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## Detailed reference

### General information

#### Initialization

bjam's first job upon startup is to load the Jam code that implements the build system. To do this, it searches for a file called `boost-build.jam`, first in the invocation directory, then in its parent and so forth up to the filesystem root, and finally in the directories specified by the environment variable BOOST_BUILD_PATH. When found, the file is interpreted, and should specify the build system location by calling the boost-build rule:

    rule boost-build ( location ? )

If location is a relative path, it is treated as relative to the directory of `boost-build.jam`. The directory specified by that location and the directories in BOOST_BUILD_PATH are then searched for a file called `bootstrap.jam`, which is expected to bootstrap the build system. This arrangement allows the build system to work without any command-line or environment variable settings. For example, if the build system files were located in a directory "build-system/" at your project root, you might place a `boost-build.jam` at the project root containing:

    boost-build build-system ;

In this case, running bjam anywhere in the project tree will automatically find the build system.

The default `bootstrap.jam`, after loading some standard definitions, loads two files, which can be provided/customised by user: `site-config.jam` and `user-config.jam`.

Locations where those files are searched are summarized below:

Table 25.2. Search paths for configuration files

| site-config.jam | user-config.jam
--- | --- | ---
Linux | `/etc` `$HOME` `$BOOST_BUILD_PATH` | `$HOME` `$BOOST_BUILD_PATH`
Windows | `%SystemRoot%` `%HOMEDRIVE%%HOMEPATH%` `%HOME%` `%BOOST_BUILD_PATH%` | `%HOMEDRIVE%%HOMEPATH%` `%HOME%` `%BOOST_BUILD_PATH%`

Boost.Build comes with default versions of those files, which can serve as templates for customized versions.

#### Command line

The command line may contain:

- Jam options,
- Boost.Build options,
- Command line arguments

##### Command line arguments

Command line arguments specify targets and build request using the following rules.

- An argument that does not contain slashes or the `=` symbol is either a value of an implicit feature or of a target to be built. It is taken to be value of a feature if an appropriate feature exists. Otherwise, it is considered a target id. Building the special target name “clean” has the same effect as using the `--clean` option.

- An argument containing either slashes or the `=` symbol specifies a number of build request elements (see the section called “Build Request”). In its simplest form, it's just a set of properties, separated by slashes, which become a single build request element, for example:

      borland/<runtime-link>static

  A more complex form can be used to save typing. For example, instead of

      borland/runtime-link=static borland/runtime-link=dynamic

  one can use

      borland/runtime-link=static,dynamic

  Exactly, the conversion from argument to build request elements is performed by (1) splitting the argument at each slash, (2) converting each split part into a set of properties and (3) taking all possible combinations of the property sets. Each split part should have either the form

      feature-name=feature-value1[","feature-valueN]*

  or, in case of implicit features

      feature-value1[","feature-valueN;]*

  will be converted into the property set

      <feature-name>feature-value1 .... <feature-name>feature-valueN

For example, the command line

    target1 debug gcc/runtime-link=dynamic,static

would cause target called `target1` to be rebuilt in debug mode, except that for gcc, both dynamically and statically linked binaries would be created.

##### Command line options

All of the Boost.Build options start with the "--" prefix. They are described in the following table.

FIXME: That table has moved into "User documentation" section and there's nothing we can add here. Remove this part?

### Builtin rules

This section contains the list of all rules that can be used in Jamfile—both rules that define new targets and auxiliary rules.

`exe`

  Creates an executable file. See the section called “Programs”.

`lib`

  Creates an library file. See the section called “Libraries”.

`install`

  Installs built targets and other files. See the section called “Installing”.

`alias`

  Creates an alias for other targets. See the section called “Alias”.

`unit-test`

  Creates an executable that will be automatically run. See the section called “Testing”.

`compile`, `compile-fail`, `link`, `link-fail`, `run`, `run-fail`

  Specialized rules for testing. See the section called “Testing”.

