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## 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 b2 anywhere in the project tree will automatically find the build system.

The default `bootstrap.jam`, after loading some standard definitions, loads two `site-config.jam` and `user-config.jam`.

### 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.

`preprocessed`

  Creates an preprocessed source file. The arguments follow the common syntax.

`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 Subversion 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.

`always`

  The `always` funciton takes a single parameter—a list of metatarget names. The top-level targets produced by the named metatargets will be always considered out of date. Consider this example:

      exe hello : hello.cpp ;
      exe bye : bye.cpp ;
      always hello ;

  If a build of `hello` is requested, then the binary will always be relinked. The object files will not be recompiled, though. Note that if a build of `hello` is not requested, for example you specify just `bye` on the command line, `hello` will not be relinked.

`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 b2 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 b2 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

This section documents the features that are built-in into Boost.Build. For features with a fixed set of values, that set is provided, with the default value listed first.

`variant`

  A feature combining 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

  Users can define their own build variants using the `variant` rule from the `common` module.

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

`link`

  Allowed values: `shared`, `static`

  A feature controling how libraries are built.

`runtime-link`

  Allowed values: `shared`, `static`

  Controls if a static or shared C/C++ runtime should be used. There are some restrictions how this feature can be used, for example on some compilers an application using static runtime should not use shared libraries at all, and on some compilers, mixing static and shared runtime requires extreme care. Check your compiler documentation for more details.

`threading`

  Allowed values: `single`, `multi`

  Controls if the project should be built in multi-threaded mode. This feature does not necessary change code generation in the compiler, but it causes the compiler to link to additional or different runtime libraries, and define additional preprocessor symbols (for example, `_MT` on Windows and `_REENTRANT` on Linux). How those symbols affect the compiled code depends on the code itself.

`source`
  The `<source>X` feature has the same effect on building a target as putting X in the list of sources. It is 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.
`implicit-dependency`
  Indicates that the target named by the value of this feature may produce files that are included by the sources of the target being declared. See the section called “Generated headers” for more information.
`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 do not 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 is 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 is both the C and C++ compilers, for `cxxflags` that is the C++ compiler and for `linkflags` that is the linker. The features are handy when you are trying to do something special that cannot be achieved by a higher-level feature in Boost.Build.
`include`
  Specifies an additional include path that is to be passed to C and C++ compilers.
`define`
  Specifies an preprocessor symbol that should be defined on the command line. You may either specify just the symbol, which will be defined without any value, or both the symbol and the value, separated by equal sign.
`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 you 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 an 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.

`target-os`

  The operating system for which the code is to be generated. The compiler you used should be the compiler for that operating system. This option causes Boost.Build to use naming conventions suitable for that operating system, and adjust build process accordingly. For example, with gcc, it controls if import libraries are produced for shared libraries or not.

  The complete list of possible values for this feature is: aix, bsd, cygwin, darwin, freebsd, hpux, iphone, linux, netbsd, openbsd, osf, qnx, qnxnto, sgi, solaris, unix, unixware, windows.

  See the section called “Cross-compilation” for details of crosscompilation

`architecture`

  The `architecture` features specifies the general processor familty to generate code for.

`instruction-set`

  Allowed values: depend on the used toolset.

  The `instruction-set` specifies for which specific instruction set the code should be generated. The code in general might not run on processors with older/different instruction sets.

  While Boost.Build allows a large set of possible values for this features, whether a given value works depends on which compiler you use. Please see the section called “C++ Compilers” for details.

`address-model`

  Allowed values: `32`, `64`.

  The `address-model` specifies if 32-bit or 64-bit code should be generated by the compiler. Whether this feature works depends on the used compiler, its version, how the compiler is configured, and the values of the `architecture` `instruction-set` features. Please see the section called “C++ Compilers” for details.

`c++-template-depth`

  Allowed values: Any positive integer.

  This feature allows configuring a C++ compiler with the maximal template instantiation depth parameter. Specific toolsets may or may not provide support for this feature depending on whether their compilers provide a corresponding command-line option.

