Adding a new Test Problem.

This is the best place to start your Enzo Development Career. Even if you’re not interested in actually writing a new problem generator, in this page I’ll discuss the basic Enzo data structures and programming patterns.

One deficiency in this tutorial is the lack of Particles. This is not an oversight, but due to the fact that the author of the article doesn’t really use particles, as he’s not a cosmologist. These will be added in the future, but particles are really not that big of a deal when it comes to the general Enzo data structures. All the information herein is still essential.


Essentially, you need two write files: MyProblemInitialize.C and Grid_MyProblemInitializeGrid.C. We’ll be discussing these two files. MyProblemInitialize is the basic setup code that sets up parameters and the hierarchy, and MyProblemInitializeGrid is a member function of the grid class, and actually allocates and assigns data. There are several pitfalls to setting up these files, so read these pages carefully.

We strongly recommend reading everything that proceeds this page on the Getting Started with Enzo page and the page about version control and regression testing, Introduction to Enzo Modification.

Lastly, please give your problem a reasonable name. I’ll be using MyProblem throughout this tutorial. Please change this to something that reflects the problem you’re installing.

Adding Setup Files and Defining a New Problem Type

Please follow the general Enzo naming convention and call your routines MyProblemInitialize and store it in MyProblemInitialize.C, and MyProblemInitializeGrid and store it in Grid_MyProblemInitializeGrid.C

You’ll need to install your code in three places.

  1. Make.config.objects

    is the file that lists all the source file objects needed to build Enzo. Put


    somewhere in the list of objects. If you want to make things really clean, you can add your own variable to the Makefile and have it driven by a command line switch, but this isn’t necessary.

  2. Grid.h. You’ll need to put

    MyProblemInitializeGrid in this the grid class definition. Put it with the rest of the *InitializeGrid routines.

  3. InitializeNew.C. Put

    MyProblemInitialize in InitializeNew. At the end of the large block of *Initialize, take the next unused ProblemType number and install your code. It should look something like this:

    // 61) Protostellar Collapse
    if (ProblemType == 61)
      ret = ProtostellarCollapseInitialize(fptr, Outfptr, TopGrid, MetaData);
    // 62) My New Problem
    if ( ProblemType == 62 )
      ret = MyProblemInitialize(fptr, Outfptr, TopGrid, MetaData);
    // Insert new problem intializer here...
    if (ret == INT_UNDEFINED) {
      fprintf(stderr, "Problem Type %"ISYM" undefined.\n", ProblemType);
      return FAIL;

To call your problem generator, make sure ProblemType = 62 is in your parameter file. (Or, if 62 is taken, whatever the next unused value is.)

The return value ret is used to check for errors and invalid values of ProblemType. The function signature will be discussed in the next section.

Also, don’t forget to put the proto type at the top:

int MyProblemInitialize(FILE *fptr, FILE *Outfptr,
                                   HierarchyEntry &TopGrid,
                                   TopGridData &MetaData);

We will revisit InitializeNew at the end. For almost all problems, this will be all you do for these three files.


The primary drive routine is called MyProblemInitialize. It basically sets up some global values, problem specific values, and the hierarchy before calling MyProblemInitializeGrid.

Function Signature

The function signature of MyProblemInitialize is fairly rigid. It should look exactly like the prototype you installed in InitializeNew. There are 4 arguments that you’ll almost certainly need, and one additional argument that only rare problems will need. You won’t likely have any need to add any other arguments. In order, they are:

  1. FILE *fptr This is the pointer to the parameter file argument to
    Enzo. It’s opened and closed in InitializeNew You can read parameters if you like, see below.
  2. FILE *Outfptr This is the output pointer, a file called “amr.out.”
    This file contains the derived details of your problem setup for your record. There is no necessary output for this, it’s for the users convenience.
  3. HierarchyEntry &TopGrid This is the pointer to the top of the
    Hierarchy Linked List. For details of the linked list, Getting Around the Hierarchy: Linked Lists in Enzo. For most problem types, it points to the undivided root grid, which is a grid the full size of the top grid, where you will be initializing your data. For problems that are too large for the entire root grid to be allocated, we use the ParallelRootGridIO functionality, to be discussed later. (Please read everything between here and there.)
  4. TopGridData &MetaData This is the structure that contains the meta
    data describing the Top Grid. Things like boundary condition, problem domain size, rank, and dimension are stored here. See TopGridData.h for a complete list of the contents.

