Question

I wrote a quick generic linked list, simple stuff. But I have a bug and I cannot spot what it is complaining about. Pertinent code:

typedef struct _node {
    void *data;
    struct _node *next;
} node;

typedef struct _queue {
    node *root;
    node *last;
    unsigned int length;
} queue;

node * find_node(void *data, int size, queue *q)
{
    node *n;

    for(n=q->root;n;n=n->next)
        if(memcmp(data, n->data, size)==0)
            return (n);

    return (NULL);
}

Testing it:

queue q = {NULL, NULL, 0};
node *n;
int data[QUEUEMAX];
int i;

/* insert bunch of ints into queue */
for(i=0;i<QUEUEMAX;i++) {
    data[i] = give_me_a_number();
    n = alloc_node();
    n->data = data[i];
    insert_into(n, &q);
}

printf("list size = %d.\n", q.length);

/* print out, make sure they're there */   
for(n=q.root;n;n=n->next)
    printf("data = %d\n", (int)n->data); //*(int *)n->data didn't work, segfault?

/* find a specific node */
node *nd = find_node(&data[10], sizeof(int), &q);
/* remove it */
rm_node(nd, &q);

Running it:

$ ./test
list size = 256.
data = 10
data = 11
data = 12
data = 13
data = 14
data = 15
data = 16
... blah blah (256 lines)
Segmentation Fault

gdb says the problem is the memcmp() in find_node(). I think gcc is whining about the n->data being passed to memcmp(). Any ideas? Also, I was getting a segfault trying to do int x = *(int *)n->data but this seems valid to me, non?

Answer 1

In this code:

n->data = data[i];

You are currently setting the void* data pointer to be data[i] but you really want to set it to the address of data[i] so you need to do:

n->data = &data[i];

That is also why you got a segfault on your cast.