  Note: Due to some internal details in the current Boost Build implementation it is not possible to have features whose valid values are all positive integer. As a workaround a large set of allowed values has been defined for this feature and, if a different one is needed, user can easily add it by calling the feature.extend rule.

`embed-manifest`

  Allowed values: on, off.

  This feature is specific to the msvc toolset (see the section called “Microsoft Visual C++”), and controls whether the manifest files should be embedded inside executables and shared libraries, or placed alongside them. This feature corresponds to the IDE option found in the project settings dialog, under Configuration Properties → Manifest Tool → Input and Output → Embed manifest.

### 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 the tool's configuration. This is done by calling the `using` rule, typically in your `user-config.jam`, 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 are 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. The name of support module for compiler is also the value for the `toolset` feature that can be used to explicitly request that compiler.

##### GNU C++

The `gcc` module supports the GNU C++ compiler on Linux, a number of Unix-like system including SunOS and on Windows (either Cygwin or MinGW). On Mac OSX, it is recommended to use system gcc, see the section called “Apple Darwin gcc”.

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 is 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.

In order to compile 64-bit applications, you have to specify `address-model=64`, and the `instruction-set` feature should refer to a 64 bit processor. Currently, those include `nocona`, `opteron`, `athlon64` and `athlon-fx`.

##### Apple Darwin gcc

The `darwin` module supports the version of gcc that is modified and provided by Apple. The configuration is essentially identical to that of the gcc module.

The darwin toolset can generate so called "fat" binaries—binaries that can run support more than one architecture, or address mode. To build a binary that can run both on Intel and PowerPC processors, specify `architecture=combined`. To build a binary that can run both in 32-bit and 64-bit modes, specify `address-model=32_64`. If you specify both of those properties, a "4-way" fat binary will be generated.

##### 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 2010—10.0

- Visual Studio 2008—9.0

- 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.

`assembler`

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

`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.

`compiler-filter`

  Command through which to pipe the output of running the compiler. For example to pass the output to STLfilt.

`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.

`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.

`mc-compiler`

  The command that compiles Microsoft message catalog files. If not specified, mc 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.

`setup`

  The filename of the global environment setup script to run before invoking any of the tools defined in this toolset. Will not be used in case a target platform specific script has been explicitly specified for the current target platform. Used setup script will be passed the target platform identifier (x86, x86_amd64, x86_ia64, amd64 or ia64) as a arameter. If not specified a default script is chosen based on the used compiler binary, e.g. vcvars32.bat or vsvars32.bat.

`setup-amd64`, `setup-i386`, `setup-ia64`

  The filename of the target platform specific environment setup script to run before invoking any of the tools defined in this toolset. If not specified the global environment setup script is used.

###### 64-bit support

Starting with version 8.0, Microsoft Visual Studio can generate binaries for 64-bit processor, both 64-bit flavours of x86 (codenamed AMD64/EM64T), and Itanium (codenamed IA64). In addition, compilers that are itself run in 64-bit mode, for better performance, are provided. The complete list of compiler configurations are as follows (we abbreviate AMD64/EM64T to just AMD64):

- 32-bit x86 host, 32-bit x86 target

- 32-bit x86 host, 64-bit AMD64 target

- 32-bit x86 host, 64-bit IA64 target

- 64-bit AMD64 host, 64-bit AMD64 target

- 64-bit IA64 host, 64-bit IA64 target

The 32-bit host compilers can be always used, even on 64-bit Windows. On the contrary, 64-bit host compilers require both 64-bit host processor and 64-bit Windows, but can be faster. By default, only 32-bit host, 32-bit target compiler is installed, and additional compilers need to be installed explicitly.

To use 64-bit compilation you should:

1. Configure you compiler as usual. If you provide a path to the compiler explicitly, provide the path to the 32-bit compiler. If you try to specify the path to any of 64-bit compilers, configuration will not work.

