Step 0 Hello World

Step 0 - Hello World

In this first tutorial we write the Hello World equivalent of t8code. We will initalize t8code and use it to print a short message on the root process. You will find the code in tutorials/general/t8_step0_helloworld.cxx, and it creates the executable tutorials/general/t8_step0_helloworld.

To initalize t8code we need to initialize (in this order) MPI, libsc and t8code.

We then use t8_global_productionf to print a message on the root process.

At the end of our program, we need to finalize libsc and then MPI.

Note that libsc wraps all MPI calls and data structures, which is why we prefix these with sc or SC.

#include <t8.h>

int
main (int argc, char **argv)
{
  int                 mpiret;

  /* Initialize MPI. This has to happen before we initialize sc or t8code. */
  mpiret = sc_MPI_Init (&argc, &argv);
  /* Error check the MPI return value. */
  SC_CHECK_MPI (mpiret);

  /* Initialize the sc library, has to happen before we initialize t8code. */
  sc_init (sc_MPI_COMM_WORLD, 1, 1, NULL, SC_LP_ESSENTIAL);
  /* Initialize t8code with log level SC_LP_PRODUCTION. See sc.h for more info on the log levels. */
  t8_init (SC_LP_PRODUCTION);

  /* Print a message on the root process. */
  t8_global_productionf (" [step0] \n");
  t8_global_productionf (" [step0] Hello, this is t8code :)\n");
  t8_global_productionf (" [step0] \n");

  sc_finalize ();

  mpiret = sc_MPI_Finalize ();
  SC_CHECK_MPI (mpiret);

  return 0;
}

Logging

To output a message on the root process in this Hello World example we use the function t8_global_productionf.

t8code offers various logging functions and logging levels via libsc.

In short, logging functions of the form t8_global_* (t8_global_essentialf, t8_global_errorf, etc.) only print on the root process, while logging functions without the global print on each process with the MPI rank as part of the message (t8_productionf, t8_essentialf, t8_errorf, t8_debugf). For a description of all logging functions see t8.h.

The logging level is set with t8_init (in the example above it is SC_LP_PRODUCTION) and determines which of these get printed. The different logging levels are described in sc.h in detail. The most common are

Log level	description
SC_LP_DEBUG	logs almost everything, in particular t8_debugf
SC_LP_PRODUCTION	logs *_production, *_essential and *_error
SC_LP_SILENT	never logs anything