FARGO3D has many different kinds of outputs, each one with different information. They are:

  • Scalar fields.
  • Domain files.
  • Variables file.
  • Grid files.
  • Legacy file.
  • Planetary files.
  • Monitoring files.

This section is devoted to a brief explanation of each kind of files

Scalar fields

These files have a .dat extension. They are unformatted binary files. The structure of each file is a sequence of doubles (8 bytes), or floats (4 bytes) if the FLOAT option was activated at build time (see the section .opt files). The number of bytes stored in a field file is:

  • \(8 \times N_x\times N_y\times N_z\)
  • \(4\times N_x\times N_y\times N_z\) if the option FLOAT was activated.

Remember that \(N_x\), \(N_y\), \(N_z\) are the global variables “NX NY NZ” defined in the .par file (the size of the mesh).

For a correct reading of the file, you must be careful with the order of the data. The figure below shows how the data is stored in each file (for 2D simulations, but the concept is the same in 3D).


The fast (“innermost”) index is always the x-index (index i inside the code). The next index is the y-index (j) and the last one is the z-index (k). If one direction is not used (eg: 2D YZ simulation), the indices used follow the same rule. Note that scalar fields files do not contain information about coordinates. It is only a cube of data, without any additional information. The coordinates of each cell are stored in additional files, called domain_[xyz].dat (see the section below).

When you use MPI, the situation becomes more complex, because each processor writes its piece of mesh. If you want to merge the files manually, you need the information of the grid files, detailed below. In practice, all the runs are done with the run time flag -m (merge), in order to avoiding the need for a manual merge. If your cluster does not have a global storage, you have to do the merge manually after having copied all files to a common directory.

The fields may be written with a different output format, called VTK-format. This format is a little bit more complicated and is discussed in the VTK section.

Here you have some minimalist reading examples with different tools (additional material can be found in the First Steps section and in the utils/ directory):


FILE *fi;
double f[nx*ny*nz];
fi = fopen(filename, "r");
fread(f,sizeof(double), nx*ny*nz, fi);


from pylab import *
rho = fromfile("gasdens120.dat").reshape(nz,ny,nx)x


GDL> openr, 10, 'gasdens10.dat'
GDL> rho = dblarr(nx,ny)
GDL> readu, 10, rho
GDL> close, 10


real*8  :: data(nx*ny*nz)
open(unit=100, status="old", file=filename, &
&     form="unformatted", access="direct", recl = NX*NY*NZ*8)
read(100,rec=1) data


plot filename binary format="%lf" array=(nx,ny) w image

Domain files

Another important piece of output is the domain files:

  • domain_x.dat
  • domain_y.dat
  • domain_z.dat

These three files are created after any run, except if they existed in the output directory before running your simulation. The content of these files are the coordinates of the lower face of each cell ([xyz]min inside the code). They can also be considered as the coordinates of the interfaces between cells. It is important to note that domain_[yz].dat are written with the ghost cells. The format is ASCII, and the total number of lines is:

  • domain_x.dat: Nx lines
  • domain_y.dat: Ny+2NGHY lines
  • domain_z.dat: Nz+2NGHZ lines

where NGHY=NGHZ=3 by default. The active mesh starts at line 4 and has Ny+1/Nz+1 lines (up to the upper boundary of the active mesh).

If you want to use a logarithmic spacing of the domain, you could set the parameter Spacing to log (see the section “Default parameters”). If you include in the output directory files with the name domain_[xyz].dat, their content will be read, which enables you to handcraft any kind of non constant zone size.


When you run the code, two files called variables.par and IDL.var are created inside the output directory. These files are ASCII files containing the same information in two different formats: the name of all parameters and their corresponding values. IDL.var is properly formatted to simplifying the reading process in an IDL/GDL script:

IDL> @IDL.var
IDL> print, input_par.nx
IDL> print,input_par.xmax

The standard .par format is used in variables.par. It may be used again as the input parameter file of FARGO3D, should you have erased the original parameter file.

Grid files

One grid file is created per processor. Inside each file, there is information stored about the submesh relative to each processor. The current format is on 7 columns, with the data:

  • CPU_Rank: Index of the cpu.
  • Y0: Initial Y index for the submesh.
  • YN: Final Y index for the submesh.
  • Z0: Initial Z index for the submesh.
  • ZN: Final Z index for the submesh.
  • IndexY: The Y index of the processor in a 2D mesh of processors.
  • IndexZ: The Z index of the processor in a 2D mesh of processors.

For an explanation of the last two items, go to the section about MPI.