If you want to write a problem with Dirichlet boundary conditions, for instance jet inflow, you will need to add a fifth argument to the function (and, of course, it’s called in InitializeNew). This is the external boundary, ExternalBoundary &Exterior. This is the External Boundary object, which you will need to deal with. We will not be discussing this here. If you need to be doing a problem with boundary conditions other than the big 3 (periodic, reflecting, outflow) then we recommend you read the entirety of this tutorial, then follow what’s done with the DoubleMach problem, which is problem type 4. You will also need to examine Grid_SetExternalBoundaryValues.C

Necessary Headers

The essential header files for MyProblemInitialize are the following:

#include <stdio.h>
#include <string.h>
#include "macros_and_parameters.h"
#include "typedefs.h"
#include "global_data.h"
#include "Fluxes.h"
#include "GridList.h"
#include "ExternalBoundary.h"
#include "Grid.h"
#include "Hierarchy.h"
#include "TopGridData.h"

These should be in this order, to ensure proper definitions across different header files. You should be familiar with the two standard headers <stdio.h> and <string.h>

In brief, these are:

  • macros_and_parameters.h The standard set of macros. This takes
    care of the float promotion so its inclusion is ABSOLUTELY ESSENTIAL
  • typedefs.h This takes
    care of enumerates for parameters like the hydro method.
  • global_data.h There
    are a lot of global parameters in Enzo. This houses them.
  • Fluxes.h Definition of the
    flux object. Not necessary for your objects, but I think its necessary for the later
  • GridList.h I don’t think
    this is necessary, but it’s usually included.
  • ExternalBoundary.h This defines the external boundary object. Even
    if you’re not including the external boundary, it’s necessary for the following headers.
  • Grid.h This defines the grid
    class, which you’ll definitely need.
  • Hierarchy.h This defines the Hierarchy Entry linked list.
  • TopGridData.h This defines the meta data object.

More information can be found in Header files in Enzo.

Initializing Baryon Fields

At some point in your problem type setup, it is essential that the arrays to hold BaryonField data are initialized. To do this, you must tell Enzo what baryon fields exist in the problem, allocate the BaryonField arrays, and label the fields so that they can be written out.

The easiest way to set up and allocate the fields is through a call to InitializeUniformGrid. This function, found in Grid_InitializeUniformGrid.C, takes care of setting up the fields and allocating the arrays and initializes the gas to a uniform state. You will still need to add labels for output as described below.

The call to InitializeUniformGrid can simply be added in MyProblemInitialize.C before the call to the problem initializer as follows:

if (TopGrid.GridData->InitializeUniformGrid(MyProblemUniformDensity,
                                      MyProblemUniformBField) == FAIL) {
                                         ENZO_FAIL("Error in InitializeUniformGrid.");

Note that it is in theory possible to set up the BaryonFields manually in your test problem without the call to InitializeUniformGrid. This is done in some test problems in the code base, but is discouraged as a call to InitializeUniformGrid is cleaner, simpler, and is already implemented. If for some reason you do need to do the setup and allocation manually, look at Grid_InitializeUniformGrid.C to see how it is done. The field information must be done on every grid on every processor, so it is essential that you do this step before exiting on remote grids.

Setting up Data Labels

There are two arrays that need to be filled in MyProblemInitialize. One of them is ABSOLUTELY ESSENTIAL for the functioning of the code. These are DataLabel and DataUnits. Both of these are arrays of strings that will be used to label the HDF5 output files. Each element of the array corresponds to an element of the BaryonField array and MUST be defined in the same order as fields are entered in InitializeUniformGrid or elsewhere in your problem initializer. There is not a mechanism to ensure that you do this right, so don’t screw it up.


