C singly linked list swap two nodes

go through the list to find p's pre node and q's pre node.
if p not the head, p_pre node's next = q 
if q not the head, q_pre node's next = p
swap p->next and q->next with a temp node

typedef struct node{
    char *data; 
    struct node *next; 
}node_t; 

int swap(node_t *first, node_t *p, node_t *q)
{
    node_t *m = first, *p_pre = 0, *q_pre = 0; 

    for (; m; m = m->next){
        if (m->next == p) p_pre = m; 
        if (m->next == q) q_pre = m; 
    }

    if (q != first){
        if (!q_pre) return -1; 
        q_pre->next = p; 
    }

    if (p != first){
        if (!p_pre) return -1; 
        p_pre->next = q; 
    }

    m = q->next; 
    q->next = p->next; 
    p->next = m;
    return 0; 
}

//test
int main(int argc, const char *argv[])
{
    node_t d = {"d", 0}, c = {"c", &d}, b = {"b", &c}, a = {"a", &b}, *t;
    swap(&a, &a, &c); 
    for (t = &c; t; t = t->next)//print list
        printf("%s\n", t->data);
    return 0;
}