p_context_t context;
p_context_init(&context, input, input_length);
size_t result = p_parse(&context);
if (p_parse(&context) == P_UNEXPECTED_TOKEN)
{
    p_position_t error_position = p_position(&context);
    fprintf(stderr, "Error: unexpected token `%s' at row %u column %u\n",
        p_token_names[context.token],
        error_position.row + 1, error_position.col + 1);
}