====== Printkeys ======

When writing output in cp2k, to the screen or a file, one should use a //printkey//. 

A printkey allows the user to control the output. He can e.g. turn if off, redirect it to a different file, or output after a certain number of iteration (see [[dev:iter_levels]]).

To each printkey belongs an input-section, which is always structured in a similar way. For example, the printkey used to write trajectories is controlled by [[http://manual.cp2k.org/trunk/CP2K_INPUT/MOTION/PRINT/TRAJECTORY.html|this input-section]].

====== Creating a Printkey Input-section ======

<code Fortran>
 USE cp_output_handling,              ONLY: cp_print_key_section_create
 !...
 CALL section_create(subsection,name="print",&
      description="Controls the printing properties during an MD run",&
      n_keywords=0, n_subsections=1, repeats=.TRUE., required=.FALSE.,error=error)
 CALL cp_print_key_section_create(print_key,"TRAJECTORY",&
      description="Controls the output of the trajectory",&
      print_level=low_print_level, common_iter_levels=1,&
      filename="",unit_str="angstrom",error=error)
 CALL section_add_subsection(subsection,print_key,error=error)
 CALL section_release(print_key,error=error)
 !...
</code>

====== Using a Printkey for output ======

<code Fortran>
 USE cp_output_handling,              ONLY: cp_print_key_finished_output,&
                                            cp_print_key_unit_nr
 !...
 TYPE(cp_logger_type), POINTER :: logger
 INTEGER                       :: traj_unit
 !...
 logger => cp_error_get_logger(error)
 traj_unit = cp_print_key_unit_nr(logger,root_section,"MOTION%PRINT%TRAJECTORY",error=error)
 IF (traj_unit > 0) THEN
   WRITE(traj_unit, *) "here goes the output"
 ENDIF
 CALL cp_print_key_finished_output(traj_unit,logger,root_section,"MOTION%PRINT%TRAJECTORY",error=error)
</code>