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 example/tutorials/t8_step0_helloworld.c.

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.

#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 leg 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 examples we use the function t8_global_productionf.

t8code offers various logging function 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 (here 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