BazelToolchain

Important

Some of the features used in this section are still under development, while they are recommended and usable and we will try not to break them in future releases, some breaking changes might still happen if necessary to prepare for the Conan 2.0 release.

Available since: 1.37.0

BazelToolchain

The BazelToolchain is the toolchain generator for Bazel. It will generate a conan_bzl.rc file that contains a build configuration conan-config to inject all the parameters into the bazel build command.

The BazelToolchain generator can be used by name in conanfiles:

conanfile.py

class Pkg(ConanFile):
    generators = "BazelToolchain"

conanfile.txt

[generators]
BazelToolchain

And it can also be fully instantiated in the conanfile generate() method:

conanfile.py

from conan import ConanFile
from conan.tools.google import BazelToolchain

class App(ConanFile):
    settings = "os", "arch", "compiler", "build_type"

    def generate(self):
        tc = BazelToolchain(self)
        tc.generate()

After running conan install command, the BazelToolchain generates the conan_bzl.rc file that contains Bazel build parameters (it will depend on your current Conan settings and options from your default profile):

conan_bzl.rc

# Automatic bazelrc file created by Conan

build:conan-config --cxxopt=-std=gnu++17

build:conan-config --dynamic_mode=off
build:conan-config --compilation_mode=opt

The Bazel build helper will use that conan_bzl.rc file to perform a call using this configuration. The outcoming command will look like this bazel --bazelrc=/path/to/conan_bzl.rc build --config=conan-config <target>.

The toolchain supports the following methods and attributes:

constructor

def __init__(self, conanfile, namespace=None):

conanfile: the current recipe object. Always use self.
namespace: Deprecated since Conan 1.62. It only keeps backward compatibility.

Attributes

You can change some attributes before calling the generate() method if you want to change some of the precalculated values:

from conan import ConanFile
from conan.tools.google import BazelToolchain

class App(ConanFile):
    settings = "os", "arch", "compiler", "build_type"

    def generate(self):
        tc = BazelToolchain(self)
        tc.cxxopt.append("--my_flag")
        tc.generate()

These attributes are processed and passed as part of build:conan-config:

force_pic (defaulted to fpic if options.shared == False and options.fpic == True else None): Injected to the --force_pic parameter.
dynamic_mode (defaulted to fully if shared, else off): Injected to the --dynamic_mode parameter.
cppstd (defaulted to None if your settings does not have settings.compiler.cppstd
copt (defaulted to []): They will be part of the --copt parameter.
conlyopt (defaulted to []): They will be part of the --conlyopt parameter.
cxxopt (defaulted to []): They will be part of the --cxxopt parameter.
linkopt (defaulted to []): They will be part of the --linkopt parameter.
compilation_mode (defaulted to opt if settings.build_type == "Release", otherwise, if settings.build_type == "Debug", it’ll be dbg): Injected to the --compilation_mode parameter.
compiler (defaulted to None): Injected to the --compiler parameter.
cpu (defaulted to None): Injected to the --cpu parameter.
crosstool_top (defaulted to None): Injected to the --crosstool_top parameter.

conf

BazelToolchain is affected by these [conf] variables:

tools.build:cxxflags list of extra C++ flags that will be used by cxxopt.
tools.build:cflags list of extra of pure C flags that will be used by conlyopt.
tools.build:sharedlinkflags list of extra linker flags that will be used by linkopt.
tools.build:exelinkflags list of extra linker flags that will be used by linkopt.
tools.build:linker_scripts list of linker scripts, each of which will be prefixed with -T and added to linkopt.