2. When compiling, use `address-model=64`, to generate AMD64 code.

3. To generate IA64 code, use `architecture=ia64`

The (AMD64 host, AMD64 target) compiler will be used automatically when you are generating AMD64 code and are running 64-bit Windows on AMD64. The (IA64 host, IA64 target) compiler will never be used, since nobody has an IA64 machine to test.

It is believed that AMD64 and EM64T targets are essentially compatible. The compiler options `/favor:AMD64` and `/favor:EM64T`, which are accepted only by AMD64 targeting compilers, cause the generated code to be tuned to a specific flavor of 64-bit x86. Boost.Build will make use of those options depending on the value of the`instruction-set` feature.

##### 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 is 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 : [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, the aCC 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.

##### 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 is 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 dmc 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.

Starting with Sun Studio 12, you can create 64-bit applications by using the `address-model=64` property.

##### 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.

#### Third-party libraries

Boost.Build provides special support for some third-party C++ libraries, documented below.

##### STLport library

The STLport library is an alternative implementation of C++ runtime library. Boost.Build supports using that library on Windows platfrom. Linux is hampered by different naming of libraries in each STLport version and is not officially supported.

Before using STLport, you need to configure it in `user-config.jam` using the following syntax:

    using stlport : [version] : header-path : [library-path] ;

Where `version` is the version of STLport, for example `5.1.4`, `headers` is the location where STLport headers can be found, and `libraries` is the location where STLport libraries can be found. The version should always be provided, and the library path should be provided if you're using STLport's implementation of iostreams. Note that STLport 5.* always uses its own iostream implementation, so the library path is required.

When STLport is configured, you can build with STLport by requesting `stdlib=stlport` on the command line.

##### zlib

Provides support for the zlib library. zlib can be configured either to use precompiled binaries or to build the library from source.

zlib can be initialized using the following syntax

    using zlib : [version] : [options] : [condition] : [is-default] ;

Options for using a prebuilt library:

`search`

  The directory containing the zlib binaries.

`name`

  Overrides the default library name.

`include`

  The directory containing the zlib headers.

If none of these options is specified, then the environmental variables ZLIB_LIBRARY_PATH, ZLIB_NAME, and ZLIB_INCLUDE will be used instead.

Options for building zlib from source:

`source`

  The zlib source directory. Defaults to the environmental variable ZLIB_SOURCE.

`tag`

  Sets the tag property to adjust the file name of the library. Ignored when using precompiled binaries.

`build-name`

  The base name to use for the compiled library. Ignored when using precompiled binaries.

Examples:

    # Find zlib in the default system location
    using zlib ;
    # Build zlib from source
    using zlib : 1.2.7 : <source>/home/steven/zlib-1.2.7 ;
    # Find zlib in /usr/local
    using zlib : 1.2.7 : <include>/usr/local/include <search>/usr/local/lib ;
    # Build zlib from source for msvc and find
    # prebuilt binaries for gcc.
    using zlib : 1.2.7 : <source>C:/Devel/src/zlib-1.2.7 : <toolset>msvc ;
    using zlib : 1.2.7 : : <toolset>gcc ;

#### Documentation tools

Boost.Build support for the Boost documentation tools is documented below.

##### xsltproc

To use xsltproc, you first need to configure it using the following syntax:

    using xsltproc : [xsltproc] ;

Where `xsltproc` is the xsltproc executable. If `xsltproc` is not specified, and the variable XSLTPROC is set, the value of XSLTPROC will be used. Otherwise, xsltproc will be searched for in PATH.

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

`xsl:param`

  Values should have the form `name`=`value`

`xsl:path`

  Sets an additional search path for xi:include elements.

`catalog`

  A catalog file used to rewrite remote URL's to a local copy.

The xsltproc module provides the following rules. Note that these operate on jam targets and are intended to be used by another toolset, such as boostbook, rather than directly by users.

`xslt`

      rule xslt ( target : source stylesheet : properties * )

  Runs xsltproc to create a single output file.

`xslt-dir`

      rule xslt-dir ( target : source stylesheet : properties * : dirname )

  Runs xsltproc to create multiple outputs in a directory. `dirname` is unused, but exists for historical reasons. The output directory is determined from the target.

