# Library interface to LAMMPS

As described on the [Build basics](Build_basics) doc page, LAMMPS can be
built as a static or shared library, so that it can be called by another
code, used in a [coupled manner](Howto_couple) with other codes, or
driven through a [Python interface](Python_head).

At the core of LAMMPS is the `LAMMPS` class, which encapsulates the
state of the simulation program through the state of the various class
instances that it is composed of. So a calculation using LAMMPS requires
creating an instance of the `LAMMPS` class and then send it (text)
commands, either individually or from a file, or perform other
operations that modify the state stored inside that instance or drive
simulations. This is essentially what the `src/main.cpp` file does as
well for the standalone LAMMPS executable, reading commands either from
an input file or the standard input.

Creating a LAMMPS instance can be done by using C++ code directly or
through a C-style interface library to LAMMPS that is provided in the
files `src/library.cpp` and `src/library.h`. This [C language
API](lammps_c_api), can be used from C and C++, and is also the basis
for the [Python](Python_module) and [Fortran](Fortran) interfaces or the
[SWIG based wrappers](swig) included in the LAMMPS source code.

The `examples/COUPLE` and `python/examples` directories contain some
example programs written in C++, C, Fortran, and Python, which show how
a driver code can link to LAMMPS as a library, run LAMMPS on a subset of
processors (so the others are available to run some other code
concurrently), grab data from LAMMPS, change it, and send it back into
LAMMPS.

A detailed documentation of the available APIs and examples of how to
use them can be found in the [Programmer
Guide](programmer_documentation) section of this manual.