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+Building Third-party Software
+=============================
+
+Third-party projects usually ship with their own build description,
+which often happens to be not compatible with justbuild. Nevertheless,
+it is highly desireable to include external projects via their source
+code base, instead of relying on the integration of out-of-band binary
+distributions. justbuild offers a flexible approach to provide the
+required build description via an overlay layer without the need to
+touch the original code base.
+
+For the remainder of this section, we expect to have the project files
+available resulting from successfully completing the tutorial section on
+*Building C++ Hello World*. We will demonstrate how to use the
+open-source project [fmtlib](https://github.com/fmtlib/fmt) as an
+example for integrating third-party software to a justbuild project.
+
+Creating the target overlay layer for fmtlib
+--------------------------------------------
+
+Before we construct the overlay layer for fmtlib, we need to determine
+its file structure ([tag
+8.1.1](https://github.com/fmtlib/fmt/tree/8.1.1)). The relevant header
+and source files are structured as follows:
+
+ fmt
+ |
+ +--include
+ | +--fmt
+ | +--*.h
+ |
+ +--src
+ +--format.cc
+ +--os.cc
+
+The public headers can be found in `include/fmt`, while the library's
+source files are located in `src`. For the overlay layer, the `TARGETS`
+files should be placed in a tree structure that resembles the original
+code base's structure. It is also good practice to provide a top-level
+`TARGETS` file, leading to the following structure for the overlay:
+
+ fmt-layer
+ |
+ +--TARGETS
+ +--include
+ | +--fmt
+ | +--TARGETS
+ |
+ +--src
+ +--TARGETS
+
+Let's create the overlay structure:
+
+``` sh
+$ mkdir -p fmt-layer/include/fmt
+$ mkdir -p fmt-layer/src
+```
+
+The directory `include/fmt` contains only header files. As we want all
+files in this directory to be included in the `"hdrs"` target, we can
+safely use the explicit `TREE` reference[^1], which collects, in a
+single artifact (describing a directory) *all* directory contents from
+`"."` of the workspace root. Note that the `TARGETS` file is only part
+of the overlay, and therefore will not be part of this tree.
+Furthermore, this tree should be staged to `"fmt"`, so that any consumer
+can include those headers via `<fmt/...>`. The resulting header
+directory target `"hdrs"` in `include/fmt/TARGETS` should be described
+as:
+
+``` {.jsonc srcname="fmt-layer/include/fmt/TARGETS"}
+{ "hdrs":
+ { "type": ["@", "rules", "data", "staged"]
+ , "srcs": [["TREE", null, "."]]
+ , "stage": ["fmt"]
+ }
+}
+```
+
+The actual library target is defined in the directory `src`. For the
+public headers, it refers to the previously created `"hdrs"` target via
+its fully-qualified target name (`["include/fmt", "hdrs"]`). Source
+files are the two local files `format.cc`, and `os.cc`. The final target
+description in `src/TARGETS` will look like this:
+
+``` {.jsonc srcname="fmt-layer/src/TARGETS"}
+{ "fmt":
+ { "type": ["@", "rules", "CC", "library"]
+ , "name": ["fmt"]
+ , "hdrs": [["include/fmt", "hdrs"]]
+ , "srcs": ["format.cc", "os.cc"]
+ }
+}
+```
+
+Finally, the top-level `TARGETS` file can be created. While it is
+technically not strictly required, it is considered good practice to
+*export* every target that may be used by another project. Exported
+targets are subject to high-level target caching, which allows to skip
+the analysis and traversal of entire subgraphs in the action graph.
+Therefore, we create an export target that exports the target
+`["src", "fmt"]`, with only the variables in the field
+`"flexible_config"` being propagated. The top-level `TARGETS` file
+contains the following content:
+
+``` {.jsonc srcname="fmt-layer/TARGETS"}
+{ "fmt":
+ { "type": "export"
+ , "target": ["src", "fmt"]
+ , "flexible_config": ["CXX", "CXXFLAGS", "ADD_CXXFLAGS", "AR", "ENV"]
+ }
+}
+```
+
+After adding the library to the multi-repository configuration (next
+step), the list of configuration variables a target, like `["src",
+"fmt"]`, actually depends on can be obtained using the `--dump-vars`
+option of the `analyse` subcommand. In this way, an informed decision
+can be taken when deciding which variables of the export target to make
+tunable for the consumer.