##### boostbook

To use boostbook, you first need to configure it using the following syntax:

    using boostbook : [docbook-xsl-dir] : [docbook-dtd-dir] : [boostbook-dir] ;

`docbook-xsl-dir` is the DocBook XSL stylesheet directory. If not provided, we use DOCBOOK_XSL_DIR from the environment (if available) or look in standard locations. Otherwise, we let the XML processor load the stylesheets remotely.

`docbook-dtd-dir` is the DocBook DTD directory. If not provided, we use DOCBOOK_DTD_DIR From the environment (if available) or look in standard locations. Otherwise, we let the XML processor load the DTD remotely.

`boostbook-dir` is the BoostBook directory with the DTD and XSL subdirs.

The boostbook module depends on xsltproc. For pdf or ps output, it also depends on fop.

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

`format`

  Allowed values: `html`, `xhtml`, `htmlhelp`, `onehtml`, `man`, `pdf`, `ps`, `docbook`, `fo`, `tests`.

  The `format` feature determines the type of output produced by the boostbook rule.

The boostbook module defines a rule for creating a target following the common syntax.

`boostbook`

      rule boostbook ( target-name : sources * : requirements * : default-build * )

  Creates a boostbook target.

##### doxygen

To use doxygen, you first need to configure it using the following syntax:

    using doxygen : [name] ;

`name` is the doxygen command. If it is not specified, it will be found in the PATH.

The doxygen module depends on the boostbook module when generating BoostBook XML.

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

`doxygen:param`

  All the values of `doxygen:param` are added to the doxyfile.

`prefix`

  Specifies the common prefix of all headers when generating BoostBook XML. Everything before this will be stripped off.

`reftitle`

  Specifies the title of the library-reference section, when generating BoostBook XML.

`doxygen:xml-imagedir`

  When generating BoostBook XML, specifies the directory in which to place the images generated from LaTex formulae.

  | Warning
  The path is interpreted relative to the current working directory, not relative to the Jamfile. This is necessary to match the behavior of BoostBook.

The doxygen module defines a rule for creating a target following the common syntax.

`doxygen`

      rule doxygen ( target : sources * : requirements * : default-build * : usage-requirements * )

  Creates a doxygen target. If the target name ends with .html, then this will generate an html directory. Otherwise it will generate BoostBook XML.

##### quickbook

The quickbook module provides a generator to convert from Quickbook to BoostBook XML.

To use quickbook, you first need to configure it using the following syntax:

    using quickbook : [command] ;

`command` is the quickbook executable. If it is not specified, Boost.Build will compile it from source. If it is unable to find the source it will search for a quickbook executable in PATH.

##### fop

The fop module provides generators to convert from XSL formatting objects to Postscript and PDF.

To use fop, you first need to configure it using the following syntax:

    using fop : [fop-command] : [java-home] : [java] ;

`fop-command` is the command to run fop. If it is not specified, Boost.Build will search for it in PATH and FOP_HOME.

Either `java-home` or `java` can be used to specify where to find java.

### Builtin modules

This section describes the modules that are provided by Boost.Build. The import rule allows rules from one module to be used in another module or Jamfile.

#### modules

The `modules` module defines basic functionality for handling modules.

A module defines a number of rules that can be used in other modules. Modules can contain code at the top level to initialize the module. This code is executed the first time the module is loaded.

| Note
A Jamfile is a special kind of module which is managed by the build system. Although they cannot be loaded directly by users, the other features of modules are still useful for Jamfiles.

Each module has its own namespaces for variables and rules. If two modules A and B both use a variable named X, each one gets its own copy of X. They won't interfere with each other in any way. Similarly, importing rules into one module has no effect on any other module.

Every module has two special variables. `$(__file__)` contains the name of the file that the module was loaded from and `$(__name__)` contains the name of the module.

| Note
`$(__file__)` does not contain the full path to the file. If you need this, use `modules.binding`.