Planet files

These files are output whenever a given setup includes a planetary system (which may consist of one or several planets). This, among others, is the case of the fargo and p3diso setups. There are three such files per planet, named planet[i].dat, bigplanet[i].dat and orbit[i].dat, where i is the planet number in the planetary system file specified by the parameter PLANETCONFIG. This number starts at 0. For the vast majority of runs in which one planet only is considered, three files are therefore output: planet0.dat, bigplanet0.dat, and orbit0.dat. The last two files correspond to fine grain sampling (that is, they are updated every DT, see also Monitoring). In contrast, planet[i].dat is updated at each coarse grain output (every time the 3D arrays are dumped), for restart purposes. This file is essentially a subset of bigplanet[i].dat.

At each update, a new line is appended to each of these files. In the file bigplanet[i].dat, a line contains the 10 following columns:

  1. An integer which corresponds to the current output number.
  2. The x coordinate of the planet.
  3. The y coordinate of the planet.
  4. The z coordinate of the planet.
  5. The x component of the planet velocity.
  6. The y component of the planet velocity.
  7. The z component of the planet velocity.
  8. The mass of the planet.
  9. The date.
  10. The instantaneous rotation rate of the frame.

In the file orbit[i].dat, a line contains the 10 following columns:

  1. the date \(t\),
  2. the eccentricity \(e\),
  3. the semi-major axis \(a\),
  4. the mean anomaly \(M\) (in radians),
  5. the true anomaly \(V\) (in radians),
  6. the argument of periastron \(\psi\) (in radians, measured from the ascending node),
  7. the angle \(\varphi\) between the actual and initial position of the x axis (in radians; useful to keep track of how much a rotating frame, in particular with varying rotation rate, has rotated in total).
  8. The inclination \(i\) of the orbit (in radians),
  9. the longitude \(\omega\) of the ascending node (with respect to the actual x axis),
  10. the position angle \(\alpha\) of perihelion (the angle of the projection of perihelion onto the x-y plane, with respect to the -actual- x axis)

Note that in the limit of vanishing inclination, we have

\[\alpha \approx \omega+\psi\]

The information of column 7 is very useful to determine precession rates, whenever the frame is non inertial. For instance, the precession rate of the line of nodes is given by \(d(\varphi+\omega)/dt\).


The file(s) planet[i].dat are emptied every time a new run is started. This is because these files are needed for restart, so we want to avoid that out of date, incorrect information be used upon restart. In contrast, lines accumulate in the files orbit[i].dat and bigplanet[i].dat until those (or the directory containing them) are manually suppressed.


All primitive variables (density, velocity components, internal energy density (or sound speed for isothermal setups), and magnetic field components (for MHD setups) are written every NINTERM steps of length DT (each of those being sliced in as many timesteps as required by the CFL condition). In addition, some selected arrays can be written every NSNAP steps of length DT. These arrays names are controlled by the boolean parameters WriteDensity, WriteEnergy, WriteVx, WriteVy, WriteVz, WriteBx, WriteBy, WriteBz. These allows the user either to oversample one of these fields (for an animation, for instante), or to dump to the disk only some variables (by setting NINTERM to a very large value and using NSNAP instead). Files created through the NSNAP mechanism obey the same numbering convention as those normally written. In order to avoid conflicts in filenames, the files created through NSNAP are written in the subdirectory snaps in the output directory.

Besides, the runtime graphical representation with matplotlib is performed using the files created in the snaps directory.



FARGO3D, much as its predecessor FARGO, has two kinds of outputs: coarse grain outputs, in which the data cubes of primitive variables are dumped to the disk, and fine grain outputs, in which a variety of other (usually lightweight) data is written to the disk. As their names indicate, fine grain outputs are more frequent than coarse grain outputs. Note that a coarse grain output is required to restart a run. In this manual we refer to the fine grain output as monitoring. The time interval between two fine grain outputs is given by the real parameter DT. This time interval is sliced in as many smaller intervals as required to fulfill the Courant (or CFL) condition. Note that the last sub-interval may be smaller than what is allowed by the CFL condition, so that the time difference between two fine grain outputs is exactly DT. As for its predecessor FARGO, NINTERM fine grain outputs are performed for each coarse grain output, where NINTERM is an integer parameter. Fine grain outputs or monitoring may be used to get the torque onto a planet with a high temporal resolution, or it may be used to get the evolution of Maxwell’s or Reynolds’ stress tensor, or it may be used to monitor the total mass, momentum or energy of the system as a function of time, etc. The design of the monitoring functions in FARGO3D is such that a lot of flexibility is offered, and the user can in no time write new functions to monitor the data of his choice. The monitoring functions provided with the distribution can run on the GPU, and it is extremely easy to implement a new monitoring function that will run straightforwardly on the GPU, using the functions already provided as templates.