`obj`

  Creates an object file. Useful when a single source file must be compiled with special properties.

`glob`

  The `glob` rule takes a list shell pattern and returns the list of files in the project's source directory that match the pattern. For example:

      lib tools : [ glob *.cpp ] ;

  It is possible to also pass a second argument—the list of exclude patterns. The result will then include the list of files patching any of include patterns, and not matching any of the exclude patterns. For example:

      lib tools : [ glob *.cpp : file_to_exclude.cpp bad*.cpp ] ;

`glob-tree`

  The `glob-tree` is similar to the `glob` except that it operates recursively from the directory of the containing Jamfile. For example:

      ECHO [ glob-tree *.cpp : .svn ] ;

  will print the names of all C++ files in your project. The `.svn` exclude pattern prevents the `glob-tree` rule from entering administrative directories of the Subverion version control system.

`project`

  Declares project id and attributes, including project requirements. See the section called “Projects”.

`use-project`

  Assigns a symbolic project ID to a project at a given path. This rule must be better documented!

`explicit`

  The `explicit` rule takes a single parameter—a list of target names. The named targets will be marked explicit, and will be built only if they are explicitly requested on the command line, or if their dependents are built. Compare this to ordinary targets, that are built implicitly when their containing project is built.

`constant`

  Sets project-wide constant. Takes two parameters: variable name and a value and makes the specified variable name accessible in this Jamfile and any child Jamfiles. For example:

      constant VERSION : 1.34.0 ;

`path-constant`

  Same as `constant` except that the value is treated as path relative to Jamfile location. For example, if bjam is invoked in the current directory, and Jamfile in `helper` subdirectory has:

      path-constant DATA : data/a.txt ;

  then the variable `DATA` will be set to `helper/data/a.txt`, and if bjam is invoked from the `helper` directory, then the variable `DATA` will be set to `data/a.txt`.

`build-project`

  Cause some other project to be built. This rule takes a single parameter—a directory name relative to the containing Jamfile. When the containing Jamfile is built, the project located at that directory will be built as well. At the moment, the parameter to this rule should be a directory name. Project ID or general target references are not allowed.

`test-suite`

  This rule is deprecated and equivalent to `alias`.

### Builtin features

`variant`

  A feature that combines several low-level features, making it easy to request common build configurations.

  Allowed values: `debug`, `release`, `profile`.

  The value `debug` expands to

      <optimization>off <debug-symbols>on <inlining>off <runtime-debugging>on

  The value `release` expands to

      <optimization>speed <debug-symbols>off <inlining>full <runtime-debugging>off

  The value `profile` expands to the same as `release`, plus:

      <profiling>on <debug-symbols>on

  User can define his own build variants using the `variant` rule from the `common` module.

  Notee: Runtime debugging is on in debug builds to suit the expectations of people used to various IDEs.

`link`

  A feature that controls how libraries are built.

  Allowed values: `shared`, `static`

`source`
  The `<source>X` feature has the same effect on building a target as putting X in the list of sources. It's useful when you want to add the same source to all targets in the project (you can put <source> in requirements) or to conditionally include a source (using conditional requirements, see the section called “Conditions and alternatives”) See also the `<library>` feature.
`library`
  This feature is almost equivalent to the `<source>` feature, except that it takes effect only for linking. When you want to link all targets in a Jamfile to certain library, the `<library>` feature is preferred over `<source>X` -- the latter will add the library to all targets, even those that have nothing to do with libraries.
`dependency`
  Introduces a dependency on the target named by the value of this feature (so it will be brought up-to-date whenever the target being declared is). The dependency is not used in any other way. For example, in application with plugins, the plugins are not used when linking the application, application might have dependency on its plugins, even though , and adds its usage requirements to the build properties of the target being declared. The primary use case is when you want the usage requirements (such as `#include` paths) of some library to be applied, but don't want to link to it.
`use`
  Introduces a dependency on the target named by the value of this feature (so it will be brought up-to-date whenever the target being declared is), and adds its usage requirements to the build properties of the target being declared. The dependency is not used in any other way. The primary use case is when you want the usage requirements (such as `#include` paths) of some library to be applied, but don't want to link to it.
`dll-path`
  Specify an additional directory where the system should look for shared libraries when the executable or shared library is run. This feature only affects Unix compilers. Plase see the section called “Why are the `dll-path` and `hardcode-dll-paths` properties useful? ” in the section called “Frequently Asked Questions” for details.
`hardcode-dll-paths`

  Controls automatic generation of dll-path properties.

