bazel

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Translated

Bazel build system skill for C/C++ projects. Use when writing BUILD files with cc_library and cc_binary rules, registering toolchains, configuring remote execution, debugging sandbox issues, using query and cquery for dependency graphs, or migrating from CMake to Bazel. Activates on queries about Bazel, BUILD files, cc_library, cc_binary, Bzlmod, bazel query, remote execution, or Bazel toolchain registration.

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npx skill4agent add mohitmishra786/low-level-dev-skills bazel

Tags

Translated version includes tags in frontmatter
bazel-buildc-cpp-developmentdependency-managementtoolchain-registrationremote-execution

SKILL.md Content

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Bazel

Purpose

Guide agents through Bazel for C/C++ projects: writing BUILD files, cc_library/cc_binary rules, toolchain registration, remote execution, dependency graph queries, Bzlmod dependency management, and sandbox debugging.

Triggers

  • "How do I write a Bazel BUILD file for a C++ library?"
  • "How do I add external dependencies with Bzlmod?"
  • "How do I debug Bazel sandbox errors?"
  • "How do I query the Bazel dependency graph?"
  • "How do I set up remote execution with Bazel?"
  • "How do I register a custom C++ toolchain?"

Workflow

1. Workspace structure

my-project/
├── MODULE.bazel         # Bzlmod dependency file (Bazel ≥6)
├── WORKSPACE            # legacy (still needed for some features)
├── BUILD                # root build file
├── src/
│   ├── BUILD
│   └── main.cc
└── lib/
    ├── BUILD
    ├── mylib.cc
    └── mylib.h

2. Basic BUILD file — cc_library / cc_binary

python
# lib/BUILD
cc_library(
    name = "mylib",
    srcs = ["mylib.cc"],
    hdrs = ["mylib.h"],
    copts = ["-Wall", "-Wextra", "-std=c++17"],
    visibility = ["//visibility:public"],
    deps = [
        "@com_google_absl//absl/strings",     # external dep
        "//util:helpers",                      # internal dep
    ],
)

cc_test(
    name = "mylib_test",
    srcs = ["mylib_test.cc"],
    deps = [
        ":mylib",
        "@com_google_googletest//:gtest_main",
    ],
)
python
# src/BUILD
cc_binary(
    name = "main",
    srcs = ["main.cc"],
    deps = ["//lib:mylib"],
    linkopts = ["-lpthread"],
)
bash
# Build
bazel build //src:main
bazel build //...           # build everything

# Test
bazel test //lib:mylib_test
bazel test //...            # run all tests

# Run
bazel run //src:main -- arg1 arg2

# Output path
bazel-bin/src/main

3. Bzlmod — modern dependency management

python
# MODULE.bazel
module(
    name = "my_project",
    version = "1.0",
)

bazel_dep(name = "abseil-cpp", version = "20240116.2")
bazel_dep(name = "googletest", version = "1.14.0")
bazel_dep(name = "rules_cc", version = "0.0.9")
bazel_dep(name = "platforms", version = "0.0.8")

# For http_archive deps not yet in BCR (Bazel Central Registry)
# Use module extensions
bash
# Check available versions
bazel mod graph | head -30

# Show dependency tree
bazel mod deps --depth=2

4. Dependency graph queries

bash
# Show all dependencies of a target
bazel query "deps(//src:main)"

# Find reverse dependencies (who depends on this?)
bazel query "rdeps(//..., //lib:mylib)"

# Find what changed between builds
bazel query "filter('//lib', deps(//src:main))"

# cquery — configuration-aware query
bazel cquery "deps(//src:main)" --output=files

# Show include paths for a target
bazel cquery "//lib:mylib" --output=build

# Find cycles in the build graph
bazel query --nohost_deps --noimplicit_deps \
  "somepath(//src:main, //lib:mylib)"

# aquery — action graph (shows compiler flags, inputs, outputs)
bazel aquery "//src:main"
bazel aquery "//src:main" --output=jsonproto | jq '.actions[0].arguments'

5. Toolchain registration

python
# platforms/BUILD
platform(
    name = "linux_x86_64",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
)

# toolchains/BUILD
cc_toolchain_suite(
    name = "my_toolchain",
    toolchains = {
        "k8": ":my_k8_toolchain",
    },
)

cc_toolchain(
    name = "my_k8_toolchain",
    all_files = ":empty",
    compiler_files = ":compiler_wrapper",
    # ... other file groups
)

toolchain(
    name = "my_cc_toolchain",
    toolchain = ":my_k8_toolchain",
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
    target_compatible_with = ["@platforms//os:linux"],
)
python
# MODULE.bazel — register toolchain
register_toolchains("//toolchains:my_cc_toolchain")

6. Remote execution

bash
# Use --remote_executor flag
bazel build //... \
  --remote_executor=grpc://buildgrid.example.com:50051 \
  --remote_instance_name=main

# Google Cloud Build Remote Execution
bazel build //... \
  --remote_executor=remotebuildexecution.googleapis.com \
  --remote_instance_name=projects/my-project/instances/default \
  --google_default_credentials

# Caching (without remote execution)
bazel build //... \
  --remote_cache=grpc://cache.example.com:9092

# Show what's cached
bazel build //... --remote_upload_local_results=true

7. Sandbox debugging

bash
# Show sandbox inputs and outputs
bazel build //src:main --sandbox_debug

# Run build with verbose sandboxing
bazel build //src:main --verbose_failures --sandbox_debug

# Disable sandbox entirely (for debugging)
bazel build //src:main --spawn_strategy=local

# Common sandbox errors
# "No such file or directory" → missing data dependency
#   Fix: add file to `data = [...]` attribute
# "Permission denied" → trying to write outside sandbox
#   Fix: use genrule output paths instead of hardcoded paths
# "FAILED: Build did NOT complete successfully" → check verbose output

# See the exact command Bazel ran
bazel build //src:main --subcommands
For Bazel C++ toolchain configuration, see references/bazel-cpp-toolchain.md.

Related skills

  • Use 
    skills/build-systems/cmake
    for CMake as an alternative build system
  • Use 
    skills/build-systems/build-acceleration
    for sccache integration with Bazel remote cache
  • Use 
    skills/compilers/gcc
    or 
    skills/compilers/clang
    for compiler flags used in Bazel 
    copts