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Boost Build System


Boost.Build is a system for large project software construction built on Boost.Jam, a descendant of "Perforce Jam", an open-source make replacement[1]. Key features are:


Boost.Build v1 is a useful, mature system. However, its design and structure are not easily adapted to support some of the features we'd like to see in the future. To this end, the Boost.Build developers are working on Boost.Build v2, which is pretty usable already. If you are interested in contributing to this effort or you wish to discuss the design of Boost.Build, please post inquiries to the jamboost mailing list at yahoogroups.

Here are some of the design criteria that led to these features.

Table of Contents

Getting Started

Installing Boost.Jam

Follow these instructions to acquire a bjam executable for your platform. Install it somewhere in your path.

Initiating a Build

Boost.Build responds to several global variable settings. The easiest way to get going is usually to use environment variables, though you can also set them on the command-line, using -sVARIABLE_NAME=value. In addition to the toolset configuration variables, you can use the TOOLS variable to indicate which toolset(s) to build with, and the BUILD variable to describe how you want things built. In many cases it should be sufficient to invoke bjam with no variable settings.

Some example Boost.Jam invocations:

Command Line(s) Effects
bjam -sTOOLS=gcc my_target
default (debug) BUILD of my_targetwith GCC
bjam "-sTOOLS=msvc gcc"
default-build all with msvc and gcc
set TOOLS=msvc
Set an NT environment variable to always build with MSVC
default-build all.
bjam "-sBUILD=release <debug-symbols>on"
release build with debug symbols of all using default TOOLS
bjam "-sBUILD=debug release"
debug and release build all.
set TOOLS=msvc
bjam "-sBUILD=<cxxflags>-G6"
Set an NT environment variable to always build with MSVC
default-build all, adding a compiler command line switch
set TOOLS=msvc gcc
bjam "-sBUILD=<msvc><*><cxxflags>-G6"
Set an NT environment variable to always build with MSVC and GCC
default-build all, adding a MSVC-specific compiler command line switch
bjam "-sBUILD=<define>BOOST_POSIX" build all, with the macro BOOST_POSIX defined for all compilers

Setting Jam Variables

The "-s" options in the command lines above are passing variable settings to the build system. There are actually three ways to do that:

An Example Jamfile

Here is an example of a simple subproject Jamfile. In this example, it is assumed that the user has set BOOST_ROOT, either as an environment variable, on the command-line or in the project's Jamrules file:
subproject foo/bar/baz ; # path to here from project root

# A static library called 'baz'
lib baz : baz1.cpp baz2.cpp # C++ sources
          parser/src/baz4.ll # Lex->C++ sources
          parser/src/baz5.yy  # Yacc->C++ sources
        : <include>$(BOOST_PARENT_DIRECTORY)  # Put boost in #include path

# An executable called 'test'
exe test : <lib>baz # use the 'baz' library
           baz_test.cpp   # C++ source
         : <include>$(BOOST_ROOT)

That's it! The build system takes care of the rest. If the you want to be able to build all subprojects from the project root directory, you can add a Jamfile at the root:

project-root ; # declare this to be the project root directory
# Read subproject Jamfiles
subinclude foo/bar/baz foo/bar/... ;
subinclude a/b/c ... ; # more subincludes

Support Files

To use the build system, the following must be located in your project's root directory, or in a directory specified in the BOOST_BUILD_PATH variable. It is usually convenient to specify the BOOST_BUILD_PATH in your project's Jamrules file. The Boost Jamrules file shows an example.

Filename(s) Meaning
toolset-tools.jam Feature-to-command-line mapping for toolset.
features.jam Abstract toolset feature descriptions.
boost-base.jam Boost build system-specific rule definitions.
unit-tests.jam Unit tests and assertions for boost Jam code.
The boost-base.jam file is temporary, and will eventually be compiled into our Jam executable.

Basic Design and Terminology

This section gives an overview of the way that the system works, outlining the system's capabilities and overall design. It also introduces the terminology and concepts necessary to understand the sections on writing Jamfiles and command-line invocations.