  Allowed values: `true`, `false`. This property is specific to Unix systems. If an executable is built with `<hardcode-dll-paths>true`, the generated binary will contain the list of all the paths to the used shared libraries. As the result, the executable can be run without changing system paths to shared libraries or installing the libraries to system paths. This is very convenient during development. Plase see the FAQ entry for details. Note that on Mac OSX, the paths are unconditionally hardcoded by the linker, and it's not possible to disable that behaviour.

`cflags`, `cxxflags`, `linkflags`
  The value of those features is passed without modification to the corresponding tools. For `cflags` that's both the C and C++ compilers, for `cxxflags` that's the C++ compiler and for `linkflags` that's the linker. The features are handy when you're trying to do something special that cannot be achieved by higher-level feature in Boost.Build.
`warnings`
  The `<warnings>` feature controls the warning level of compilers. It has the following values:

  - `off` - disables all warnings.

  - `on` - enables default warning level for the tool.

  - `all` - enables all warnings.

  Default value is `all`.
`warnings-as-errors`
  The `<warnings-as-errors>` makes it possible to treat warnings as errors and abort compilation on a warning. The value `on` enables this behaviour. The default value is `off`.
`build`

  Allowed values: `no`

  The `build` feature is used to conditionally disable build of a target. If `<build>no` is in properties when building a target, build of that target is skipped. Combined with conditional requirements this allows to skip building some target in configurations where the build is known to fail.

`tag`

  The `tag` feature is used to customize the name of the generated files. The value should have the form:

      @rulename

  where `rulename` should be a name of a rule with the following signature:

      rule tag ( name : type ? : property-set )

  The rule will be called for each target with the default name computed by Boost.Build, the type of the target, and property set. The rule can either return a string that must be used as the name of the target, or empty string, in which case the default name will be used.

  Most typical use of the `tag` feature is to encode build properties, or library version in library target names. You should take care to return non-empty string from the tag rule only for types you care about — otherwise, you might end up modifying names of object files, generated header file and other targets for which changing names does not make sense.

`debug-symbols`

  Allowed values: `on`, `off`.

  The `debug-symbols` feature specifies if produced object files, executables and libraries should include debug information. Typically, the value of this feature is implicitly set by the `variant` feature, but it can be explicitly specified by the user. The most common usage is to build release variant with debugging information.

### Builtin tools

Boost.Build comes with support for a large number of C++ compilers, and other tools. This section documents how to use those tools.

Before using any tool, you must declare your intention, and possibly specify additional information about tool's configuration. This is done with the `using` rule, for example:

    using gcc ;

additional parameters can be passed just like for other rules, for example:

    using gcc : 4.0 : g++-4.0 ;

The options that can be passed to each tool will be documented in the subsequent sections.

#### C++ Compilers

This section lists all Boost.Build modules that support C++ compilers and documents how each one can be initialized.

##### GNU C++

The `gcc` module supports the GNU C++ compiler on Linux, a number of Unix-like system including MacOS X, SunOS and BeOS, and on Windows (either Cygwin or MinGW).

The `gcc` module is initialized using the following syntax:

    using gcc : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the version is not explicitly specified, it will be automatically detected by running the compiler with the `-v` option. If the command is not specified, the g++ binary will be searched in `PATH`.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

`root`

  Specifies root directory of the compiler installation. This option is necessary only if it's not possible to detect this information from the compiler command—for example if the specified compiler command is a user script.

