lammps/examples/COUPLE/plugin

This directory has a simple C code that shows how LAMMPS can be included
in an application as a shared library loaded at runtime. The code is
essentially the same as in the "simple" example that links to LAMMPS
either with a static or shared library. The purpose is to illustrate
how another code could be built without having a LAMMPS library present
and then load the (separately compiled) shared library.

simple.c           is the C driver
liblammpsplugin.c  is the LAMMPS library plugin loader

You can then build the driver executable codes with a compile line
like below.

mpicc -c -O -Wall -g -I$HOME/lammps/src liblammpsplugin.c
mpicc -c -O -Wall -g simple.c
mpicc simple.o liblammsplugin.o -ldl -o simpleC

You also need to build LAMMPS as a shared library
(see examples/COUPLE/README), e.g. 

cd $HOME/lammps/src
make mode=shlib mpi

or 

cd $HOME/lammps
mkdir build-shared
cd build-shared
cmake -D BUILD_LIB=on -D BUILD_SHARED_LIBS=on ../cmake
make

You then run simpleC on a parallel machine
on some number of processors Q with 3 arguments:

% mpirun -np Q simpleC P in.lj $HOME/lammps/src/liblammps.so

or

% mpirun -np Q simpleC P in.lj $HOME/lammps/build-shared/liblammps.so

P is the number of procs you want LAMMPS to run on (must be <= Q) and
in.lj is a LAMMPS input script and the last argument is the path to
the LAMMPS shared library. This either has to be an absolute path, or
liblammps.so has to be in a folder that is included in the environment
variable LD_LIBRARY_PATH so it will be found by the dynamic object loader.

The driver will launch LAMMPS on P procs, read the input script a line
at a time, and pass each command line to LAMMPS.  The final line of
the script is a "run" command, so LAMMPS will run the problem.

The driver then requests all the atom coordinates from LAMMPS, moves
one of the atoms a small amount "epsilon", passes the coordinates back
to LAMMPS, and runs LAMMPS again.  If you look at the output, you
should see a small energy change between runs, due to the moved atom.

The C driver is calling C-style routines in the src/library.cpp file
of LAMMPS through the function pointers in the liblammpsplugin_t struct.
This has the benefit that your binary is not linked to liblammps.so directly
and thus you can change the name of the shared library (e.g. to have 
different variants compiled, or to load a different LAMMPS versions without
having to update your executable). The shared library still has to be
compatible with the compilation settings the plugin code.