Boost C++ Libraries

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

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 #included 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>, ^[11] augments) the build request.

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

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

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

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. ^[12] 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/.

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 ;

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.

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>NEWORK_LIB_SHARED
     <variant>release:<define>EXTRA_FAST
    ;

In the example above, whenever network is built with <link>shared, <define>NEWORK_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.

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”.