`rc`

  Specifies the resource compiler command that will be used with the version of gcc that is being configured. This setting makes sense only for Windows and only if you plan to use resource files. By default windres will be used.

`rc-type`

  Specifies the type of resource compiler. The value can be either `windres` for msvc resource compiler, or `rc` for borland's resource compiler.

##### Microsoft Visual C++

The `msvc` module supports the Microsoft Visual C++ command-line tools on Microsoft Windows. The supported products and versions of command line tools are listed below:

- Visual Studio 2005—8.0

- Visual Studio .NET 2003—7.1

- Visual Studio .NET—7.0

- Visual Studio 6.0, Service Pack 5—6.5

The `msvc` module is initialized using the following syntax:

    using msvc : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the version is not explicitly specified, the most recent version found in the registry will be used instead. If the special value `all` is passed as the version, all versions found in the registry will be configured. If a version is specified, but the command is not, the compiler binary will be searched in standard installation paths for that version, followed by `PATH`.

The compiler command should be specified using forward slashes, and quoted.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

`setup`

  The filename of the environment setup scripts to run before invoking the compiler. If not specified, vcvars32.bat alongside the compiler binary will be used.

`compiler`

  The command that compiles C and C++ sources. If not specified, cl will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

`linker`

  The command that links executables and dynamic libraries. If not specified, link will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

`assembler`

  The command that compiles assember files. If not specified, cl will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

`resource-compiler`

  The command that compiles resource files. If not specified, rc will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

`idl-compiler`

  The command that compiles Microsoft COM interface definition files. If not specified, midl will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

`mc-compiler`

  The command that compiles Microsoft message catalog files. If not specified, mt will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

##### Intel C++

The `intel-linux` and `intel-win` modules support the Intel C++ command-line compiler—the Linux and Windows versions respectively.

The module is initialized using the following syntax:

    using intel-linux : [version] : [c++-compile-command] : [compiler options] ;

or

    using intel-win : [version] : [c++-compile-command] : [compiler options] ;

respectively.

This statement may be repeated several times, if you want to configure several versions of the compiler.

If compiler command is not specified, then Boost.Build will look in `PATH` for an executable icpc (on Linux), or icc.exe (on Windows).

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

The Linux version supports the following additional options:

`root`

  Specifies root directory of the compiler installation. This option is necessary only if it's not possible to detect this information from the compiler command—for example if the specified compiler command is a user script.

##### HP aC++ compiler

The `acc` module supports the HP aC++ compiler for the HP-UX operating system.

The module is initialized using the following syntax:

    using acc ;

There are no configuration options. The compiler will always be invoked as aCC and should be in `PATH`.

##### Borland C++ Compiler

The `borland` module supports the command line C++ compiler included in C++ Builder 2006 product and earlier version of it, running on Microsoft Windows.

The supported products are listed below. The version reported by the command lines tools is also listed for reference.:

- C++ Builder 2006—5.8.2

- CBuilderX—5.6.5, 5.6.4 (depending on release)

- CBuilder6—5.6.4

- Free command line tools—5.5.1

The module is initialized using the following syntax:

    using borland : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a binary named bcc32 in `PATH`.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

##### Comeau C/C++ Compiler

The `como-linux` and the `como-win` modules supports the Comeau C/C++ Compiler on Linux and Windows respectively.

The module is initialized using the following syntax:

    using como-linux : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a binary named como in `PATH`.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

Before using the windows version of the compiler, you need to setup necessary environment variables per compiler's documentation. In particular, the `COMO_XXX_INCLUDE` variable should be set, where `XXX` corresponds to the used backend C compiler.

##### Code Warrior

The `cw` module support CodeWarrior compiler, originally produced by Metrowerks and presently developed by Freescale. Boost.Build supports only the versions of the compiler that target x86 processors. All such versions were released by Metrowerks before aquisition and are not sold any longer. The last version known to work is 9.4

The module is initialized using the following syntax:

    using cw : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a binary named mwcc in default installation paths and in `PATH`.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

`root`

  Specifies root directory of the compiler installation. This option is necessary only if it's not possible to detect this information from the compiler command—for example if the specified compiler command is a user script.

