and y values and append them to the linked list.

This is that part of the code.

static void _mouse_CB(int button, int state, int x, int y)
{
    if(state==GLUT_DOWN)
    {
        switch(button)
        {
        case GLUT_LEFT_BUTTON :
            px=x;py=y;
            P.x=x;
            P.y=y;
            prev=VL_new1(P);

            append(&prev,P);
            break;

the append function here i wrote from geeksforgeeks and added a while loop at the end to check if the values are added correctly, but i am getting overflow.

void append(node_v** head_ref, Vector A)
{ 
    node_v* new_node = (node_v*) malloc(sizeof(node_v)); 

    node_v *last = *head_ref;  

    new_node->X.x  = A.x; 
      new_node->X.y  = A.y; 
    new_node->next = NULL; 

    if (*head_ref == NULL) 
    { 
       *head_ref = new_node; 
       return; 
    }   
    while (last->next != NULL) 
        last = last->next; 
    last->next = new_node; 
    last = *head_ref;
    while(last){
      printf("%d %d\n", last->X.x,last->X.y);
      last = last->next;
    }
    return;     
} 

To create a node i wrote this function

node_v* VL_new1(Vector A){
        node_v *new = (node_v*)malloc(sizeof(node_v));
        if(new==NULL){exit(1);}
        else{
            new->X.x  = A.x; 
            new->X.y  = A.y; 
            new->next = NULL;
        }
        return new;
}

Each time i run this program and click on the appeared window, on the terminal the printf inside append function outputs this

-732680176 -729092496
0 -1344244448

What changes should i make to not get overflow and successfully add the current values?

Answer 1

The new node is created in the function append:

node_v* new_node = (node_v*) malloc(sizeof(node_v));

The instruction prev=VL_new1(P); generates a new list head. Every time when the code is executed, then prev is set and the previous content of prev is lost.

Remove if:

case GLUT_LEFT_BUTTON :
    px=x;py=y;
    P.x=x;
    P.y=y;
    append(&prev,P);

Note, the function VL_new1 can be called in append, instead:

void append(node_v** head_ref, Vector A)
{ 
    node_v *last = *head_ref;
    node_v* new_node = VL_new1(A); 

    if (*head_ref == NULL) 
    { 
       *head_ref = new_node; 
       return; 
    }

    while (last->next != NULL) 
        last = last->next; 
    last->next = new_node;
}