+
+Adding fmtlib to the Multi-Repository Configuration
+---------------------------------------------------
+
+Based on the *hello world* tutorial, we can extend the existing
+`repos.json` by the layer definition `"fmt-targets-layer"` and the
+repository `"fmtlib"`, which is based on the Git repository with its
+target root being overlayed. Furthermore, we want to use `"fmtlib"` in
+the repository `"tutorial"`, and therefore need to introduce an
+additional binding `"format"` for it:
+
+``` {.jsonc srcname="repos.json"}
+{ "main": "tutorial"
+, "repositories":
+ { "rules-cc":
+ { "repository":
+ { "type": "git"
+ , "branch": "master"
+ , "commit": "123d8b03bf2440052626151c14c54abce2726e6f"
+ , "repository": "https://github.com/just-buildsystem/rules-cc.git"
+ , "subdir": "rules"
+ }
+ , "target_root": "tutorial-defaults"
+ , "rule_root": "rules-cc"
+ }
+ , "tutorial":
+ { "repository": {"type": "file", "path": "."}
+ , "bindings": {"rules": "rules-cc", "format": "fmtlib"}
+ }
+ , "tutorial-defaults":
+ { "repository": {"type": "file", "path": "./tutorial-defaults"}
+ }
+ , "fmt-targets-layer":
+ { "repository": {"type": "file", "path": "./fmt-layer"}
+ }
+ , "fmtlib":
+ { "repository":
+ { "type": "git"
+ , "branch": "8.1.1"
+ , "commit": "b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9"
+ , "repository": "https://github.com/fmtlib/fmt.git"
+ }
+ , "target_root": "fmt-targets-layer"
+ , "bindings": {"rules": "rules-cc"}
+ }
+ }
+}
+```
+
+This `"format"` binding can you be used to add a new private dependency
+in `greet/TARGETS`:
+
+``` {.jsonc srcname="greet/TARGETS"}
+{ "greet":
+ { "type": ["@", "rules", "CC", "library"]
+ , "name": ["greet"]
+ , "hdrs": ["greet.hpp"]
+ , "srcs": ["greet.cpp"]
+ , "stage": ["greet"]
+ , "private-deps": [["@", "format", "", "fmt"]]
+ }
+}
+```
+
+Consequently, the `fmtlib` library can now be used by `greet/greet.cpp`:
+
+``` {.cpp srcname="greet/greet.cpp"}
+#include "greet.hpp"
+#include <fmt/format.h>
+
+void greet(std::string const& s) {
+ fmt::print("Hello {}!\n", s);
+}
+```
+
+Due to changes made to `repos.json`, building this tutorial requires to
+rerun `just-mr`, which will fetch the necessary sources for the external
+repositories:
+
+``` sh
+$ just-mr build helloworld
+INFO: Requested target is [["@","tutorial","","helloworld"],{}]
+INFO: Analysed target [["@","tutorial","","helloworld"],{}]
+INFO: Export targets found: 0 cached, 0 uncached, 1 not eligible for caching
+INFO: Discovered 7 actions, 3 trees, 0 blobs
+INFO: Building [["@","tutorial","","helloworld"],{}].
+INFO: Processed 7 actions, 1 cache hits.
+INFO: Artifacts built, logical paths are:
+ helloworld [0ec4e36cfb5f2c3efa0fff789349a46694a6d303:132736:x]
+$
+```
+
+Note to build the `fmt` target alone, its containing repository `fmtlib`
+must be specified via the `--main` option:
+
+``` sh
+$ just-mr --main fmtlib build fmt
+INFO: Requested target is [["@","fmtlib","","fmt"],{}]
+INFO: Analysed target [["@","fmtlib","","fmt"],{}]
+INFO: Export targets found: 0 cached, 0 uncached, 1 not eligible for caching
+INFO: Discovered 3 actions, 1 trees, 0 blobs
+INFO: Building [["@","fmtlib","","fmt"],{}].
+INFO: Processed 3 actions, 3 cache hits.
+INFO: Artifacts built, logical paths are:
+ libfmt.a [513b2ac17c557675fc841f3ebf279003ff5a73ae:240914:f]
+ (1 runfiles omitted.)
+$
+```
+
+Employing high-level target caching
+-----------------------------------
+
+The make use of high-level target caching for exported targets, we need
+to ensure that all inputs to an export target are transitively
+content-fixed. This is automatically the case for `"type":"git"`
+repositories. However, the `libfmt` repository also depends on
+`"rules-cc"`, `"tutorial-defaults"`, and `"fmt-target-layer"`. As the
+latter two are `"type":"file"` repositories, they must be put under Git
+versioning first:
+
+``` sh
+$ git init .
