Question

I use vmalloc to allocate a contiguous virtual memory block. I then set all the bytes of this block to 0 by using memset. I then, fill this memory region with a data structure that I created that is 16 bytes in size.

Then, during one of my functions, I place a pointer to one of these structures, and begin to walk down the memory region to find the first 0 byte so that I can place another data structure here. My first pointer correctly gets the previously placed structure, and I can see the address. This is the printout:

point’s filename / //Filename
ffffc9001095b500 //Address

That is the correct memory region for this structure (since I placed it here).

I then increment my pointer, which should take me to memory region ffffc9001095b510,
but instead my pointer comes back with an address of NULL. Why is this?

Here’s the code:

void * check_aux(char * upPath, int index, int location){
struct directory * point;       
int i = 0;
int dirnum = 0;
//int lastaddr;
printk("Inside check_aux\n");   

if(location > 117){
    return NULL;
}
else if(upPath == NULL){
    return NULL;
}
else{
    point = getLocation(index, location);
    printk("This is point's filename %s\n%p\n", point->filename, point);        
    while(point != NULL && dirnum < 16){

        while(point->filename[i] == upPath[i] || (point->filename[i] == 0 && (upPath[i] =='/' || upPath[i] == 0))){
            printk("%c == %c\n", point->filename[i], upPath[i]);
            if(point->filename[i] == 0 && upPath[i] == 0){
                return NULL;
            }
            else if(point->filename[i] == 0 && upPath[i] == '/'){
                if(nodes[point->index].type[0] == 'd'){
                    return check_aux(&upPath[i+1], point->index, 0);
                }
                else{
                    return NULL;
                }


            }
            else{
                i++;
            }

        }
        dirnum++;
        (point++);
    }
    if(dirnum == 16){
        return check_aux(upPath, index, location+1);
    }
    printk("Returning point %p\n", point);
    return point;
}

}

The data struct is defined as

struct directory{
char filename[14];
short index;

};

And filename cannot start with 0.

Answer 1

You leave the loop in 2 cases:

• if point is NULL, in which case you return NULL.
• if dirnum is 16. In which case you call the function recursively, and will also return NULL.

All other return statements return NULL, too.