This is the actual name of the field in the HDF5 file. Messing this up is asking for trouble. If you’re not using chemistry, you’ll want something that looks like this. If you change the actual names, you guarantee that an analysis tool somewhere will break, so don’t do it. See CosmologySimulationInitialize.C for a more complete list, including extra chemical species.

char *DensName = "Density";
char *TEName   = "TotalEnergy";
char *GEName   = "GasEnergy";
char *Vel1Name = "x-velocity";
char *Vel2Name = "y-velocity";
char *Vel3Name = "z-velocity";
i = 0;
DataLabel[i++] = DensName;
DataLabel[i++] = TEName;
if (DualEnergyFormalism)
  DataLabel[i++] = GEName;
DataLabel[i++] = Vel1Name;
DataLabel[i++] = Vel2Name;
DataLabel[i++] = Vel3Name;


The units really don’t matter very much. They’re usually set to NULL

Reading from the Parameter File

You may want to read in problem specific parameters. PLEASE do not put problem specific parameters in the main parameter file reader.

The usual pattern reads each line of the parameter file, and tries to match each line with a parameter. This allows the parameter file to be independent of of order. The typical pattern looks like this:

float MyVelocity, MyDensity;
char line[MAX_LINE_LENGTH];
while (fgets(line, MAX_LINE_LENGTH, fptr) != NULL) {
 ret = 0;

  /* read parameters */

  ret += sscanf(line, "MyProblemVelocity      = %"FSYM,
  ret += sscanf(line, "MyProblemDensity      = %"FSYM,
  if (ret == 0 && strstr(line, "=") && strstr(line, "MyProblem") &&
      line[0] != '#' && MyProcessorNumber == ROOT_PROCESSOR)
       "warning: the following parameter line was not interpreted:\n%s\n",

If you’re not familiar with these functions, here is a good list of standard C functions.

The last line checks for errors in parameters that start with MyProblem. Everything involving this routine should be prepended with MyProblem. In the file ReadParameterFile.C, the parameter file is read and any lines not recognized are thrown as errors; this is the section identified with

\* check to see if the line belongs to one of the test problems \*/.

You must add your prefix (in this case, MyProblem) to the list of test problem prefixes considered in this section:

if (strstr(line, "MyProblem")           ) ret++;

or else it will register as an error.

Calling the Grid Initializer: Unigrid

For a small, unigrid problem, the problem initializer is called using the standard Enzo function call procedure.

if( TopGrid.GridData->MyProblemInitializeGrid(MyVelocity, MyDensity) == FAIL ){
  fprintf(stderr,"MyProblemInitialize: Error in MyProblemInitializeGrid\n");
  return FAIL;

TopGrid is the HierarchyEntry that starts the hierarchy linked list. It’s member GridData is a pointer to the actual grid object that you will be modifying.

We will be discussing AMR problems, and large problems that require parallel startup later.


MyProblemInitializeGrid is the member function of the grid class. As a member function, it can access the private data, most importantly BaryonField. BaryonField is an array of pointers that stores the actual data that the simulator is interested in.


When setting up a new test problem, make sure to only set field values on Grids which live on the current processor. In Enzo, each Grid is a ‘real Grid’ on one processor and a ‘remote Grid’, storing only metadata, on other processors. Therefore, your problem initializer should include

if (ProcessorNumber != MyProcessorNumber)
  return SUCCESS;

before setting field values.

Finally, set up your test problem by setting the BaryonField values. See the page on Baryon Field Access for details. Accessing Data in BaryonField

Initializing AMR problems

For problems that you want to initialize in an AMR fashion, all the previous steps apply. However, instead of simply calling the problem initializer on the Top Grid, one must now initialize a HierarchyEntry linked list (of which TopGrid is the head) and call the problem initializer on each subgrid. There are several ways to do this, depending on the complexity of the code. One first needs to understand the HierarchyEntry linked list. This Page gives a tutorial on the linked lists, and links to examples in the code.

Using ParallelRootGridIO

Main article: Using Parallel Root Grid IO

ParallelRootGridIO is a fairly complex piece of code. If you absolutely must do this in the code, it is recommended that you read the description of the inner workings of ParallelRootGridIO and then cloning what’s done for the CosmologyInitialize routines.