# System properties

This section documents the following functions:

-   `lammps_get_natoms`{.interpreted-text role="cpp:func"}
-   `lammps_get_thermo`{.interpreted-text role="cpp:func"}
-   `lammps_last_thermo`{.interpreted-text role="cpp:func"}
-   `lammps_extract_box`{.interpreted-text role="cpp:func"}
-   `lammps_reset_box`{.interpreted-text role="cpp:func"}
-   `lammps_memory_usage`{.interpreted-text role="cpp:func"}
-   `lammps_get_mpi_comm`{.interpreted-text role="cpp:func"}
-   `lammps_extract_setting`{.interpreted-text role="cpp:func"}
-   `lammps_extract_global_datatype`{.interpreted-text role="cpp:func"}
-   `lammps_extract_global`{.interpreted-text role="cpp:func"}

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The library interface allows the extraction of different kinds of
information about the active simulation instance and also - in some
cases - to apply modifications to it. This enables combining of a LAMMPS
simulation with other processing and simulation methods computed by the
calling code, or by another code that is coupled to LAMMPS via the
library interface. In some cases the data returned is direct reference
to the original data inside LAMMPS, cast to a void pointer. In that case
the data needs to be cast to a suitable pointer for the calling program
to access it, and you may need to know the correct dimensions and
lengths. This also means you can directly change those value(s) from the
calling program (e.g., to modify atom positions). Of course, changing
values should be done with care. When accessing per-atom data, please
note that these data are the per-processor **local** data and are
indexed accordingly. Per-atom data can change sizes and ordering at
every neighbor list rebuild or atom sort event as atoms migrate between
subdomains and processors.

``` c
#include "library.h"
#include <stdio.h>

int main(int argc, char **argv)
{
  void *handle;
  int i;

  handle = lammps_open_no_mpi(0, NULL, NULL);
  lammps_file(handle,"in.sysinit");
  printf("Running a simulation with %g atoms.\n",
         lammps_get_natoms(handle));

  printf(" %d local and %d ghost atoms. %d atom types\n",
         lammps_extract_setting(handle,"nlocal"),
         lammps_extract_setting(handle,"nghost"),
         lammps_extract_setting(handle,"ntypes"));

  double  *dt = (double *)lammps_extract_global(handle,"dt");
  printf("Changing timestep from %g to 0.5\n", *dt);
  *dt = 0.5;

  lammps_command(handle,"run 1000 post no");

  for (i=0; i < 10; ++i) {
    lammps_command(handle,"run 100 pre no post no");
    printf("PE = %g\nKE = %g\n",
           lammps_get_thermo(handle,"pe"),
           lammps_get_thermo(handle,"ke"));
  }
  lammps_close(handle);
  return 0;
}
```

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::: {.doxygenfunction project="progguide"}
lammps_get_natoms
:::

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::: {.doxygenfunction project="progguide"}
lammps_get_thermo
:::

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::: {.doxygenfunction project="progguide"}
lammps_last_thermo
:::

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::: {.doxygenfunction project="progguide"}
lammps_extract_box
:::

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::: {.doxygenfunction project="progguide"}
lammps_reset_box
:::

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::: {.doxygenfunction project="progguide"}
lammps_memory_usage
:::

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::: {.doxygenfunction project="progguide"}
lammps_get_mpi_comm
:::

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::: {.doxygenfunction project="progguide"}
lammps_extract_setting
:::

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::: {.doxygenfunction project="progguide"}
lammps_extract_global_datatype
:::

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::: {.doxygenfunction project="progguide"}
lammps_extract_global
:::