Hello World

Now that you have HELICS installed, you are ready to create your first HELICS federation. Let’s create a simple Hello, World example with 2 federates.

Note: This tutorial assumes basic familiarity with the command line. The HELICS co-simulation framework itself makes no specific demands about your editing, tooling, or where your code lives. Feel free to use whatever editor or IDE you are comfortable with.

Create a federations directory

Linux and Mac:

$ mkdir -p ~/federations/hello_world
$ cd ~/federations/hello_world

Windows CMD:

> mkdir %USERPROFILE%\federations
> cd %USERPROFILE%\federations
> mkdir hello_world
> cd hello_world

Writing your first federation

Next, make a new source file and call it hello_world_sender.c. Copy the contents from hello_world_sender.c and paste it into the file.

Next, create a new source file and call it hello_world_receiver.c. Copy the contents from hello_world_receiver.c and paste it into the file.

We will go through in more detail the contents of these files. For now, save the files and open two terminals.

Compiling the federates

To compile the federates, you can use the following commands.

Linux and Mac:

$ cc hello_world_sender.c -o ./hello_world_sender -lhelicsSharedLib
$ cc hello_world_receiver.c -o ./hello_world_receiver -lhelicsSharedLib

You may need to include additional include paths and library paths in the above command.

Running a federation

Linux and Mac:

Next, open three terminals. In the first terminal, run the following command.

$ ./helics_broker -f2

In the second terminal, run the following command.

$ ./hello_world_sender

In a third terminal, run the following command.

$ ./hello_world_receiver

You should see Hello, World printed out in the terminal where you ran the hello_world_receiver.

Anatomy of a HELICS federation

Now, let’s go over what just happened in the hello_world_sender.c part of the “Hello, World” program in detail.

The following block creates a ValueFederate. We will discuss what FederateInfo is and what a ValueFederate is, along with other types of Federates in more detail in other documents.

fedinfo = helicsCreateFederateInfo ();
helicsFederateInfoSetCoreTypeFromString (fedinfo,"zmq",&err);
helicsFederateInfoSetCoreInitString (fedinfo,fedinitstring,&err);
helicsFederateInfoSetTimeProperty (fedinfo,helicsGetPropertyIndex("period"), 1.0,&err);
vfed = helicsCreateValueFederate ("hello_world_sender",fedinfo,&err);

The following registers a global publication.

pub = helicsFederateRegisterGlobalPublication (vfed, "hello", helics_data_type_string, "",&err);

The following ensures that the federation has entered execution mode. If helicsFederateEnterInitializingMode is not included the call to helicsFederateEnterExecutingMode will automatically make the call in the background.

helicsFederateEnterInitializingMode (vfed,&err);
helicsFederateEnterExecutingMode (vfed,&err);

These functions publish a String and make a RequestTime function call to advance time in the simulation.

helicsPublicationPublishString(pub, "Hello, World",&err);
currenttime=helicsFederateRequestTime(vfed, 1.0, &err);

And finally, these functions free the Federate and close the HELICS library.

helicsFederateFinalize (vfed,&err);
helicsFederateFree (vfed);
helicsCloseLibrary ();

You can see that the hello_world_receiver.c is also very similar, but uses a Subscription instead. A snippet of the code is shown below.

fedinfo = helicsCreateFederateInfo ();
helicsFederateInfoSetCoreTypeFromString (fedinfo, "zmq",&err);
helicsFederateInfoSetCoreInitString (fedinfo, fedinitstring,&err);
helicsFederateInfoSetTimeProperty (fedinfo,helics_property_time_period, 1.0,&err);

vfed = helicsCreateValueFederate ("hello_world_receiver",fedinfo,&err);
sub = helicsFederateRegisterSubscription (vfed, "hello",NULL,&err);

helicsFederateEnterInitializingMode (vfed,&err);
helicsFederateEnterExecutingMode (vfed,&err);

/** request that helics grant the federate a time of 1.0
    the new time will be returned in currentime*/
currenttime=helicsFederateRequestTime (vfed, 1.0,&err);

isUpdated = helicsInputIsUpdated (sub);
helicsInputGetString (sub, value, 128,&actualLen,&err)
printf("%s\n", value);

helicsFederateFinalize (vfed,&err);
helicsFederateFree (vfed);
helicsCloseLibrary ();

A note on the &err term Many functions in the C API take a pointer to a helics_error structure. This can be created by a call to helicsErrorInitialize and can be reset by helicsErrorClear(helics_error *err). If an error occurs during the execution of a function or some inputs were invalid an error code in the helics_error structure will be set and a message included. For all functions if an error structure that already has an error in place is passed as an argument the function short circuits and does nothing. So checks can be done after a sequence of calls if desired with no worry about side effects. In the C++98 API an error triggers an exception, and in the base C++ API these originate as exceptions.