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## Tutorial

This section will guide you though the most basic features of Boost.Build. We will start with the “Hello, world” example, learn how to use libraries, and finish with testing and installing features.

### Hello, world

The simplest project that Boost.Build can construct is stored in `example/hello/` directory. The project is described by a file called `Jamroot` that contains:

    exe hello : hello.cpp ;

Even with this simple setup, you can do some interesting things. First of all, just invoking b2 will build the `hello` executable by compiling and linking `hello.cpp` . By default, the debug variant is built. Now, to build the release variant of `hello`, invoke

    b2 release

Note that the debug and release variants are created in different directories, so you can switch between variants or even build multiple variants at once, without any unnecessary recompilation. Let us extend the example by adding another line to our project's `Jamroot`:

    exe hello2 : hello.cpp ;

Now let us build both the debug and release variants of our project again:

    b2 debug release

Note that two variants of `hello2` are linked. Since we have already built both variants of `hello`, hello.cpp will not be recompiled; instead the existing object files will just be linked into the corresponding variants of `hello2`. Now let us remove all the built products:

    b2 --clean debug release

It is also possible to build or clean specific targets. The following two commands, respectively, build or clean only the debug version of `hello2`.

    b2 hello2
    b2 --clean hello2

### Properties

To represent aspects of target configuration such as debug and release variants, or single- and multi-threaded builds portably, Boost.Build uses features with associated values. For example, the `debug-symbols` feature can have a value of `on` or `off`. A property is just a (feature, value) pair. When a user initiates a build, Boost.Build automatically translates the requested properties into appropriate command-line flags for invoking toolset components like compilers and linkers.

There are many built-in features that can be combined to produce arbitrary build configurations. The following command builds the project's `release` variant with inlining disabled and debug symbols enabled:

    b2 release inlining=off debug-symbols=on

Properties on the command-line are specified with the syntax:

    feature-name=feature-value

The `release` and `debug` that we have seen in b2 invocations are just a shorthand way to specify values of the `variant` feature. For example, the command above could also have been written this way:

    b2 variant=release inlining=off debug-symbols=on

`variant` is so commonly-used that it has been given special status as an implicit feature— Boost.Build will deduce its identity just from the name of one of its values.

A complete description of features can be found in the section called “Features and properties”.

#### Build Requests and Target Requirements

The set of properties specified on the command line constitutes a build request—a description of the desired properties for building the requested targets (or, if no targets were explicitly requested, the project in the current directory). The actual properties used for building targets are typically a combination of the build request and properties derived from the project's `Jamroot` (and its other Jamfiles, as described in the section called “Project Hierarchies”). For example, the locations of `#include`d header files are normally not specified on the command-line, but described in Jamfiles as target requirements and automatically combined with the build request for those targets. Multithread-enabled compilation is another example of a typical target requirement. The Jamfile fragment below illustrates how these requirements might be specified.

    exe hello
        : hello.cpp
        : <include>boost <threading>multi
        ;

When `hello` is built, the two requirements specified above will always be present. If the build request given on the b2 command-line explictly contradicts a target's requirements, the target requirements usually override (or, in the case of “free”” features like `<include>`, [13] augments) the build request.

| Tip
The value of the `<include>` feature is relative to the location of `Jamroot` where it is used.

#### Project Attributes

If we want the same requirements for our other target, `hello2`, we could simply duplicate them. However, as projects grow, that approach leads to a great deal of repeated boilerplate in Jamfiles. Fortunately, there's a better way. Each project can specify a set of attributes, including requirements:

    project
        : requirements <include>/home/ghost/Work/boost <threading>multi
        ;

    exe hello : hello.cpp ;
    exe hello2 : hello.cpp ;

The effect would be as if we specified the same requirement for both `hello` and `hello2`.

### Project Hierarchies

So far we have only considered examples with one project, with one user-written Boost.Jam file, `Jamroot`. A typical large codebase would be composed of many projects organized into a tree. The top of the tree is called the project root. Every subproject is defined by a file called `Jamfile` in a descendant directory of the project root. The parent project of a subproject is defined by the nearest `Jamfile` or `Jamroot` file in an ancestor directory. For example, in the following directory layout:

    top/
      |
      +-- Jamroot
      |
      +-- app/
      |    |
      |    +-- Jamfile
      |    `-- app.cpp
      |
      `-- util/
           |
           +-- foo/
           .    |
           .    +-- Jamfile
           .    `-- bar.cpp

the project root is `top/`. The projects in `top/app/` and `top/util/foo/` are immediate children of the root project.

| Note
When we refer to a “Jamfile,” set in normal type, we mean a file called either `Jamfile` or `Jamroot`. When we need to be more specific, the filename will be set as “`Jamfile`” or “`Jamroot`.”

