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4.11. Writing plugins

Plugins provide a mechanism to add functionality to a LAMMPS executable without recompiling LAMMPS. The functionality for this and the plugin command are implemented in the PLUGIN package which must be installed to use plugins.

Plugins use the operating system’s capability to load dynamic shared object (DSO) files in a way similar shared libraries and then reference specific functions in those DSOs. Any DSO file with plugins has to include an initialization function with a specific name, “lammpsplugin_init”, that has to follow specific rules described below. When loading the DSO with the “plugin” command, this function is looked up and called and will then register the contained plugin(s) with LAMMPS.

When the environment variable LAMMPS_PLUGIN_PATH is set, then LAMMPS will search the directory (or directories) listed in this path for files with names that end in plugin.so (e.g. helloplugin.so) and will try to load the contained plugins automatically at start-up. For plugins that are loaded this way, the behavior of LAMMPS should be identical to a binary where the corresponding code was compiled in statically as a package.

From the programmer perspective this can work because of the object oriented design of LAMMPS where all pair style commands are derived from the class Pair, all fix style commands from the class Fix and so on and usually only functions present in those base classes are called directly. When a pair_style command command or fix command command is issued a new instance of such a derived class is created. This is done by a so-called factory function which is mapped to the style name. Thus when, for example, the LAMMPS processes the command pair_style lj/cut 2.5, LAMMPS will look up the factory function for creating the PairLJCut class and then execute it. The return value of that function is a Pair * pointer and the pointer will be assigned to the location for the currently active pair style.

A DSO file with a plugin thus has to implement such a factory function and register it with LAMMPS so that it gets added to the map of available styles of the given category. To register a plugin with LAMMPS an initialization function has to be present in the DSO file called lammpsplugin_init which is called with three void * arguments: a pointer to the current LAMMPS instance, a pointer to the opened DSO handle, and a pointer to the registration function. The registration function takes two arguments: a pointer to a lammpsplugin_t struct with information about the plugin and a pointer to the current LAMMPS instance. Please see below for an example of how the registration is done.

4.11.1. Members of `lammpsplugin_t`

Member	Description
version	LAMMPS Version string the plugin was compiled for
style	Style of the plugin (pair, bond, fix, command, etc.)
name	Name of the plugin style
info	String with information about the plugin
author	String with the name and email of the author
creator.v1	Pointer to factory function for pair, bond, angle, dihedral, improper, kspace, or command styles
creator.v2	Pointer to factory function for compute, fix, or region styles
handle	Pointer to the open DSO file handle

Only one of the two alternate creator entries can be used at a time and which of those is determined by the style of plugin. The “creator.v1” element is for factory functions of supported styles computing forces (i.e. pair, bond, angle, dihedral, or improper styles) or command styles and the function takes as single argument the pointer to the LAMMPS instance. The factory function is cast to the lammpsplugin_factory1 type before assignment. The “creator.v2” element is for factory functions creating an instance of a fix, compute, or region style and takes three arguments: a pointer to the LAMMPS instance, an integer with the length of the argument list and a char ** pointer to the list of arguments. The factory function pointer needs to be cast to the lammpsplugin_factory2 type before assignment.

4.11.2. Pair style example

As an example, a hypothetical pair style plugin “morse2” implemented in a class PairMorse2 in the files pair_morse2.h and pair_morse2.cpp with the factory function and initialization function would look like this:

#include "lammpsplugin.h"
#include "version.h"
#include "pair_morse2.h"

using namespace LAMMPS_NS;

static Pair *morse2creator(LAMMPS *lmp)
{
  return new PairMorse2(lmp);
}

extern "C" void lammpsplugin_init(void *lmp, void *handle, void *regfunc)
{
  lammpsplugin_regfunc register_plugin = (lammpsplugin_regfunc) regfunc;
  lammpsplugin_t plugin;

  plugin.version = LAMMPS_VERSION;
  plugin.style   = "pair";
  plugin.name    = "morse2";
  plugin.info    = "Morse2 variant pair style v1.0";
  plugin.author  = "Axel Kohlmeyer (akohlmey@gmail.com)";
  plugin.creator.v1 = (lammpsplugin_factory1 *) &morse2creator;
  plugin.handle  = handle;
  (*register_plugin)(&plugin,lmp);
}

The factory function in this example is called morse2creator(). It receives a pointer to the LAMMPS class as only argument and thus has to be assigned to the creator.v1 member of the plugin struct and cast to the lammpsplugin_factory1 function pointer type. It returns a pointer to the allocated class instance derived from the Pair class. This function may be declared static to avoid clashes with other plugins. The name of the derived class, PairMorse2, however must be unique inside the entire LAMMPS executable.

