How to build on different operating systems?

.build_template:
  stage: build
  script:
    - gmake -C src -j11
  artifacts:
    name: "executable-$CI_JOB_NAME-$CI_COMMIT_SHORT_SHA"
    paths:
      - src/executable
    expire_in: 14 days

build:rhel7:
  extends: .build_template
  tags: [ "C++", "Boost", "rhel7" ]

build:rhel8:
  extends: .build_template
  tags: [ "C++", "Boost", "rhel8" ]

build:freebsd14:
  extends: .build_template
  tags: [ "C++", "Boost", "freebsd14" ]

.variables_base:
  variables:
    BUILD_TARGET: executable

.build_template:
  stage: build
  script:
    - gmake -C src -j11
  artifacts:
    name: "${BUILD_TARGET}-${CI_JOB_NAME}-${CI_COMMIT_SHORT_SHA}"
    paths: [ "src/executable" ]

.build_rhel7:
  extends: [ .variables_base, .build_template ]
  tags: [ "C++", "Boost", "rhel7" ]

.build_rhel8:
  extends: [ .variables_base, .build_template ]
  tags: [ "C++", "Boost", "rhel8" ]

.build_freebsd14:
  extends: [ .variables_base, .build_template ]
  tags: [ "C++", "Boost", "freebsd14" ]

You need to modify it most likely as I cant test it for obvious resons. But it should (more less) work

To keep your pipelines nice and DRY this is a perfect time to use gitlabs parallel.matrix:

executable:
  parallel:
    matrix:
     - PLATFORM: [ rhel7, rhel8, freebsd14 ] 
  stage: build
  tags:
    - C++
    - Boost
    - $PLATFORM
  script:
    - gmake -C src -j11
  artifacts:
    name: the executable
    paths:
    - src/executable

To explain: "parallel.matrix" creates named job variants for each combination of the supplied variables so in this case it will generate a pipeline largely equivalent to this ci.yaml:

"executable: [rhel7]":
  variables:
    PLATFORM: rhel7
  ...

"executable: [rhel8]":
  variables:
    PLATFORM: rhel8
  ...
  
"executable: [freebsd14]":
  variables:
    PLATFORM: freebsd14
  stage: build
  tags:
   ...

If you need the artifacts to be unique and gmake doesn't automatically include things like the platform in the name, the variable is injected for your use.

So, each platform has to be explicitly enumerated, eh? I was hoping for builds to automatically happen on all runners of all platforms, that have the "C++" and "Boost" tags...

So that, for example, if someone stands up a freebsd15 and an ubuntu24 runners in the future, I wouldn't need to modify the .gitlab-ci.yml...