When developing Linux kernel modules, specifically ones that tie into procfs via the seq_file interfaces and have circular buffers, it’s handy to be able to take an offset into your buffer and use the modulo operator (%) to make it wrap around easily. In the kernel you’re restricted to a smaller set of available operators however and modulo only works on certain types.

One type that it doesn’t work on that happens to be used in the seq_file interface is loff_t. If you try something like this:

static void *my_procfs_seq_start(struct seq_file *s, loff_t *pos)
{
  return global_pointer + ((*pos) % CIRCULAR_BUFFER_SIZE);
}

You will probably get an error about the unresolved symbol “_moddi3”. The quick fix for this is to do the following instead:

static void *my_procfs_seq_start(struct seq_file *s, loff_t *pos)
{
  return global_pointer + (((int) (*pos)) % CIRCULAR_BUFFER_SIZE);
}

This simply converts your dereferenced loff_t pointer to an int. Make sure your value will fit into your architecture’s int size!. If it doesn’t, you’ll get strange results.