Hengjiu Kang

• Introduction
• Overview
• Sample Project
• Links

Introduction

Bazel is a building system designed by google, and is widely used as native building system in Google’s projects including Protobuf, glog and etc. However Bazel is optimized for Java-like languages and it emphasized on “packages” and “modules”, so at first glance Bazel is strange, even “weird” to me. Adding external library support becomes a “pin point” in the Bazel project, especially when most of the third party software supports CMake but not Bazel.

In this blog, I will how to add external non-bazel library into a Bazel project.

Overview

Bazel gives more weights on the package-based design philosophy, so the dependencies are in some logical directories not physical locations in the hard drive. “cc_library” is the function/command to make the library files into a usable library object in the bazel world.

If the library we wanted has bazel support (WORKSPACE file and BUILD files), we can directly add them as a library:

• Depending on other Bazel project

  local_repository(
    name = "coworkers_project",
    path = "/path/to/coworkers-project",
  )
  

• Then we can use this library in the BUILD file:

  cc_binary(
    name = "main",
    srcs = ["main.cc"],
    deps = ["@coworkers_project//:libcoworkers"]
        )
  

However the real project is when we want to import a non-bazel library, for example, libzmq in my case.

Sample Project

I created a sample project with the following directory structure:

the directory third_party/libzmq/zeromq-4.3.0 is extracted directory from libzmq release zeromq-4.3.0.gz.tar

Work Flow

We need to modify three files to add libzmq library:

Codes

./WORKSPACE

new_local_repository(
    name = "libzmq",
    path = "third_party/libzmq/zeromq-4.3.0",
    build_file = "third_party/libzmq/libzmq.BUILD",
    )

Using new_local_repository is intended here. It would be ideal to have .tar file in the ./third_party/libzmq directory, however then we need to use new_http_archive command, and this command does not support file:// URL very well.

build_file is necessary and important here. new_local_repository only creates reference to an external library, and this command requires some script to help compiling and get the library files we need.

./third_party/libzmq/libzmq.BUILD

licenses(["notice"])

package(default_visibility = ["//visibility:public"])

include_files = [
    "include/zmq.h",
    "include/zmq_utils.h",
]

lib_files = [
    "lib/libzmq.a",
]

genrule(
    name = "libzmq-srcs",
    outs = include_files + lib_files,
    cmd = "\n".join([
        'export INSTALL_DIR=$$(pwd)/$(@D)',
        'export TMP_DIR=$$(mktemp -d -t libzmq.XXXXX)',
        'mkdir -p $$TMP_DIR',
        'cp -R $$(pwd)/../../../../../external/libzmq/* $$TMP_DIR',
        'cd $$TMP_DIR',
        'mkdir build',
        'cd build',
        'cmake ../ -DCMAKE_INSTALL_PREFIX=$$INSTALL_DIR',
        'make install',
        'rm -rf $$TMP_DIR',
    ]),
)

cc_library(
    name = "libzmq",
    srcs = lib_files,
    hdrs = include_files,
    includes=["include"],
    linkstatic = 1,
)

include_files gives all the necessary .h files from libzmq. Notice that directories used here are relating to the root directory of install directory. I will explain this in minute.
lib_files gives all the necessary .a/.so files from libzmq. After compiling, different library files will be generated inclding .dylib files in my MacOS, but we only need libzmq.a here.
genrule has customized commands that essentially executes “cmd” content in bash/sh.

Links