`setup`

  The command that sets up environment variables prior to invoking the compiler. If not specified, cwenv.bat alongside the compiler binary will be used.

`compiler`

  The command that compiles C and C++ sources. If not specified, mwcc will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

`linker`

  The command that links executables and dynamic libraries. If not specified, mwld will be used. The command will be invoked after the setup script was executed and adjusted the `PATH` variable.

##### Digital Mars C/C++ Compiler

The `dmc` module supports the Digital Mars C++ compiler.

The module is initialized using the following syntax:

    using dmc : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a binary named como in `PATH`.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

##### HP C++ Compiler for Tru64 Unix

The `hp_cxx` modules supports the HP C++ Compiler for Tru64 Unix.

The module is initialized using the following syntax:

    using hp_cxx : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a binary named hp_cxx in `PATH`.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

##### Sun Studio

The `sun` module supports the Sun Studio C++ compilers for the Solaris OS.

The module is initialized using the following syntax:

    using sun : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a binary named CC in `/opt/SUNWspro/bin` and in `PATH`.

When using this compiler on complex C++ code, such as the Boost C++ library, it is recommended to specify the following options when intializing the `sun` module:

    -library=stlport4 -features=tmplife -features=tmplrefstatic

See the Sun C++ Frontend Tales for details.

The following options can be provided, using `<`option-name`>`option-value`` syntax:

`cflags`

  Specifies additional compiler flags that will be used when compiling C sources.

`cxxflags`

  Specifies additional compiler flags that will be used when compiling C++ sources.

`compileflags`

  Specifies additional compiler flags that will be used when compiling both C and C++ sources.

`linkflags`

  Specifies additional command line options that will be passed to the linker.

##### IBM Visual Age

The `vacpp` module supports the IBM Visual Age C++ Compiler, for the AIX operating system. Versions 7.1 and 8.0 are known to work.

The module is initialized using the following syntax:

    using vacpp ;

The module does not accept any initialization options. The compiler should be installed in the `/usr/vacpp/bin` directory.

Later versions of Visual Age are known as XL C/C++. They were not tested with the the `vacpp` module.

### Build process

The general overview of the build process was given in the user documentation. This section provides additional details, and some specific rules.

To recap, building a target with specific properties includes the following steps:

1. applying default build,

2. selecting the main target alternative to use,

3. determining "common" properties,

4. building targets referred by the sources list and dependency properties,

5. adding the usage requirements produces when building dependencies to the "common" properties,

6. building the target using generators,

7. computing the usage requirements to be returned.

#### Alternative selection

When there are several alternatives, one of them must be selected. The process is as follows:

1. For each alternative condition is defined as the set of base properies in requirements. [Note: it might be better to specify the condition explicitly, as in conditional requirements].
2. An alternative is viable only if all properties in condition are present in build request.
3. If there's one viable alternative, it's choosen. Otherwise, an attempt is made to find one best alternative. An alternative a is better than another alternative b, iff the set of properties in b's condition is a strict subset of the set of properities of 'a's condition. If there's one viable alternative, which is better than all others, it's selected. Otherwise, an error is reported.

#### Determining common properties

The "common" properties is a somewhat artificial term. Those are the intermediate property set from which both the build request for dependencies and properties for building the target are derived.

Since default build and alternatives are already handled, we have only two inputs: build requests and requirements. Here are the rules about common properties.

1. Non-free feature can have only one value

2. A non-conditional property in requirement in always present in common properties.

3. A property in build request is present in common properties, unless (2) tells otherwise.

4. If either build request, or requirements (non-conditional or conditional) include an expandable property (either composite, or property with specified subfeature value), the behaviour is equivalent to explicitly adding all expanded properties to build request or requirements.

5. If requirements include a conditional property, and condiiton of this property is true in context of common properties, then the conditional property should be in common properties as well.