Flavors of monitoring

The monitoring of a quantity can be done in several flavors:

  • scalar monitoring, in which the sum (or average) of the quantity over the whole computational domain is performed. The corresponding output is a unique, two-column file, the left column being the date and the right column being the integrated or averaged scalar. A new line is appended to this file at each fine grain output.
  • 1D monitoring, either in Y (i.e. radius in cylindrical or spherical coordinates) or Z (i.e. colatitude in spherical coordinates). In this case the integral or average is done over the two other dimensions only, so as to get, respectively, radial or vertical profiles in each output. Besides, the 1D monitoring comes itself in two flavors:
    • a raw format, for which a unique file is written, in which a row of bytes is appended at every fine grain output. This file can be readily used for instance with IDL (using openr & readu commands) or Python (using numpy’s fromfile command). For example, this allows to plot a map of the vertically and azimuthally averaged Maxwell’s tensor, as a function of time and radius. This map allows to estimate when the turbulence has reached a saturated state at all radii. In another vein, one can imagine a map of the azimuthally and radially averaged torque, which provides the averaged torque dependence on time and on z.
    • a formatted output. In this case a new file is written at each fine grain output. It is a two-column file, in which the first column represents the Y or Z value, as appropriate, and the second column the integrated or averaged value. The simultaneous use of both formats is of course redundant. They have been implemented for the user’s convenience.
  • 2D monitoring. In this case the integral (or averaging) is performed exclusively in X (or azimuth), so that 2D maps in Y and Z of the quantity are produced. In this case, a new file in raw format is written at each fine grain output.

Monitoring a quantity

Thus far in this section we have vaguely used the expression “the quantity”. What is the quantity and how is it evaluated ?

The quantity is any scalar value, which is stored in a dedicated 3D array. It is the user’s responsibility to determine an adequate expression for the quantity, and to write a routine that fills, for each zone, the array with the corresponding quantity. For instance, if one is interested in monitoring the mass of the system, the quantity of interest is the product of the density in a zone by the volume of the zone. The reader may have a look at the C file mon_dens.c. Toward the end of that file, note how the interm[] array is precisely filled with this value. This array will further be integrated in X, and, depending on what has been requested by the user, possibly in Y and/or Z, as explained above. Note that the same function is used for the three flavors of monitoring (scalar, 1D profiles and 2D maps).

Monitoring in practice

We now know the principles of monitoring: it simply consists in having a C function that evaluates some quantity of interest for each cell. No manual averaging or integration is required if you program your custom function. But how do we request the monitoring of given quantities, what are the names of the corresponding files, and how do we include new monitoring functions to the code ? We start by answering the first question.

The monitoring (quantities and flavors) is requested at build time, through the .opt file. There, you can define up to 6 variables, which are respectively:


Each of these variables is a bitwise OR of the different quantities of interest that are defined in define.h around line 100. These variables are labeled with a short, self-explanatory, upper case preprocessor variable.

For instance, assume that you want to monitor the total mass (scalar monitoring) and total angular momentum (also scalar monitoring), that you want to have a formatted output of the radial torque density, plus a 2D map of the azimuthally averaged angular momentum. You would have to write in your .opt file the following lines:


Note the pipe symbol | on the first line, which stands for the bitwise OR. It can be thought of as “switching on” simultaneously several bits in the binary representation of MONITOR_SCALAR, which triggers the corresponding request for each bit set to one. The condition for that, naturally, is that the different variables defined around line 100 in define.h are in geometric progression with a factor of 2: each of them corresponds to a given specific bit set to one. In our example we therefore activate the scalar monitoring of the mass and of the angular momentum (we will check in a minute that MOM_X corresponds to the angular momentum in cylindrical and spherical coordinates). This example also shows that a given variable may be used simultaneously for different flavors of monitoring: MOM_X (the angular momentum) is used both for scalar monitoring and 2D maps.

The answer to the second question above (file naming conventions) is as follows:

  • Unique files are written directly in the output directory. Their name has a radix which indicates which quantity is monitored (e.g. mass, momx, etc.), then a suffix which indicates the kind of integration or averaging performed ( _1d_Z_raw or _1d_Y_raw, or nothing for scalar monitoring) and the extension .dat.
  • Monitoring flavors that require new files at each fine grain output do not write the files directly in the output directory, in order not to clutter this directory. Instead, they are written in subdirectories which are named FG000..., like “Fine Grain”, plus the number of the current coarse grain output, with a zero padding on the left. In these directories, the files are written following similar conventions as above, plus a unique (zero padded) fine grain output number. It is a good idea to have a look at one of the outputs of the public distribution (choose a setup that requests some monitoring, by looking at its .opt file), in order to understand in depth these file naming conventions.
  • Some monitoring functions depend on the planet (such as the torque). In this case, the code performs automatically a loop on the different planets, and the corresponding file name has a suffix which indicates in a self-explanatory manner the planet it corresponds to.