Projects and Subprojects

A project is a source directory tree containing at least one Jamfile. The root directory of the project is known as the project root. The root directory of a project may contain a Jamrules file, which contains project-specific Jam code. If the Jamrules file is not present when Jam is invoked, a warning will be issued.

Subdirectories containing Jamfiles are called subproject directories. Each such Jamfile describes a subproject.

The build system installation directory is a directory containing Jam files describing compilers and build variants. The installation directory can be specified in a file called boost-build.jam in the project root directory. This file should contain a line boost-build $path-to-installation-dir. If no such file is available the environment variable BOOST_BUILD_PATH will be used. This lists a set of directories to search for the files comprising the build system. If the installation directory is not specified, it is the same as the project root, and BOOST_BUILD_PATH is set to include that directory.


Each Jamfile describes one or more main targets.

Each main target is an abstract description of one or more built targets which are expressions of the corresponding main target under particular compilers and build variants. Intermediate files such as .o/.obj files generated by compiling .cpp files as a consequence of building a main target are also referred to as built targets. The term build directory tree refers to the location of built target files.

For each main target, there is a corresponding location in the build directory tree known as the target's build root, where all intermediate and final targets resulting from that main target are located.

Features and Properties

A feature is a normalized (toolset-independent) description of an individual build parameter, such as whether inlining is enabled. Each feature usually corresponds to a command-line option of one or more build tools. Features come in four varieties:

  1. Simple features can take on any of several predetermined values. For example, the feature optimization might take one of the values off, speed, or space. Simple features have a default value. The key aspect of simple features is that they are assumed to affect link compatibility: object files generated with different values for a simple feature are generated into a separate directories, and (with a few exceptions) main targets generated with different values won't be linked together.
  2. Free features can either be single-valued, as above, or may take on any number of user-specified values simultaneously. For example, the define feature for a release build might have the values NDEBUG and BOOST_RELEASE_BUILD. Free features are assumed not to affect link compatibility.
  3. Path features are free features whose values describe paths which may be relative to the subproject (such as linked libraries or #include search directories). The build system treats the values of these features specially to ensure that they are interpreted relative to the subproject directory regardless of the directory where Jam was invoked.
  4. Dependency features are path features whose values describe a dependency of built targets. For example, an external library might be specified with a dependency-feature: if the library is updated, the target will be updated also. The <library-file> feature works this way [2].

A feature-value pair is known as a build property, or simply property. The prefixes simple, free, path, and dependency apply to properties in an analogous way to features.

Build Variants

A build variant, or simply variant is a named set of build properties describing how targets should be built. Typically you'll want at least two separate variants: one for debugging, and one for your release code.

Built targets for distinct build variants and toolsets are generated in separate parts of the build directory tree, known as the variant directories. For example, a (sub)project with main targets foo and bar, compiled with both GCC and KAI for debug and release variants might generate the following structure (target directories in bold).

 +-foo  <--- foo's build root
 | +-gcc
 | | +-debug
 | | `-release
 | `-kai
 |   +-debug
 |   `-release
 `-bar  <--- bar's build root
   | +-debug
   | `-release

The properties constituting a variant may differ according to toolset, so debug may mean a slightly different set of properties for two different compilers.


When a target is built with simple properties that don't exactly match those specified in a build variant, the non-matching features are called subvariant features and the target is located in a subvariant directory beneath the directory of the base variant. This can occur for two reasons:

  1. Some features are only relevant to certain compilers. When relevant simple features have no value specified in the build variant, a value must be chosen. Even when the default value is used, the target is generated into a subvariant directory. For example, the runtime-link feature may be unspecified in the debug variant, but relevant to MSVC. In that case, a fragment of the target tree might look like:
     +-foo  <--- foo's build root
     | +-msvc
     | | +-debug
     . . . `-runtime-link-dynamic
     . . .
    Because the default value of runtime-link is dynamic, when the debug variant is requested, the runtime-link-dynamic subvariant of foo is built.