6. If no value for a feature is given by other rules here, it has default value in common properties.

Those rules are declarative, they don't specify how to compute the common properties. However, they provide enough information for the user. The important point is the handling of conditional requirements. The condition can be satisfied either by property in build request, by non-conditional requirements, or even by another conditional property. For example, the following example works as expected:

    exe a : a.cpp
          : <toolset>gcc:<variant>release
            <variant>release:<define>FOO ;

### Definitions

#### Features and properties

A feature is a normalized (toolset-independent) aspect of a build configuration, such as whether inlining is enabled. Feature names may not contain the '`>`' character.

Each feature in a build configuration has one or more associated values. Feature values for non-free features may not contain the '`<`', '`:`', or '`=`' characters. Feature values for free features may not contain the '`<`' character.

A property is a (feature,value) pair, expressed as <feature>value.

A subfeature is a feature that only exists in the presence of its parent feature, and whose identity can be derived (in the context of its parent) from its value. A subfeature's parent can never be another subfeature. Thus, features and their subfeatures form a two-level hierarchy.

A value-string for a feature F is a string of the form `value-subvalue1-subvalue2`...`-subvalueN`, where `value` is a legal value for F and `subvalue1`...`subvalueN` are legal values of some of F's subfeatures. For example, the properties `<toolset>gcc <toolset-version>3.0.1` can be expressed more conscisely using a value-string, as `<toolset>gcc-3.0.1`.

A property set is a set of properties (i.e. a collection without duplicates), for instance: `<toolset>gcc <runtime-link>static`.

A property path is a property set whose elements have been joined into a single string separated by slashes. A property path representation of the previous example would be `<toolset>gcc/<runtime-link>static`.

A build specification is a property set that fully describes the set of features used to build a target.

##### Property Validity

For free features, all values are valid. For all other features, the valid values are explicitly specified, and the build system will report an error for the use of an invalid feature-value. Subproperty validity may be restricted so that certain values are valid only in the presence of certain other subproperties. For example, it is possible to specify that the `<gcc-target>mingw` property is only valid in the presence of `<gcc-version>2.95.2`.

##### Feature Attributes

Each feature has a collection of zero or more of the following attributes. Feature attributes are low-level descriptions of how the build system should interpret a feature's values when they appear in a build request. We also refer to the attributes of properties, so that an incidental property, for example, is one whose feature has the incidental attribute.

- incidental

  Incidental features are assumed not to affect build products at all. As a consequence, the build system may use the same file for targets whose build specification differs only in incidental features. A feature that controls a compiler's warning level is one example of a likely incidental feature.

  Non-incidental features are assumed to affect build products, so the files for targets whose build specification differs in non-incidental features are placed in different directories as described in "target paths" below. [ where? ]

- propagated

  Features of this kind are propagated to dependencies. That is, if a main target is built using a propagated property, the build systems attempts to use the same property when building any of its dependencies as part of that main target. For instance, when an optimized exectuable is requested, one usually wants it to be linked with optimized libraries. Thus, the `<optimization>` feature is propagated.

- free

  Most features have a finite set of allowed values, and can only take on a single value from that set in a given build specification. Free features, on the other hand, can have several values at a time and each value can be an arbitrary string. For example, it is possible to have several preprocessor symbols defined simultaneously:

      <define>NDEBUG=1 <define>HAS_CONFIG_H=1

- optional

  An optional feature is a feature that is not required to appear in a build specification. Every non-optional non-free feature has a default value that is used when a value for the feature is not otherwise specified, either in a target's requirements or in the user's build request. [A feature's default value is given by the first value listed in the feature's declaration. -- move this elsewhere - dwa]

- symmetric

  A symmetric feature's default value is not automatically included in build variants. Normally a feature only generates a subvariant directory when its value differs from the value specified by the build variant, leading to an assymmetric subvariant directory structure for certain values of the feature. A symmetric feature, when relevant to the toolset, always generates a corresponding subvariant directory.