+$ git add tutorial-defaults fmt-layer
+$ git commit -m"fix compile flags and fmt targets layer"
+```
+
+Note that `rules-cc` already is under Git versioning.
+
+Now, to instruct `just-mr` to use the content-fixed, committed source
+trees of those `"type":"file"` repositories the pragma `"to_git"` must
+be set for them in `repos.json`:
+
+``` {.jsonc srcname="repos.json"}
+{ "main": "tutorial"
+, "repositories":
+ { "rules-cc":
+ { "repository":
+ { "type": "git"
+ , "branch": "master"
+ , "commit": "123d8b03bf2440052626151c14c54abce2726e6f"
+ , "repository": "https://github.com/just-buildsystem/rules-cc.git"
+ , "subdir": "rules"
+ }
+ , "target_root": "tutorial-defaults"
+ , "rule_root": "rules-cc"
+ }
+ , "tutorial":
+ { "repository": {"type": "file", "path": "."}
+ , "bindings": {"rules": "rules-cc", "format": "fmtlib"}
+ }
+ , "tutorial-defaults":
+ { "repository":
+ { "type": "file"
+ , "path": "./tutorial-defaults"
+ , "pragma": {"to_git": true}
+ }
+ }
+ , "fmt-targets-layer":
+ { "repository":
+ { "type": "file"
+ , "path": "./fmt-layer"
+ , "pragma": {"to_git": true}
+ }
+ }
+ , "fmtlib":
+ { "repository":
+ { "type": "git"
+ , "branch": "master"
+ , "commit": "b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9"
+ , "repository": "https://github.com/fmtlib/fmt.git"
+ }
+ , "target_root": "fmt-targets-layer"
+ , "bindings": {"rules": "rules-cc"}
+ }
+ }
+}
+```
+
+Due to changes in the repository configuration, we need to rebuild and
+the benefits of the target cache should be visible on the second build:
+
+``` sh
+$ just-mr build helloworld
+INFO: Requested target is [["@","tutorial","","helloworld"],{}]
+INFO: Analysed target [["@","tutorial","","helloworld"],{}]
+INFO: Export targets found: 0 cached, 1 uncached, 0 not eligible for caching
+INFO: Discovered 7 actions, 3 trees, 0 blobs
+INFO: Building [["@","tutorial","","helloworld"],{}].
+INFO: Processed 7 actions, 7 cache hits.
+INFO: Artifacts built, logical paths are:
+ helloworld [0ec4e36cfb5f2c3efa0fff789349a46694a6d303:132736:x]
+$
+$ just-mr build helloworld
+INFO: Requested target is [["@","tutorial","","helloworld"],{}]
+INFO: Analysed target [["@","tutorial","","helloworld"],{}]
+INFO: Export targets found: 1 cached, 0 uncached, 0 not eligible for caching
+INFO: Discovered 4 actions, 2 trees, 0 blobs
+INFO: Building [["@","tutorial","","helloworld"],{}].
+INFO: Processed 4 actions, 4 cache hits.
+INFO: Artifacts built, logical paths are:
+ helloworld [0ec4e36cfb5f2c3efa0fff789349a46694a6d303:132736:x]
+$
+```
+
+Note that in the second run the export target `"fmt"` was taken from
+cache and its 3 actions were eliminated, as their result has been
+recorded to the high-level target cache during the first run.
+
+Combining overlay layers for multiple projects
+----------------------------------------------
+
+Projects typically depend on multiple external repositories. Creating an
+overlay layer for each external repository might unnecessarily clutter
+up the repository configuration and the file structure of your
+repository. One solution to mitigate this issue is to combine the
+`TARGETS` files of multiple external repositories in a single overlay
+layer. To avoid conflicts, the `TARGETS` files can be assigned different
+file names per repository. As an example, imagine a common overlay layer
+with the files `TARGETS.fmt` and `TARGETS.gsl` for the repositories
+`"fmtlib"` and `"gsl-lite"`, respectively:
+
+ common-layer
+ |
+ +--TARGETS.fmt
+ +--TARGETS.gsl
+ +--include
+ | +--fmt
+ | | +--TARGETS.fmt
+ | +--gsl
+ | +--TARGETS.gsl
+ |
+ +--src
+ +--TARGETS.fmt
+
+Such a common overlay layer can be used as the target root for both
+repositories with only one difference: the `"target_file_name"` field.