  2. It is possible to request (either on the command-line, or as part of a main target description) that particular subvariants be built. For example, it may be desirable to generate builds that link to the runtime both statically and dynamically. In that case, both subvariant directories in the example above would be generated:
     +-foo  <--- foo's build root
     | +-msvc
     | | +-debug
     . . . +-runtime-link-dynamic
     . . . `-runtime-link-static
     . . .
In no case will targets be built directly into bin/foo/msvc/debug, since the debug variant doesn't include the runtime-link feature, which is relevant to MSVC.

When a subvariant includes multiple subvariant features, targets are built into a subvariant directory whose path is determined by concatenating the properties sorted in order of their feature names. For example, the borland compiler, which uses different libraries depending on whether the target is a console or GUI program, might create the following structure for a DLL:

 +-foo  <--- foo's build root
 | +-msvc
 | | +-debug
 | | | +-runtime-link-dynamic
 | | | | +-user-interface-console
 | | | | `-user-interface-gui
 . . . `-runtime-link-static
 . . .   +-user-interface-console
 . . .   `user-interface-gui

Any configuration of properties for which a target is built, whether base variant or subvariant, is known as a build configuration, or simply a build.

Dependent Targets

When a main target depends on the product of a second main target (as when an executable depends on and links to a static library), each build configuration of the dependent target is depends on the corresponding build of the dependency. Because only simple features participate in build identity, the dependent and dependency targets may have completely different free features. This puts the onus on the user for ensuring link-compatibility when certain free properties are used. For example, when assert() is used in header files, the preprocessor symbol NDEBUG can impact link-compatibility of separate compilation units. This danger can be minimized by encapsulating such feature differences inside of build variants.


This section describes how to start a build from the command-line and how to write project and subproject Jamfiles. It also describes the other files written in the Jam language: build-tool specification files, feature descriptions files.

The Command Line

This section describes in detail how the build system can be invoked.

User Targets

The Jam command line ends with an optional list of target names; if no target names are supplied, the built-in pseudotarget all is built. In a large project, naming targets can be dicey because of collisions. Jam uses a mechanism called grist to distinguish targets that would otherwise have the same name. Fortunately, you won't often have to supply grist at the command-line. When you declare a main target, a Jam pseudotarget of the same name is created which depends on all of the subvariants requested for your invocation of the build system. For example, if your subproject declares:

exe my_target : my_source1.cpp my_source2.c ;
and you invoke Jam with -sBUILD="debug release" my_target, you will build both the debug and release versions of my_target.

These simple, ungristed names are called user targets, and are only available for the subproject where Jam is invoked. That way, builds from the top level (which may include many Jamfiles through the subinclude rule) and builds of library dependencies (which may live in other subprojects), don't collide. If it is necessary to refer more explicitly to a particular target from the command-line, you will have to add ``grist''. Please see this section for a more complete description of how to name particular targets in a build.

Global Variables

This is a partial list of global variables that can be set on the command-line. Of course you are free to write your own Jam rules which interpret other variables from the command-line. This list just details some of the variables used by the build system itself. Note also that if you don't like the default values you can override them in your project's Jamrules file.

Variable Default Example Notes
TOOLS Platform-dependent "-sTOOLS=gcc msvc" build with gcc and msvc
-sTOOLS=gcc build with gcc
BUILD debug -sBUILD=release build the release variant
"-sBUILD=debug release" build both debug and release variants
"-sBUILD=<optimization>speed" build a subvariant of the default variant (debug) with optimization for speed.
"-sBUILD=debug release <runtime-link>static/dynamic" build subvariants of the debug and release variants that link to the runtime both statically and dynamically.
ALL_LOCATE_TARGET empty -sALL_LOCATE_TARGET=~/build Generate all build results in the build subdirectory of the user's home directory (UNIX).
This section describes how to write a Jamfile for a subproject.

SubProject Jamfiles

The subproject rule

A subproject's Jamfile begins with an invocation of the subproject rule that specifies the subproject's location relative to the top of the project tree:

subproject path-from-top ;

The subproject rule tells the build system where to place built targets from the subproject in case ALL_LOCATE_TARGET is used to specify the build directory tree. If there is a Jamfile in the project root directory, you should use the project-root rule instead:

project-root ;

Describing Main Targets

A main target is described using the following syntax:

target-type name : sources
    [ : requirements [ : default-BUILD ] ] ;

NOTE: for simple features in both requirements and default-BUILD, more-specific qualification overrides less-specific.