- path

  The value of a path feature specifies a path. The path is treated as relative to the directory of Jamfile where path feature is used and is translated appropriately by the build system when the build is invoked from a different directory

- implicit

  Values of implicit features alone identify the feature. For example, a user is not required to write "<toolset>gcc", but can simply write "gcc". Implicit feature names also don't appear in variant paths, although the values do. Thus: bin/gcc/... as opposed to bin/toolset-gcc/.... There should typically be only a few such features, to avoid possible name clashes.

- composite

  Composite features actually correspond to groups of properties. For example, a build variant is a composite feature. When generating targets from a set of build properties, composite features are recursively expanded and added to the build property set, so rules can find them if necessary. Non-composite non-free features override components of composite features in a build property set.

- dependency

  The value of dependency feature if a target reference. When used for building of a main target, the value of dependency feature is treated as additional dependency.

  For example, dependency features allow to state that library A depends on library B. As the result, whenever an application will link to A, it will also link to B. Specifying B as dependency of A is different from adding B to the sources of A.

Features that are neither free nor incidental are called base features.

##### Feature Declaration

The low-level feature declaration interface is the `feature` rule from the `feature` module:

    rule feature ( name : allowed-values * : attributes * )

A feature's allowed-values may be extended with the `feature.extend` rule.

#### Build Variants

A build variant, or (simply variant) is a special kind of composite feature that automatically incorporates the default values of features that . Typically you'll want at least two separate variants: one for debugging, and one for your release code. [ Volodya says: "Yea, we'd need to mention that it's a composite feature and describe how they are declared, in pacticular that default values of non-optional features are incorporated into build variant automagically. Also, do we wan't some variant inheritance/extension/templates. I don't remember how it works in V1, so can't document this for V2.". Will clean up soon -DWA ]

#### Property refinement

When a target with certain properties is requested, and that target requires some set of properties, it is needed to find the set of properties to use for building. This process is called property refinement and is performed by these rules

1. Each property in the required set is added to the original property set
2. If the original property set includes property with a different value of non free feature, that property is removed.

#### Conditional properties

Sometime it's desirable to apply certain requirements only for a specific combination of other properties. For example, one of compilers that you use issues a pointless warning that you want to suppress by passing a command line option to it. You would not want to pass that option to other compilers. Conditional properties allow you to do just that. Their syntax is:

            property ( "," property ) * ":" property

For example, the problem above would be solved by:

    exe hello : hello.cpp : <toolset>yfc:<cxxflags>-disable-pointless-warning ;

The syntax also allows several properties in the condition, for example:

    exe hello : hello.cpp : <os>NT,<toolset>gcc:<link>static ;

#### Target identifiers and references

Target identifier is used to denote a target. The syntax is:

    target-id -> (project-id | target-name | file-name )
                  | (project-id | directory-name) "//" target-name
    project-id -> path
    target-name -> path
    file-name -> path
    directory-name -> path

This grammar allows some elements to be recognized as either

- project id (at this point, all project ids start with slash).
- name of target declared in current Jamfile (note that target names may include slash).
- a regular file, denoted by absolute name or name relative to project's sources location.

To determine the real meaning a check is made if project-id by the specified name exists, and then if main target of that name exists. For example, valid target ids might be:

    a                                    -- target in current project
    lib/b.cpp                            -- regular file
    /boost/thread                        -- project "/boost/thread"
    /home/ghost/build/lr_library//parser -- target in specific project

Rationale:Target is separated from project by special separator (not just slash), because:

- It emphasises that projects and targets are different things.
- It allows to have main target names with slashes.

Target reference is used to specify a source target, and may additionally specify desired properties for that target. It has this syntax:

    target-reference -> target-id [ "/" requested-properties ]
    requested-properties -> property-path

For example,

              exe compiler : compiler.cpp libs/cmdline/<optimization>space ;

would cause the version of `cmdline` library, optimized for space, to be linked in even if the `compiler` executable is build with optimization for speed.

