list *buckets = (list*)malloc(n*sizeof(list));

insert(buckets[0], data);

for(i = 0; i < n; ++i)
    *(buckets + i) = NULL;

list **buckets = ( list ** )malloc( n * sizeof( list * ) );

#include <string.h>

//...

memset( buckets, 0, n * sizeof( list *) );

for ( i = 0; i < n; ++i) buckets[i] = NULL;

List * buckets[n] = { 0 };