3.3. Link LAMMPS as a library to another code

LAMMPS is designed as a library of C++ objects that can be integrated into other applications including Python scripts. The files src/library.cpp and src/library.h define a C-style API for using LAMMPS as a library. See the Library interface to LAMMPS page for a description of the interface and how to use it for your needs.

The Basic build options page explains how to build LAMMPS as either a shared or static library. This results in a file in the compilation folder called liblammps.a or liblammps_<name>.a in case of building a static library. In case of a shared library the name is the same only that the suffix is going to be either .so or .dylib or .dll instead of .a depending on the OS. In some cases the .so file may be a symbolic link to a file with the suffix .so.0 (or some other number).

Note

Care should be taken to use the same MPI library for the calling code and the LAMMPS library. The library.h file includes mpi.h and uses definitions from it so those need to be available and consistent. When LAMMPS is compiled with the included STUBS MPI library, then its mpi.h file needs to be included. While it is technically possible to use a full MPI library in the calling code and link to a serial LAMMPS library compiled with MPI STUBS, it is recommended to use the same MPI library for both, and then use MPI_Comm_split() in the calling code to pass a suitable communicator with a subset of MPI ranks to the function creating the LAMMPS instance.

3.3.1. Link with LAMMPS as a static library

The calling application can link to LAMMPS as a static library with compilation and link commands as in the examples shown below. These are examples for a code written in C in the file caller.c. The benefit of linking to a static library is, that the resulting executable is independent of that library since all required executable code from the library is copied into the calling executable.

CMake buildTraditional make

This assumes that LAMMPS has been configured without setting a LAMMPS_MACHINE name, installed with “make install”, and the PKG_CONFIG_PATH environment variable has been updated to include the liblammps.pc file installed into the configured destination folder. The commands to compile and link a coupled executable are then:

mpicc -c -O $(pkgconf liblammps --cflags) caller.c
mpicxx -o caller caller.o -$(pkgconf liblammps --libs)

However, it is only as simple as shown above for the case of a plain LAMMPS library without any optional packages that depend on libraries (bundled or external) or when using a shared library. Otherwise, you need to include all flags, libraries, and paths for the coupled executable, that are also required to link the LAMMPS executable.

CMake buildTraditional make

When using CMake, additional libraries with sources in the lib folder are built, but not included in liblammps.a and (currently) not installed with make install and not included in the pkgconfig configuration file. They can be found in the top level build folder, but you have to determine the necessary link flags manually. It is therefore recommended to either use the traditional make procedure to build and link with a static library or build and link with a shared library instead.

3.3.2. Link with LAMMPS as a shared library

When linking to LAMMPS built as a shared library, the situation becomes much simpler, as all dependent libraries and objects are either included in the shared library or registered as a dependent library in the shared library file. Thus those libraries need not to be specified when linking the calling executable. Only the -I flags are needed. So the example case from above of the serial version static LAMMPS library with the POEMS package installed becomes:

CMake buildTraditional make

The commands with a shared LAMMPS library compiled with the CMake build process are the same as for the static library.

mpicc -c -O $(pkgconf liblammps --cflags) caller.c
mpicxx -o caller caller.o -$(pkgconf --libs)

Locating liblammps.so at runtime

Unlike with a static link, now the liblammps.so file is required at runtime and needs to be in a folder, where the shared linker program of the operating system can find it. This would be either a folder like /usr/local/lib64 or ${HOME}/.local/lib64 or a folder pointed to by the LD_LIBRARY_PATH environment variable. You can type

printenv LD_LIBRARY_PATH

to see what directories are in that list.

Or you can add the LAMMPS src directory or the directory you performed a CMake style build in to your LD_LIBRARY_PATH environment variable, so that the current version of the shared library is always available to programs that use it.

For the Bourne or Korn shells (/bin/sh, /bin/ksh, /bin/bash etc.), you would add something like this to your ${HOME}/.profile file:

LD_LIBRARY_PATH ${LD_LIBRARY_PATH-/usr/lib64}:${HOME}/lammps/src
export LD_LIBRARY_PATH

For the csh or tcsh shells, you would equivalently add something like this to your ${HOME}/.cshrc file:

setenv LD_LIBRARY_PATH ${LD_LIBRARY_PATH}:${HOME}/lammps/src

You can verify whether all required shared libraries are found with the ldd tool. Example:

$ LD_LIBRARY_PATH=/home/user/lammps/src ldd caller
     linux-vdso.so.1 (0x00007ffe729e0000)
     liblammps.so => /home/user/lammps/src/liblammps.so (0x00007fc91bb9e000)
     libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007fc91b984000)
     libm.so.6 => /lib64/libm.so.6 (0x00007fc91b83e000)
     libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fc91b824000)
     libc.so.6 => /lib64/libc.so.6 (0x00007fc91b65b000)
     /lib64/ld-linux-x86-64.so.2 (0x00007fc91c094000)

If a required library is missing, you would get a ‘not found’ entry:

$  ldd caller
     linux-vdso.so.1 (0x00007ffd672fe000)
     liblammps.so => not found
     libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fb7c7e86000)
     libm.so.6 => /usr/lib64/libm.so.6 (0x00007fb7c7d40000)
     libgcc_s.so.1 => /usr/lib64/libgcc_s.so.1 (0x00007fb7c7d26000)
     libc.so.6 => /usr/lib64/libc.so.6 (0x00007fb7c7b5d000)
     /lib64/ld-linux-x86-64.so.2 (0x00007fb7c80a2000)