Describing Template Targets

Template targets provide a way to handle commonalities between projects targets. They have the same form as main targets but do not initiate build requests. A target that lists a template as a dependency inherits all the settings from the template, i.e. the templates sources, requirements and default build settings will be added to the targets settings. Paths mentioned in a template definition are always relative to the subdirectory of the Jamfile containing the templates definition, regardless of the subdirectory of the dependent main target. Typically a project will have at least one template target that handles defines, include paths and additional compiler flags common to all targets in the project.

Describing Stage Targets

Stage targets are a special kind of target that don't build a single file but to a collection of files. The goal is to create a directory which is composed of the various files that other targets generate, or individual files. When built a stage target creates a directory with the same name as the target, and copies the dependent files to it. The form of the target is the same as that of main targets with some differences...


This artificially complex example shows how two executables called "foo" and "fop" might be described in a Jamfile. All common settings are factored out in the templates "base" and "executable". Foo is composed of the sources ./foo.cpp and ./src/bar.cpp (specified relative to the directory in which the Jamfile resides). Fop only has one sourcefile ./fop.cpp. Both executables link against the built target which results from building the target baz as described in ../bazlib/Jamfile.

template base : 
    ## Requirements ##
    : <include>../bazlib/include 
    ## default-BUILD ##
    : debug release

template executable : <template>base <lib>../bazlib/baz ;

exe foo : <template>executable foo.cpp src/bar.cpp ;

exe fop : <template>executable fop.cpp ;

The requirements section:

The default-BUILD section:

Using External Projects

To use dependencies such as libraries from another project tree, first use the project rule to declare a project id and location for the external project. Then add the appropriate external dependency target specification to your program's list of sources. For example, if you are developing a program which uses the Boost.Threads library, you might write
project boost : /home/dave/boost-cvs ;
in your Jamrules file, and place
in your target's list of sources.

Requirement Rules

Target requirements support the use of auxiliary rules to allow for more complex decisions about the requirements. If specified, by using the name of a rule in the requirements, the rule is called with the signature: ( toolset variant : subvariant-path properties * ) and should return the modified set of properties. There are a number of built-in rules for some common tasks that Boost uses, and you can use:

Rule Effects
std::locale-support Ensures that locale support is available for the target. For example some toolsets, like CodeWarrior, locale support is only available on specific platforms using a static runtime.
std::facet-support Ensures that facet support is available for the target.

Adds a constructed prefix tag to the target to conform to the Boost common naming conventions for libraries. The tag is constructed as:

  • <toolset-tag> maps to an abbreviated name of the toolset and when possible, and applicable, the version of the toolset.
  • <thread-tag> "mt" when multi-threading is enabled.
  • <runtime-tag> adds these single letter tags:
      "s" when static linking to runtime
      "g" when linking to debug runtime
      "y" when building debug-python variants
      "d" when building debug variants
      "p" when building with stlport libraries
      "n" when building with stlport and using native iostreams
  • <version-tag> adds "major_minor" from a <version> property. Defaults to using $(BOOST_VERSION) if no version property is present.

Install Descriptions

Installable files and targets are described with:

install name type : sources... : [options]... ;

Install descriptions define files and targets that can be installed by use of a stage target.

Install descriptions are meant to be used by stage targets to collect the various sources of many install descriptions into one or more destination directories. For this there are two rules that help in getting the sources specified:

Feature Descriptions

Features are described by stating the feature type (simple features are specified with "feature"), followed by the feature name. An optional second argument can be used to list the permissible values of the feature. Examples can be found in features.jam.

Variant Descriptions

Variants are described with the following syntax:

variant name [ : parent-name] : [<toolset-name>]<feature>value... ;
The variant rule specifies the list of properties comprising a variant. Properties may be optionally qualified with a toolset name, which specifies that the property applies only to that toolset. One or more parent variants may be specified to inherit the properties  from those parent(s). For inherited properties precedence is given on a left to right order, making the immediate properties override those in the parent(s). This can be used to great effect for describing global properties that are shared amongst various variants, and therefore targets. For example:
variant my-globals : <rtti>off ;

variant my-debug : my-globals debug ;

variant my-release : my-globals release ;
More examples can be found in features.jam.