Projects inherit all attributes (such as requirements) from their parents. Inherited requirements are combined with any requirements specified by the subproject. For example, if `top/Jamroot` has

    <include>/home/ghost/local

in its requirements, then all of its subprojects will have it in their requirements, too. Of course, any project can add include paths to those specified by its parents. [14] More details can be found in the section called “Projects”.

Invoking b2 without explicitly specifying any targets on the command line builds the project rooted in the current directory. Building a project does not automatically cause its subprojects to be built unless the parent project's Jamfile explicitly requests it. In our example, `top/Jamroot` might contain:

    build-project app ;

which would cause the project in `top/app/` to be built whenever the project in `top/` is built. However, targets in `top/util/foo/` will be built only if they are needed by targets in `top/` or `top/app/`.

### Dependent Targets

When building a target `X` that depends on first building another target `Y` (such as a library that must be linked with X), `Y` is called a dependency of `X` and `X` is termed a dependent of `Y`.

To get a feeling of target dependencies, let's continue the above example and see how `top/app/Jamfile` can use libraries from `top/util/foo`. If `top/util/foo/Jamfile` contains

    lib bar : bar.cpp ;

then to use this library in `top/app/Jamfile`, we can write:

    exe app : app.cpp ../util/foo//bar ;

While `app.cpp` refers to a regular source file, `../util/foo//bar` is a reference to another target: a library `bar` declared in the Jamfile at `../util/foo`.

| Tip
Some other build system have special syntax for listing dependent libraries, for example `LIBS` variable. In Boost.Build, you just add the library to the list of sources.

Suppose we build `app` with:

    b2 app optimization=full define=USE_ASM

Which properties will be used to build `foo`? The answer is that some features are propagated—Boost.Build attempts to use dependencies with the same value of propagated features. The `<optimization>` feature is propagated, so both `app` and `foo` will be compiled with full optimization. But `<define>` is not propagated: its value will be added as-is to the compiler flags for `a.cpp`, but won't affect `foo`.

Let's improve this project further. The library probably has some headers that must be used when compiling `app.cpp`. We could manually add the necessary `#include` paths to `app`'s requirements as values of the `<include>` feature, but then this work will be repeated for all programs that use `foo`. A better solution is to modify `util/foo/Jamfile` in this way:

    project
        : usage-requirements <include>.
        ;

    lib foo : foo.cpp ;

Usage requirements are applied not to the target being declared but to its dependants. In this case, `<include>.` will be applied to all targets that directly depend on `foo`.

Another improvement is using symbolic identifiers to refer to the library, as opposed to `Jamfile` location. In a large project, a library can be used by many targets, and if they all use `Jamfile` location, a change in directory organization entails much work. The solution is to use project ids—symbolic names not tied to directory layout. First, we need to assign a project id by adding this code to `Jamroot`:

    use-project /library-example/foo : util/foo ;

Second, we modify `app/Jamfile` to use the project id:

    exe app : app.cpp /library-example/foo//bar ;

The `/library-example/foo//bar` syntax is used to refer to the target `bar` in the project with id `/library-example/foo`. We've achieved our goal—if the library is moved to a different directory, only `Jamroot` must be modified. Note that project ids are global—two Jamfiles are not allowed to assign the same project id to different directories.

| Tip
If you want all applications in some project to link to a certain library, you can avoid having to specify it directly the sources of every target by using the `<library>` property. For example, if `/boost/filesystem//fs` should be linked to all applications in your project, you can add `<library>/boost/filesystem//fs` to the project's requirements, like this:
    project
       : requirements <library>/boost/filesystem//fs
       ;

### Static and shared libaries

Libraries can be either static, which means they are included in executable files that use them, or shared (a.k.a. dynamic), which are only referred to from executables, and must be available at run time. Boost.Build can create and use both kinds.

The kind of library produced from a `lib` target is determined by the value of the `link` feature. Default value is `shared`, and to build a static library, the value should be `static`. You can request a static build either on the command line:

    b2 link=static

or in the library's requirements:

    lib l : l.cpp : <link>static ;

We can also use the `<link>` property to express linking requirements on a per-target basis. For example, if a particular executable can be correctly built only with the static version of a library, we can qualify the executable's target reference to the library as follows:

    exe important : main.cpp helpers/<link>static ;

No matter what arguments are specified on the b2 command line, `important` will only be linked with the static version of `helpers`.