1. `rule binding ( module-name )`

   Returns the filesystem binding of the given module.

   For example, a module can get its own location with:

       me = [ modules.binding $(__name__) ] ;

2. `rule poke ( module-name ? : variables + : value * )`

   Sets the module-local value of a variable.

   For example, to set a variable in the global module:

       modules.poke : ZLIB_INCLUDE : /usr/local/include ;

3. `rule peek ( module-name ? : variables + )`

   Returns the module-local value of a variable.

   For example, to read a variable from the global module:

       local ZLIB_INCLUDE = [ modules.peek : ZLIB_INCLUDE ] ;

4. `rule call-in ( module-name ? : rule-name args * : * )`

   Call the given rule locally in the given module. Use this for rules accepting rule names as arguments, so that the passed rule may be invoked in the context of the rule's caller (for example, if the rule accesses module globals or is a local rule).

   | Note
   rules called this way may accept at most 8 parameters.

   Example:

       rule filter ( f : values * )
       {
           local m = [ CALLER_MODULE ] ;
           local result ;
           for v in $(values)
           {
               if [ modules.call-in $(m) : $(f) $(v) ]
               {
                   result += $(v) ;
               }
           }
           return result ;
       }

5. `rule load ( module-name : filename ? : search * )`

   Load the indicated module if it is not already loaded.

   `module-name`

     Name of module to load.

   `filename`

     (partial) path to file; Defaults to `$(module-name).jam`

   `search`

     Directories in which to search for filename. Defaults to `$(BOOST_BUILD_PATH)`.

6. `rule import ( module-names + : rules-opt * : rename-opt * )`

   Load the indicated module and import rule names into the current module. Any members of `rules-opt` will be available without qualification in the caller's module. Any members of `rename-opt` will be taken as the names of the rules in the caller's module, in place of the names they have in the imported module. If `rules-opt = '*'`, all rules from the indicated module are imported into the caller's module. If `rename-opt` is supplied, it must have the same number of elements as `rules-opt`.

   | Note
   The `import` rule is available without qualification in all modules.

   Examples:

       import path ;
       import path : * ;
       import path : join ;
       import path : native make : native-path make-path ;

7. `rule clone-rules ( source-module target-module )`

   Define exported copies in `$(target-module)` of all rules exported from `$(source-module)`. Also make them available in the global module with qualification, so that it is just as though the rules were defined originally in `$(target-module)`.

#### path

Performs various path manipulations. Paths are always in a 'normalized' representation. In it, a path may be either:

- `'.'`, or

- `['/'] [ ( '..' '/' )* (token '/')* token ]`

In plain english, a path can be rooted, `'..'` elements are allowed only at the beginning, and it never ends in slash, except for the path consisting of slash only.

1. `rule make ( native )`

   Converts the native path into normalized form.

2. `rule native ( path )`

   Builds the native representation of the path.

3. `rule is-rooted ( path )`

   Tests if a path is rooted.

4. `rule has-parent ( path )`

   Tests if a path has a parent.

5. `rule basename ( path )`

   Returns the path without any directory components.

6. `rule parent ( path )`

   Returns the parent directory of the path. If no parent exists, an error is issued.

7. `rule reverse ( path )`

   Returns `path2` such that `[ join path path2 ] = "."`. The path may not contain `".."` element or be rooted.

8. `rule join ( elements + )`

   Concatenates the passed path elements. Generates an error if any element other than the first one is rooted. Skips any empty or undefined path elements.

9. `rule root ( path root )`

   If `path` is relative, it is rooted at `root`. Otherwise, it is unchanged.

10. `rule pwd ( )`

    Returns the current working directory.

