Compile C++ file with Python.h import using Bazel

Question

I want to compile a C++ file which uses Python embedding. Therefore I #include in my C++ source. When using g++ as compiler I would specify the following flags:

g++ -o pybridge pybridge.cc -I/usr/include/python2.7/ -lpython2.7

I want to use Bazel for compilation now and tried the following target rule:

cc_binary(
  name = "pybridge",
  srcs = ["pybridge.cc"],
  copts = ["-I/usr/include/python2.7/"],
  linkopts = ["-lpython2.7"]
)

Running bazel build gives error messages like this:

pybridge.cc:(.text+0x10): undefined reference to Py_Initialize

Answer 1

Bazel executes your build in a sandbox, to prevent it from accessing random resources on your system that won't be present on, say, your coworker's system.

This means that, unless you've declared a file (like the Python library) as a dependency, Bazel won't put it in the sandbox and your build won't be able to find it.

There are two options:

The easy one is to build with --spawn_strategy=standalone ( bazel build --spawn_strategy=standalone :pybridge ). This tells Bazel not to use sandboxing for this build. Note that, as far as Bazel knows, nothing has changed between the sandboxed and non-sandboxed run, so you'll have to clean before re-running without sandboxing, or you'll just get the cached error.

The harder option is to declare /usr/lib/libpython2.7.so as an input to your build. If you want to do that, add the following to the WORKSPACE file:

local_repository(
    name = "system_python",
    path = "/usr/lib/python2.7/config-x86_64-linux-gnu", # Figure out where it is on your system, this is where it is on mine
    build_file_content = """
cc_library(
   name = "my-python-lib",
   srcs = ["libpython2.7.so"],
   visibility = ["//visibility:public"],
)
""",
)

Then in your BUILD file, instead of linkopts = ["-lpython2.7"] , add deps = ["@system_python//:my-python-lib"] . Then Bazel will understand that your build depends on libpython2.7.so (and include it in the sandbox).

Answer 2

( Tried commenting on OP's post but I lack the required karma. )

FWIW, I've had problems linking against Python 2.7's libraries (on Windows), even when I didn't use Bazel but ran the linker manually, so this problem may be unrelated to Bazel.