Thorn SampleIO serves as an example for creating your own I/O thorns. Its code is clearly structured and well documented, and implements a very simple and light-weight but fully functioning Cactus I/O method.
Together with the documentation about I/O methods in the Cactus Users’ Guide and the chapter about thorn IOUtil in the Cactus Thorn Guide you should be able to use this code and modify/extend it according to your needs.

1 Purpose

Thorn SampleIO registers the I/O method SampleIO with the Cactus flesh I/O interface. This method prints the data values of three-dimensional, distributed Cactus grid functions/arrays at a chosen location to screen.

The implemented I/O method makes use of the Cactus Hyperslabbing API to obtain the data values to print.

2 SampleIO Parameters

Parameters to control the SampleIO I/O method are:

• SampleIO::out_vars
  This parameter denotes the variables to output as a space-separated list of full variable and/or group names.

• SampleIO::point_x, SampleIO::point_y, SampleIO::point_z
  The location of the data point to output for all variables is given in index coordinates (starting from 0) on a three-dimensional computational grid.

• SampleIO::out_every
  This parameter sets the frequency for periodic output. A positive value means to output every so many iterations. A negative value chooses the value of the general IO::out_every integer parameter to be taken. A value of zero disables SampleIO periodic output.
  The value for out_every is used for all variables by default. This can be overwritten for individual variables by appending an option string to the variable name, like in

  SampleIO::out_vars = "MyThorn::MyVar[out_every=2]".

  All parameters are steerable, ie. they can be changed at runtime.
  The code in thorn SampleIO includes the logic to check whether a parameter has been changed since the last output, and how to re-evaluate the I/O parameters.

  3 Notes

  Like any other I/O thorn should do, SampleIO inherits general I/O parameters from thorn IOUtil. Therefore this I/O helper thorn must be included in the ThornList of a Cactus configuration in order to compile thorn SampleIO, and also activated at runtime in the ActiveThorns parameter in your parameter file.

  4 Parameters

  out_every	Scope: private	INT
  Description: How often to do SampleIO output, overrides IO::out_every
  Range		Default: -1
  1:*	Every so many iterations
  0:	Disable SampleIO output
  -1:	Choose the default from IO::out_every

  out_vars	Scope: private	STRING
  Description: Variables to output by SampleIO
  Range		Default: (none)
  .+	Space-separated list of fully qualified variable/group names
  	An empty string to output nothing

  point_x	Scope: private	INT
  Description: x-index (starting from 0) locating the array element to output
  Range		Default: (none)
  0:*	An index between [0, nx)

  point_y	Scope: private	INT
  Description: y-index (starting from 0) locating the array element to output
  Range		Default: (none)
  0:*	An index between [0, ny)

  point_z	Scope: private	INT
  Description: z-index (starting from 0) locating the array element to output
  Range		Default: (none)
  0:*	An index between [0, nz)

  io_out_every	Scope: shared from IO	INT

  strict_io_parameter_check	Scope: shared from IO	BOOLEAN

  verbose	Scope: shared from IO	KEYWORD

  5 Interfaces

  General

  Implements:

  sampleio

  Inherits:

  io

  6 Schedule

  This section lists all the variables which are assigned storage by thorn CactusExamples/SampleIO. Storage can either last for the duration of the run (Always means that if this thorn is activated storage will be assigned, Conditional means that if this thorn is activated storage will be assigned for the duration of the run if some condition is met), or can be turned on for the duration of a schedule function.

  Storage

  NONE

  Scheduled Functions

  CCTK_STARTUP

    sampleio_startup

    startup routine

   

  	After:	ioutil_startup
  	Language:	c
  	Type:	function