Specifying properties in target references is especially useful if you use a library defined in some other project (one you can't change) but you still want static (or dynamic) linking to that library in all cases. If that library is used by many targets, you could use target references everywhere:

    exe e1 : e1.cpp /other_project//bar/<link>static ;
    exe e10 : e10.cpp /other_project//bar/<link>static ;

but that's far from being convenient. A better approach is to introduce a level of indirection. Create a local alias target that refers to the static (or dynamic) version of `foo`:

    alias foo : /other_project//bar/<link>static ;
    exe e1 : e1.cpp foo ;
    exe e10 : e10.cpp foo ;

The alias rule is specifically used to rename a reference to a target and possibly change the properties.

| Tip
When one library uses another, you put the second library in the source list of the first. For example:
    lib utils : utils.cpp /boost/filesystem//fs ;
    lib core : core.cpp utils ;
    exe app : app.cpp core ;
This works no matter what kind of linking is used. When `core` is built as a shared library, it is linked directly into `utils`. Static libraries can't link to other libraries, so when `core` is built as a static library, its dependency on `utils` is passed along to `core`'s dependents, causing `app` to be linked with both `core` and `utils` .

| Note
(Note for non-UNIX system). Typically, shared libraries must be installed to a directory in the dynamic linker's search path. Otherwise, applications that use shared libraries can't be started. On Windows, the dynamic linker's search path is given by the `PATH` environment variable. This restriction is lifted when you use Boost.Build testing facilities—the `PATH` variable will be automatically adjusted before running the executable.

### Conditions and alternatives

Sometimes, particular relationships need to be maintained among a target's build properties. For example, you might want to set specific `#define` when a library is built as shared, or when a target's `release` variant is built. This can be achieved using conditional requirements.

    lib network : network.cpp
        : <link>shared:<define>NETWORK_LIB_SHARED
         <variant>release:<define>EXTRA_FAST
        ;

In the example above, whenever `network` is built with `<link>shared`, `<define>NETWORK_LIB_SHARED` will be in its properties, too. Also, whenever its release variant is built, `<define>EXTRA_FAST` will appear in its properties.

Sometimes the ways a target is built are so different that describing them using conditional requirements would be hard. For example, imagine that a library actually uses different source files depending on the toolset used to build it. We can express this situation using target alternatives:

    lib demangler : dummy_demangler.cpp ;                      # alternative 1
    lib demangler : demangler_gcc.cpp : <toolset>gcc ;   # alternative 2
    lib demangler : demangler_msvc.cpp : <toolset>msvc ; # alternative 3

When building `demangler`, Boost.Build will compare requirements for each alternative with build properties to find the best match. For example, when building with `<toolset>gcc` alternative 2, will be selected, and when building with `<toolset>msvc` alternative 3 will be selected. In all other cases, the most generic alternative 1 will be built.

### Prebuilt targets

To link to libraries whose build instructions aren't given in a Jamfile, you need to create `lib` targets with an appropriate `file` property. Target alternatives can be used to associate multiple library files with a single conceptual target. For example:

    # util/lib2/Jamfile
    lib lib2
        :
        : <file>lib2_release.a <variant>release
        ;

    lib lib2
        :
        : <file>lib2_debug.a <variant>debug
        ;

This example defines two alternatives for `lib2`, and for each one names a prebuilt file. Naturally, there are no sources. Instead, the `<file>` feature is used to specify the file name.

Once a prebuilt target has been declared, it can be used just like any other target:

    exe app : app.cpp ../util/lib2//lib2 ;

As with any target, the alternative selected depends on the properties propagated from `lib2`'s dependants. If we build the release and debug versions of `app` will be linked with `lib2_release.a` and `lib2_debug.a` , respectively.

System libraries—those that are automatically found by the toolset by searching through some set of predetermined paths—should be declared almost like regular ones:

    lib pythonlib : : <name>python22 ;

We again don't specify any sources, but give a `name` that should be passed to the compiler. If the gcc toolset were used to link an executable target to `pythonlib`, `-lpython22` would appear in the command line (other compilers may use different options).

We can also specify where the toolset should look for the library:

    lib pythonlib : : <name>python22 <search>/opt/lib ;

And, of course, target alternatives can be used in the usual way:

    lib pythonlib : : <name>python22 <variant>release ;
    lib pythonlib : : <name>python22_d <variant>debug ;

A more advanced use of prebuilt targets is described in the section called “Targets in site-config.jam”.

[13] See the section called “Feature Attributes”

[14] Many features will be overridden, rather than added-to, in subprojects. See the section called “Feature Attributes” for more information