### Generators

| Warning
The information is this section is likely to be outdated and misleading.

To construct a main target with given properties from sources, it is required to create a dependency graph for that main target, which will also include actions to be run. The algorithm for creating the dependency graph is described here.

The fundamental concept is generator. If encapsulates the notion of build tool and is capable to converting a set of input targets into a set of output targets, with some properties. Generator matches a build tool as closely as possible: it works only when the tool can work with requested properties (for example, msvc compiler can't work when requested toolset is gcc), and should produce exactly the same targets as the tool (for example, if Borland's linker produces additional files with debug information, generator should also).

Given a set of generators, the fundamental operation is to construct a target of a given type, with given properties, from a set of targets. That operation is performed by rule `generators.construct` and the used algorithm is described below.

#### Selecting and ranking viable generators

Each generator, in addition to target types that it can produce, have attribute that affects its applicability in particular sitiation. Those attributes are:

1. Required properties, which are properties absolutely necessary for the generator to work. For example, generator encapsulating the gcc compiler would have <toolset>gcc as required property.
2. Optional properties, which increase the generators suitability for a particual build.

Generator's required and optional properties may not include either free or incidental properties. (Allowing this would greatly complicate caching targets).

When trying to construct a target, the first step is to select all possible generators for the requested target type, which required properties are a subset of requested properties. Generators that were already selected up the call stack are excluded. In addition, if any composing generators were selected up the call stack, all other composing generators are ignored (TODO: define composing generators). The found generators are assigned a rank, which is the number of optional properties present in requested properties. Finally, generators with highest rank are selected for futher processing.

#### Running generators

When generators are selected, each is run to produce a list of created targets. This list might include targets that are not of requested types, because generators create the same targets as some tool, and tool's behaviour is fixed. (Note: should specify that in some cases we actually want extra targets). If generator fails, it returns an empty list. Generator is free to call 'construct' again, to convert sources to the types it can handle. It also can pass modified properties to 'construct'. However, a generator is not allowed to modify any propagated properties, otherwise when actually consuming properties we might discover that the set of propagated properties is different from what was used for building sources.

For all targets that are not of requested types, we try to convert them to requested type, using a second call to `construct`. This is done in order to support transformation sequences where single source file expands to several later. See this message for details.

#### Selecting dependency graph

After all generators are run, it is necessary to decide which of successfull invocation will be taken as final result. At the moment, this is not done. Instead, it is checked whether all successfull generator invocation returned the same target list. Error is issued otherwise.

#### Property adjustment

Because target location is determined by the build system, it is sometimes necessary to adjust properties, in order to not break actions. For example, if there's an action that generates a header, say "a_parser.h", and a source file "a.cpp" which includes that file, we must make everything work as if a_parser.h is generated in the same directory where it would be generated without any subvariants.

Correct property adjustment can be done only after all targets are created, so the approach taken is:

1. When dependency graph is constructed, each action can be assigned a rule for property adjustment.

2. When virtual target is actualized, that rule is run and return the final set of properties. At this stage it can use information of all created virtual targets.

In case of quoted includes, no adjustment can give 100% correct results. If target dirs are not changed by build system, quoted includes are searched in "." and then in include path, while angle includes are searched only in include path. When target dirs are changed, we'd want to make quoted includes to be search in "." then in additional dirs and then in the include path and make angle includes be searched in include path, probably with additional paths added at some position. Unless, include path already has "." as the first element, this is not possible. So, either generated headers should not be included with quotes, or first element of include path should be ".", which essentially erases the difference between quoted and angle includes. Note: the only way to get "." as include path into compiler command line is via verbatim compiler option. In all other case, Boost.Build will convert "." into directory where it occurs.

#### Transformations cache

Under certain conditions, an attempt is made to cache results of transformation search. First, the sources are replaced with targets with special name and the found target list is stored. Later, when properties, requested type, and source type are the same, the store target list is retrieved and cloned, with appropriate change in names.