Toolset Description Files

Toolset descriptions are located in the project's root directory, or a directory specified by BOOST_BUILD_INSTALLATION, which may be set in a Jamfile or the project's Jamrules file. Each file is called toolset-name-tools.jam, where toolset-name is the name of the toolset. The toolset description file has two main jobs:

  1. redefine the following rules: These rules should simply invoke the action part of a rule whose name is uniquely defined for the toolset. For example,
    rule C++-action
        msvc-C++-action $(<) : $(>) ;
    actions msvc-C++-action
        cl -nologo -GX -c -U$(UNDEFS) -D$(DEFINES) $(CFLAGS) $(C++FLAGS) -I$(HDRS) -I$(STDHDRS) -Fo$(<) -Tp$(>)
    Note that Link-action may require special care: on platforms where the global variable gEXPORT_SUFFIX(DLL) is defined (e.g. Windows), the first argument may have two elements when linking a shared library. The first is the shared library target, and the second is the import library target, with suffix given by $(gEXPORT_SUFFIX(DLL)). It will always have a third argument which is either ``EXE'' or ``DLL''. This can be used to dispatch to different actions for linking DLLs and EXEs if necessary, but usually it will be easier to take advantage of the special <target-type> feature, which will have the same value using the flags rule described below.
  2. Translate build settings given in the global gBUILD_PROPERTIES variable into something that can be used by the toolset. The build system provides the flags rule to help translate build properties into elements of global variables which are later attached to targets so that they can affect the build actions. The flags rule is used as follows:
    flags toolset variable condition [: value...]
    The parameters are:

    Semantics only affect targets built with the specified toolset, and depend on the target's build configuration:

    1. if any specified property-set is a subset of the target's build properties, the values specified in $(3) will be appended once to variable.
    2. The value of each specified feature that participates in the target's build properties is appended to variable. In either case, the variable will be set "on" the target so it may be used in the build actions.


The description of the flags rule above is actually more complicated than it sounds. For example, the following line might be used to specify how optimization can be turned off for MSVC:

flags msvc CFLAGS <optimization>off : /Od ;
It says that the string /Od should be added to the global CFLAGS variable whenever a build configuration includes the property <optimization>off.

Similarly, in the following example,

flags msvc CFLAGS <runtime-build>release/<runtime-link>dynamic : /MD ;
we add /MD to the CFLAGS variable when both of the specified conditions are satisfied. We could grab all of the values of the free feature <include> in the HDRS variable as follows:
flags msvc HDRS <include> ;

The use of these variables should be apparent from the declaration of actions msvc-C++-action in the previous section.


Target Names

In addition to user targets, which correspond directly to the names the user writes in her subproject Jamfile, several additional targets are generated, regardless of the directory from which Jam was invoked:

Global Variables

This section describes some of the global variables used by the build system. Please note that some parts of the system (particularly those in allyourbase.jam) are heavily based on the Jambase file supplied with Jam, and as such do not follow the conventions described below.

Global variables used in the build system fall into three categories:

Please note that the build system commonly takes advantage of Jam's Dynamic Scoping feature (see the local command in the "Flow of Control" section below the link target) to temporarily "change" a global variable by declaring a local of the same name.

Many of the variables that are used to configure how Boost.Build works internally are listed here with brief descriptions.

Variables Associated with SubProject Identity

Grist Variables

Design Criteria


The requirements are driven by several basic assumptions:


This build system was designed to satisfy the following requirements:


[1] Boost Jam is actually descended directly from FTJam, which was itself a variant of Jam/MR. It is hoped that crucial features we rely on will eventually be incorporated back into the Jam/MR release.

[2] Note: right now, a dependency feature of a main target makes all resulting built targets dependent, including intermediate targets. That means that if an executable is dependent on an external library, and that library changes, all the sources comprising the executable will be recompiled as well. This behavior should probably be fixed.

Revised 8 September, 2003

Copyright © Dave Abrahams 2001.

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at