libtorch (C++-only)
===================

The core of pytorch does not depend on Python. A
CMake-based build system compiles the C++ source code into a shared
object, libtorch.so.

Building libtorch using Python
------------------------------

You can use a python script/module located in tools package to build libtorch
::
   cd <pytorch_root>

   # Make a new folder to build in to avoid polluting the source directories
   mkdir build_libtorch && cd build_libtorch

   # You might need to export some required environment variables here.
   Normally setup.py sets good default env variables, but you'll have to do
   that manually.
   python ../tools/build_libtorch.py


Alternatively, you can call setup.py normally and then copy the built cpp libraries. This method may have side effects to your active Python installation.
::
   cd <pytorch_root>
   python setup.py build

   ls torch/lib/tmp_install # output is produced here
   ls torch/lib/tmp_install/lib/libtorch.so # of particular interest

To produce libtorch.a rather than libtorch.so, set the environment variable `BUILD_SHARED_LIBS=OFF`.

To use ninja rather than make, set `CMAKE_GENERATOR="-GNinja" CMAKE_INSTALL="ninja install"`.

Note that we are working on eliminating tools/build_pytorch_libs.sh in favor of a unified cmake build.

Building libtorch using CMake
--------------------------------------

You can build C++ libtorch.so directly with cmake.  For example, to build a Release version from the master branch and install it in the directory specified by CMAKE_INSTALL_PREFIX below, you can use
::
   git clone -b master --recurse-submodule https://github.com/pytorch/pytorch.git
   mkdir pytorch-build
   cd pytorch-build
   cmake -DBUILD_SHARED_LIBS:BOOL=ON -DCMAKE_BUILD_TYPE:STRING=Release -DPYTHON_EXECUTABLE:PATH=`which python3` -DCMAKE_INSTALL_PREFIX:PATH=../pytorch-install ../pytorch
   cmake --build . --target install

To use release branch v1.6.0, for example, replace ``master`` with ``v1.6.0``.  You will get errors if you do not have needed dependencies such as Python3's PyYAML package.