4.11.3. Fix style example

If the factory function is for a fix or compute, which take three arguments (a pointer to the LAMMPS class, the number of arguments and the list of argument strings), then the pointer type is lammpsplugin_factory2 and it must be assigned to the creator.v2 member of the plugin struct. Below is an example for that:

#include "lammpsplugin.h"
#include "version.h"
#include "fix_nve2.h"

using namespace LAMMPS_NS;

static Fix *nve2creator(LAMMPS *lmp, int argc, char **argv)
{
  return new FixNVE2(lmp,argc,argv);
}

extern "C" void lammpsplugin_init(void *lmp, void *handle, void *regfunc)
{
  lammpsplugin_regfunc register_plugin = (lammpsplugin_regfunc) regfunc;
  lammpsplugin_t plugin;

  plugin.version = LAMMPS_VERSION;
  plugin.style   = "fix";
  plugin.name    = "nve2";
  plugin.info    = "NVE2 variant fix style v1.0";
  plugin.author  = "Axel Kohlmeyer (akohlmey@gmail.com)";
  plugin.creator.v2 = (lammpsplugin_factory2 *) &nve2creator;
  plugin.handle  = handle;
  (*register_plugin)(&plugin,lmp);
}

4.11.4. Command style example

Command styles also use the first variant of factory function as demonstrated in the following example, which also shows that the implementation of the plugin class may be within the same source file as the plugin interface code:

#include "lammpsplugin.h"

#include "comm.h"
#include "error.h"
#include "command.h"
#include "version.h"

#include <cstring>

namespace LAMMPS_NS {
  class Hello : public Command {
   public:
    Hello(class LAMMPS *lmp) : Command(lmp) {};
    void command(int, char **);
  };
}

using namespace LAMMPS_NS;

void Hello::command(int argc, char **argv)
{
   if (argc != 1) error->all(FLERR,"Illegal hello command");
   if (comm->me == 0)
     utils::logmesg(lmp,fmt::format("Hello, {}!\n",argv[0]));
}

static void hellocreator(LAMMPS *lmp)
{
  return new Hello(lmp);
}

extern "C" void lammpsplugin_init(void *lmp, void *handle, void *regfunc)
{
  lammpsplugin_t plugin;
  lammpsplugin_regfunc register_plugin = (lammpsplugin_regfunc) regfunc;

  plugin.version = LAMMPS_VERSION;
  plugin.style   = "command";
  plugin.name    = "hello";
  plugin.info    = "Hello world command v1.1";
  plugin.author  = "Axel Kohlmeyer (akohlmey@gmail.com)";
  plugin.creator.v1 = (lammpsplugin_factory1 *) &hellocreator;
  plugin.handle  = handle;
  (*register_plugin)(&plugin,lmp);
}

4.11.5. Additional Details

The initialization function must be called lammpsplugin_init, it must have C bindings and it takes three void pointers as arguments. The first is a pointer to the LAMMPS class that calls it and it needs to be passed to the registration function. The second argument is a pointer to the internal handle of the DSO file, this needs to be added to the plugin info struct, so that the DSO can be closed and unloaded when all its contained plugins are unloaded. The third argument is a function pointer to the registration function and needs to be stored in a variable of lammpsplugin_regfunc type and then called with a pointer to the lammpsplugin_t struct and the pointer to the LAMMPS instance as arguments to register a single plugin. There may be multiple calls to multiple plugins in the same initialization function.

To register a plugin a struct of the lammpsplugin_t needs to be filled with relevant info: current LAMMPS version string, kind of style, name of style, info string, author string, pointer to factory function, and the DSO handle. The registration function is called with a pointer to the address of this struct and the pointer of the LAMMPS class. The registration function will then add the factory function of the plugin style to the respective style map under the provided name. It will also make a copy of the struct in a list of all loaded plugins and update the reference counter for loaded plugins from this specific DSO file.

The pair style itself (i.e. the PairMorse2 class in this example) can be written just like any other pair style that is included in LAMMPS. For a plugin, the use of the PairStyle macro in the section encapsulated by #ifdef PAIR_CLASS is not needed, since the mapping of the class name to the style name is done by the plugin registration function with the information from the lammpsplugin_t struct. It may be included in case the new code is intended to be later included in LAMMPS directly.

A plugin may be registered under an existing style name. In that case the plugin will override the existing code. This can be used to modify the behavior of existing styles or to debug new versions of them without having to re-compile or re-install all of LAMMPS.

4.11.6. Compiling plugins

Plugins need to be compiled with the same compilers and libraries (e.g. MPI) and compilation settings (MPI on/off, OpenMP, integer sizes) as the LAMMPS executable and library. Otherwise the plugin will likely not load due to mismatches in the function signatures (LAMMPS is C++ so scope, type, and number of arguments are encoded into the symbol names and thus differences in them will lead to failed plugin load commands). Compilation of the plugin can be managed via both, CMake or traditional GNU makefiles. Some examples that can be used as a template are in the examples/plugins folder. The CMake script code has some small adjustments to allow building the plugins for running unit tests with them.

Another example that converts the KIM package into a plugin can be found in the examples/kim/plugin folder. No changes to the sources of the KIM package themselves are needed; only the plugin interface and loader code needs to be added. This example only supports building with CMake, but is probably a more typical example. To compile you need to run CMake with -DLAMMPS_SOURCE_DIR=<path/to/lammps/src/folder>. Other configuration setting are identical to those for compiling LAMMPS.

A second example for a plugin from a package is in the examples/PACKAGES/pace/plugin folder that will create a plugin from the ML-PACE package. In this case the bulk of the code is in a static external library that is being downloaded and compiled first and then combined with the pair style wrapper and the plugin loader. This example also contains a NSIS script that can be used to create an Installer package for Windows (the mutual licensing terms of the external library and LAMMPS conflict when distributing binaries, so the ML-PACE package cannot be linked statically, but the LAMMPS headers required to build the plugin are also available under a less restrictive license). This will automatically set the required environment variable and launching a (compatible) LAMMPS binary will load and register the plugin and the ML-PACE package can then be used as it was linked into LAMMPS.