+By specifying this field, the dispatch where to find the respective
+target description for each repository is implemented. For the given
+example, the following `repos.json` defines the overlay
+`"common-targets-layer"`, which is used by `"fmtlib"` and `"gsl-lite"`:
+
+``` {.jsonc srcname="repos.json"}
+{ "main": "tutorial"
+, "repositories":
+ { "rules-cc":
+ { "repository":
+ { "type": "git"
+ , "branch": "master"
+ , "commit": "123d8b03bf2440052626151c14c54abce2726e6f"
+ , "repository": "https://github.com/just-buildsystem/rules-cc.git"
+ , "subdir": "rules"
+ }
+ , "target_root": "tutorial-defaults"
+ , "rule_root": "rules-cc"
+ }
+ , "tutorial":
+ { "repository": {"type": "file", "path": "."}
+ , "bindings": {"rules": "rules-cc", "format": "fmtlib"}
+ }
+ , "tutorial-defaults":
+ { "repository":
+ { "type": "file"
+ , "path": "./tutorial-defaults"
+ , "pragma": {"to_git": true}
+ }
+ }
+ , "common-targets-layer":
+ { "repository":
+ { "type": "file"
+ , "path": "./common-layer"
+ , "pragma": {"to_git": true}
+ }
+ }
+ , "fmtlib":
+ { "repository":
+ { "type": "git"
+ , "branch": "8.1.1"
+ , "commit": "b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9"
+ , "repository": "https://github.com/fmtlib/fmt.git"
+ }
+ , "target_root": "common-targets-layer"
+ , "target_file_name": "TARGETS.fmt"
+ , "bindings": {"rules": "rules-cc"}
+ }
+ , "gsl-lite":
+ { "repository":
+ { "type": "git"
+ , "branch": "v0.40.0"
+ , "commit": "d6c8af99a1d95b3db36f26b4f22dc3bad89952de"
+ , "repository": "https://github.com/gsl-lite/gsl-lite.git"
+ }
+ , "target_root": "common-targets-layer"
+ , "target_file_name": "TARGETS.gsl"
+ , "bindings": {"rules": "rules-cc"}
+ }
+ }
+}
+```
+
+Using pre-built dependencies
+----------------------------
+
+While building external dependencies from source brings advantages, most
+prominently the flexibility to quickly and seamlessly switch to a
+different build configuration (production, debug, instrumented for
+performance analysis; cross-compiling for a different target
+architecture), there are also legitimate reasons to use pre-built
+dependencies. The most prominent one is if your project is packaged as
+part of a larger distribution. For that reason, just also has (in
+`etc/import.prebuilt`) target files for all its dependencies assuming
+they are pre-installed. The reason why target files are used at all for
+this situation is twofold.
+
+ - On the one hand, having a target allows the remaining targets to not
+ care about where their dependencies come from, or if it is a build
+ against pre-installed dependencies or not. Also, the top-level
+ binary does not have to know the linking requirements of its
+ transitive dependencies. In other words, information stays where it
+ belongs to and if one target acquires a new dependency, the
+ information is automatically propagated to all targets using it.
+ - Still some information is needed to use a pre-installed library and,
+ as explained, a target describing the pre-installed library is the
+ right place to collect this information.
+ - The public header files of the library. By having this explicit,
+ we do not accumulate directories in the include search path and
+ hence also properly detect include conflicts.
+ - The information on how to link the library itself (i.e.,
+ basically its base name).
+ - Any dependencies on other libraries that the library might have.
+ This information is used to obtain the correct linking order and
+ complete transitive linking arguments while keeping the
+ description maintainable, as each target still only declares its
+ direct dependencies.
+
+The target description for a pre-built version of the format library
+that was used as an example in this section is shown next; with our
+staging mechanism the logical repository it belongs to is rooted in the
+`fmt` subdirectory of the `include` directory of the ambient system.
+
+``` {.jsonc srcname="etc/import.prebuilt/TARGETS.fmt"}
+{ "fmt":
+ { "type": ["@", "rules", "CC", "library"]
+ , "name": ["fmt"]
+ , "stage": ["fmt"]
+ , "hdrs": [["TREE", null, "."]]
+ , "private-ldflags": ["-lfmt"]
+ }
+}
+```
+
+[^1]: Explicit `TREE` references are always a list of length 3, to
+ distinguish them from target references of length 2 (module and
+ target name). Furthermore, the second list element is always `null`
+ as we only want to allow tree references from the current module.
generated by cgit v1.2.3 (git 2.39.1) at 2025-07-02 06:45:18 +0000