How to register a monitoring function

You may now stop reading if you are not interested in implementing your own monitoring functions, and simply want to use the ones provided in the public distribution.

However, if you want to design custom monitoring functions for your own needs, let us explain how you include such functions to the code. Let us recall that a monitoring function is a function that fills a dedicated 3D arrays with some value of interest, left to the user. This function has no argument and must return a void. Have a look at the the file mon_dens.c and the function void mon_dens_cpu() defined in it. Note that the temporary array dedicated to the storage of the monitoring variable is the Slope array. As we enter the monitoring stage after a (M)HD time step, the Slope array is no longer used and we may use it as a temporary storage. Any custom monitoring function will have to use the Slope array to store the monitoring variable.

The monitoring function is then registered in the function InitMonitoring() in the file monitor.c. There, we call a number of times the function InitFunctionMonitoring () to register successively all the monitoring functions defined in the code.

  • The first argument is the integer (power of two) that is associated to the function, and which we use to request monitoring at build time in the .opt file.

  • The second argument is the function name itself (the observant reader will notice that this is not exactly true: in mon_dens.c the function is mon_dens_cpu(), whereas in InitMonitoring() we have:

    InitFunctionMonitoring (MASS, mon_dens, "mass",...

    instead of:

    InitFunctionMonitoring (MASS, mon_dens_cpu, "mass",...

The reason for that is that mon_dens is a function pointer itself, that points to mon_dens_cpu() or to mon_dens_gpu(), depending of whether the monitoring runs on the CPU or the GPU).

  • The third argument is a string which constitutes the radix of the corresponding output file.
  • The fourth argument is either TOTAL or AVERAGE (self-explanatory).
  • The fifth argument is a 4-character string which specifies the centering of the quantity in Y and Z. The first and third characters are always respectively Y and Z, and the second and fourth characters are either S (staggered) or C (centered). This string is used to provide the correct values of Y or Z in the formatted 1D profiles. For instance, the zone mass determined in mom_dens.c is obviously centered both in Y and Z.
  • The sixth and last argument indicates whether the monitoring function depends on the coordinates of the planet (DEP_PLANET) or not (INDEP_PLANET). In the distribution provided only the torq function depends on the planet.

The call of the InitFunctionMonitoring() therefore associates a variable such as MASS or MAXWELL to a given function. It specifies the radix of the file name to be used, and gives further details about how to evaluate the monitored value (loop on the planets, integration versus averaging, etc.) We may now check that requesting MOM_X in any of the six variables of the .opt file does indeed allow a monitoring of the angular momentum. We see in InitMonitoring() that the MOM_X variable is associated to mon_momx(). The latter is defined in the file mon_momx.c where we can see that in cylindrical or spherical coordinates the quantity evaluated is the linear azimuthal velocity in a non-rotating frame, multiplied by the cylindrical radius and by the density.

Implementing custom monitoring: a primer

In the distribution we adhere to the convention that monitoring functions are defined in files that begin with mon_. If you define your own monitoring function and you want it to run indistinctly on the CPU or on the GPU, you want to define in the mon_foo.c file the function:

void mon_foo_cpu ()

then in global.h you define a function pointer:

void (*mon_foo)();

In change_arch.c this pointer points either to the CPU function:

mon_foo = mon_foo_cpu;

or to the GPU function:

mon_foo = mon_foo_gpu;

depending on whether you want the monitoring to run on the CPU or the GPU. Finally, the syntax of the mon_foo.c file must obey the syntax described elsewhere in this manual so that its content be properly parsed into a CUDA kernel and its associated wrapper, and the object files mon_foo.o and mon_foo_gpu.o must be added respectively to the variables MAINOBJ and GPU_OBJBLOCKS of the makefile. A good starting point to implement your new mon_foo_cpu() function is to use mon_dens.c as a template.

Both the _cpu() and _gpu() functions need to be declared in prototypes.h:

ex void mon_foo_cpu(void);

in the section dedicated to the declaration of CPU prototypes, and:

ex void mon_foo_gpu(void);

in the section dedicated to the declaration of GPU prototypes. Be sure that the declaration are not at the same place in the file. The second one must be after the #ifndef __NOPROTO statement

In order to be used, you new monitoring function needs to be registered inside the function InitMonitoring () of the file monitor.c, using a syntax as follows:

InitFunctionMonitoring (FOO, mon_foo, "foo", TOTAL, "YCZC", INDEP_PLANET);

or similar, as described above. In this sentence, FOO is an integer power of 2 that must be defined in define.h. Be sure that it is unique so that it does not interfere with any other predefined monitoring variable. You are now able to request a fine grain output of your ‘’foo’’ variable, using in the .opt file expressions such as: