Make _C extension a thin C wrapper (pytorch#39375)

Summary:
It just depends on a single `torch_python` library.
C library does not depend on standard C++ library and as result it closes pytorch#36941
Pull Request resolved: pytorch#39375

Reviewed By: orionr

Differential Revision: D21840645

Pulled By: malfet

fbshipit-source-id: 777c189feee9d6fc686816d92cb9f109b8aac7ca

Author: malfet

BUILD.bazel

@@ -1961,7 +1961,7 @@ cc_library(
 
 pybind_extension(
     name = "_C",
-    srcs = ["torch/csrc/stub.cpp"],
+    srcs = ["torch/csrc/stub.c"],
     deps = [
         ":torch_python"
     ],

setup.py

@@ -579,7 +579,6 @@ def configure_extension_build():
     else:
         extra_link_args = []
         extra_compile_args = [
-            '-std=c++14',
             '-Wall',
             '-Wextra',
             '-Wno-strict-overflow',
@@ -604,9 +603,9 @@ def configure_extension_build():
     library_dirs.append(lib_path)
 
     main_compile_args = []
-    main_libraries = ['shm', 'torch_python']
+    main_libraries = ['torch_python']
     main_link_args = []
-    main_sources = ["torch/csrc/stub.cpp"]
+    main_sources = ["torch/csrc/stub.c"]
 
     if cmake_cache_vars['USE_CUDA']:
         library_dirs.append(
@@ -647,7 +646,7 @@ def make_relative_rpath(path):
     C = Extension("torch._C",
                   libraries=main_libraries,
                   sources=main_sources,
-                  language='c++',
+                  language='c',
                   extra_compile_args=main_compile_args + extra_compile_args,
                   include_dirs=[],
                   library_dirs=library_dirs,
@@ -661,7 +660,7 @@ def make_relative_rpath(path):
         extensions.append(DL)
 
     # These extensions are built by cmake and copied manually in build_extensions()
-    # inside the build_ext implementaiton
+    # inside the build_ext implementation
     extensions.append(
         Extension(
             name=str('caffe2.python.caffe2_pybind11_state'),

torch/csrc/Module.cpp

@@ -621,6 +621,7 @@ static void LogAPIUsageOnceFromPython(const std::string& event) {
   }
 }
 
+extern "C"
 #ifdef _WIN32
 __declspec(dllexport)
 #endif

torch/csrc/stub.c

@@ -0,0 +1,18 @@
+#include <Python.h>
+
+#ifdef _WIN32
+__declspec(dllimport)
+#endif
+extern PyObject* initModule(void);
+
+#ifndef _WIN32
+#ifdef __cplusplus
+extern "C"
+#endif
+__attribute__((visibility("default"))) PyObject* PyInit__C(void);
+#endif
+
+PyMODINIT_FUNC PyInit__C(void)
+{
+  return initModule();
+}