# Building a C++ App in Jenkins (Docker Agent)

Step-by-step guide to set up a Jenkins pipeline that builds a C++ project
using a Docker container as the build agent (DooD).


## 1. Push your C++ project to the git server

On the server, create a bare repo:

```bash
cd /var/git/repos
git init --bare my-cpp-app.git
chown -R 1000:1000 my-cpp-app.git
```

From your local machine, push your code:

```bash
cd /path/to/my-cpp-app
git init
git remote add origin ssh://git@<server-ip>/repos/my-cpp-app.git
git add .
git commit -m "Initial commit"
git push -u origin master
```


## 2. Add a Jenkinsfile to your project

Create a file called `Jenkinsfile` in the root of your C++ project.

### Simple build (g++ directly)

```groovy
pipeline {
    agent {
        docker { image 'gcc:latest' }
    }

    stages {
        stage('Build') {
            steps {
                sh 'g++ -o my_app main.cpp'
            }
        }

        stage('Test') {
            steps {
                sh './my_app'
            }
        }
    }
}
```

Adjust the `g++` command to match your source files.

### CMake build

```groovy
pipeline {
    agent {
        docker { image 'gcc:latest' }
    }

    stages {
        stage('Configure') {
            steps {
                sh 'cmake -B build -S .'
            }
        }

        stage('Build') {
            steps {
                sh 'cmake --build build'
            }
        }

        stage('Test') {
            steps {
                sh 'cd build && ctest --output-on-failure'
            }
        }
    }
}
```

> Note: The `gcc:latest` image includes `g++`, `gcc`, `make`, and `cmake`.
> If you need extra libraries (e.g., Boost), you can use a custom image or
> add `apt-get install` steps.

Commit and push the Jenkinsfile:

```bash
git add Jenkinsfile
git commit -m "Add Jenkinsfile for CI"
git push
```


## 3. Create the Jenkins pipeline job

1. Open Jenkins at `https://jenkins.swave.lol` (or `https://swave.lol/jenkins`)
2. Click **New Item** (top-left)
3. Enter a name, e.g. `my-cpp-app`
4. Select **Pipeline**, then click **OK**


## 4. Configure the pipeline source

On the job configuration page:

1. Scroll down to the **Pipeline** section
2. Change **Definition** from "Pipeline script" to **Pipeline script from SCM**
3. Set **SCM** to **Git**
4. In **Repository URL**, enter: `git://git-server/my-cpp-app.git`
   - This works because Jenkins and git-server are on the same `jenkins_network` (172.23.0.0/16)
   - `git-server` resolves to `172.23.0.3` via Docker DNS
   - The git daemon protocol (`git://`) on port 9418 requires no credentials
   - Alternative: `ssh://git@git-server/repos/my-cpp-app.git` (requires SSH key setup in Jenkins)
5. Set **Branch Specifier** to `*/master` (or `*/main`, whatever your default branch is)
6. **Script Path**: leave as `Jenkinsfile` (default)
7. Click **Save**

### Large repositories

For large repos (e.g., the Linux kernel), the default 10-minute fetch timeout
is too short. In the SCM config, click **Additional Behaviours** > **Advanced
clone behaviours** and set:

- **Fetch tags**: uncheck
- **Timeout (in minutes) for clone and fetch**: `120` (or more)
- **Shallow clone**: check
- **Shallow clone depth**: `1`

Shallow clone fetches only the latest commit, reducing clone size significantly.

### Automatic SCM checkout

When using **Pipeline script from SCM**, Jenkins automatically clones the
repository before running the Jenkinsfile (the "Declarative: Checkout SCM"
step). The workspace already contains the code, so you do **not** need a
`checkout` or `git clone` stage in your Jenkinsfile — adding one would clone
the repo a second time. Just use the files directly in your build stages:

```groovy
pipeline {
    agent {
        docker { image 'gcc:latest' }
    }

    stages {
        stage('Build') {
            steps {
                // Source code is already in the workspace from SCM checkout
                sh 'g++ -o my_app main.cpp'
            }
        }
    }
}
```


## 5. Run the build

1. On the job page, click **Build Now** (left sidebar)
2. A build number will appear under **Build History** — click it
3. Click **Console Output** to watch the build live

You will see Jenkins:
- Pull the `gcc:latest` Docker image (first time only)
- Spin up a temporary container on the host (DooD via `/var/run/docker.sock`)
- Clone your repo inside the container
- Run each stage (`Build`, `Test`)
- Destroy the container when done

**Important:** The build container is ephemeral — everything inside it
(including compiled binaries) is destroyed when the pipeline finishes.
To keep build artifacts, see the next section.


## 6. Saving build artifacts

By default the Docker build container is destroyed after the pipeline runs,
taking all compiled binaries with it. Use `archiveArtifacts` to copy files
into Jenkins' permanent storage before the container is removed.

### Simple build with artifacts

```groovy
pipeline {
    agent {
        docker { image 'gcc:latest' }
    }

    stages {
        stage('Build') {
            steps {
                sh 'g++ -o my_app main.cpp'
            }
        }

        stage('Test') {
            steps {
                sh './my_app'
            }
        }

        stage('Archive') {
            steps {
                archiveArtifacts artifacts: 'my_app', fingerprint: true
            }
        }
    }
}
```

### CMake build with artifacts

```groovy
pipeline {
    agent {
        docker { image 'gcc:latest' }
    }

    stages {
        stage('Configure') {
            steps {
                sh 'cmake -B build -S .'
            }
        }

        stage('Build') {
            steps {
                sh 'cmake --build build'
            }
        }

        stage('Test') {
            steps {
                sh 'cd build && ctest --output-on-failure'
            }
        }

        stage('Archive') {
            steps {
                archiveArtifacts artifacts: 'build/my_app', fingerprint: true
            }
        }
    }
}
```

Archived files are stored in `/var/jenkins_home/jobs/<job>/builds/<number>/archive/`
and can be downloaded from the build page in the Jenkins web UI under **Build Artifacts**.

You can use glob patterns to archive multiple files, e.g.:
- `archiveArtifacts artifacts: 'build/**/*.so'` — all shared libraries
- `archiveArtifacts artifacts: 'build/bin/*'` — everything in a bin directory


## 7. Automatic builds (optional)

### Option A — Poll SCM

1. Go to job config (**Configure**)
2. Under **Build Triggers**, check **Poll SCM**
3. Set schedule, e.g. `H/5 * * * *` (check every 5 minutes)
4. Click **Save**

### Option B — Git post-receive hook

Create a hook that notifies Jenkins immediately on push:

```bash
cat > /var/git/repos/my-cpp-app.git/hooks/post-receive << 'HOOK'
#!/bin/bash
curl -s "http://jenkins:8080/jenkins/git/notifyCommit?url=git://git-server/my-cpp-app.git" > /dev/null 2>&1 &
HOOK
chmod +x /var/git/repos/my-cpp-app.git/hooks/post-receive
```

This requires the **Git plugin** and **Poll SCM** to be enabled in the job
config (the schedule can be empty, e.g. just `H * * * *`).


## Incremental builds and cleaning the workspace

After the first successful build, Jenkins keeps the workspace directory
(`/var/jenkins_home/jobs/<job>/workspace/`) between builds. On subsequent
runs it does a `git fetch` + `git checkout` instead of a full clone, only
downloading new commits. This makes builds much faster, especially for
large repositories like the Linux kernel.

The same applies to Docker images: Jenkins pulls the image (e.g.,
`kernel-builder` or `gcc:latest`) once and caches it locally. Future builds
reuse the cached image. However, the Docker **container** is always created
fresh for each build and destroyed when done — only the workspace persists.

Sometimes you need a completely clean build (e.g., to rule out stale object
files causing issues). Pipeline jobs do not have a "Wipe Out Workspace"
button in the UI (only Freestyle jobs do). There are two ways to clean it:

### Option A — Delete from host

```bash
rm -rf /var/jenkins_home/jobs/<job-name>/workspace
```

The next build will do a fresh clone.

### Option B — Add cleanWs() to Jenkinsfile

Requires the **Workspace Cleanup** plugin. Add a `post` block to
automatically wipe the workspace after every build:

```groovy
pipeline {
    agent {
        docker { image 'gcc:latest' }
    }

    stages {
        stage('Build') {
            steps {
                sh 'g++ -o my_app main.cpp'
            }
        }
    }

    post {
        always {
            cleanWs()
        }
    }
}
```

Note: using `cleanWs()` means every build will do a full clone, which is
slow for large repos. Use it only when you actually need clean builds.


## How DooD works

Jenkins is configured with Docker-out-of-Docker: it mounts the host's
`/var/run/docker.sock` into the Jenkins container. When the pipeline runs
`docker { image 'gcc:latest' }`, Jenkins creates a **sibling container** on
the host (not nested), runs the build steps inside it, and removes it after.
The Jenkins container never needs compilers installed — everything runs in the
ephemeral `gcc` container.