11. `rule glob ( dirs * : patterns + : exclude-patterns * )`

    Returns the list of files matching the given pattern in the specified directory. Both directories and patterns are supplied as portable paths. Each pattern should be a non-absolute path, and can't contain "." or ".." elements. Each slash separated element of a pattern can contain the following special characters:

    - '?' matches any character

    - '*' matches an arbitrary number of characters

    A file $(d)/e1/e2/e3 (where 'd' is in $(dirs)) matches the pattern p1/p2/p3 if and only if e1 matches p1, e2 matches p2 and so on. For example:

        [ glob . : *.cpp ]
        [ glob . : */build/Jamfile ]

12. `rule glob-tree ( roots * : patterns + : exclude-patterns * )`

    Recursive version of glob. Builds the glob of files while also searching in the subdirectories of the given roots. An optional set of exclusion patterns will filter out the matching entries from the result. The exclusions also apply to the subdirectory scanning, such that directories that match the exclusion patterns will not be searched.

13. `rule exists ( file )`

    Returns true if the specified file exists.

14. `rule all-parents ( path : upper_limit ? : cwd ? )`

    Find out the absolute name of path and return the list of all the parents, starting with the immediate one. Parents are returned as relative names. If `upper_limit` is specified, directories above it will be pruned.

15. `rule glob-in-parents ( dir : patterns + : upper-limit ? )`

    Search for `patterns` in parent directories of `dir`, up to and including `upper_limit`, if it is specified, or till the filesystem root otherwise.

16. `rule relative ( child parent : no-error ? )`

    Assuming `child` is a subdirectory of `parent`, return the relative path from `parent` to `child`.

17. `rule relative-to ( path1 path2 )`

    Returns the minimal path to path2 that is relative path1.

18. `rule programs-path ( )`

    Returns the list of paths which are used by the operating system for looking up programs.

19. `rule makedirs ( path )`

    Creates a directory and all parent directories that do not already exist.

#### type

Deals with target type declaration and defines target class which supports typed targets.

1. `rule register ( type : suffixes * : base-type ? )`

   Registers a target type, possible derived from a `base-type`. Providing a list of suffixes here is a shortcut for separately calling the register-suffixes rule with the given suffixes and the set-generated-target-suffix rule with the first given suffix.

2. `rule register-suffixes ( suffixes + : type )`

   Specifies that files with suffix from `suffixes` be recognized as targets of type `type`. Issues an error if a different type is already specified for any of the suffixes.

3. `rule registered ( type )`

   Returns true iff type has been registered.

4. `rule validate ( type )`

   Issues an error if `type` is unknown.

5. `rule set-scanner ( type : scanner )`

   Sets a scanner class that will be used for this type.

6. `rule get-scanner ( type : property-set )`

   Returns a scanner instance appropriate to `type` and `property-set`.

7. `rule base ( type )`

   Returns a base type for the given type or nothing in case the given type is not derived.

8. `rule all-bases ( type )`

   Returns the given type and all of its base types in order of their distance from type.

9. `rule all-derived ( type )`

   Returns the given type and all of its derived types in order of their distance from type.

10. `rule is-derived ( type base )`

    Returns true if `type` is equal to `base` or has `base` as its direct or indirect base.

11. `rule set-generated-target-suffix ( type : properties * : suffix )`

    Sets a file suffix to be used when generating a target of `type` with the specified properties. Can be called with no properties if no suffix has already been specified for the `type`. The `suffix` parameter can be an empty string (`""`) to indicate that no suffix should be used.

    Note that this does not cause files with `suffix` to be automatically recognized as being of `type`. Two different types can use the same suffix for their generated files but only one type can be auto-detected for a file with that suffix. User should explicitly specify which one using the register-suffixes rule.

12. `rule change-generated-target-suffix ( type : properties * : suffix )`

    Change the suffix previously registered for this type/properties combination. If suffix is not yet specified, sets it.

13. `rule generated-target-suffix ( type : property-set )`

    Returns the suffix used when generating a file of `type` with the given properties.

14. `rule set-generated-target-prefix ( type : properties * : prefix )`

    Sets a target prefix that should be used when generating targets of `type` with the specified properties. Can be called with empty properties if no prefix for `type` has been specified yet.

    The `prefix` parameter can be empty string (`""`) to indicate that no prefix should be used.

    Usage example: library names use the `"lib"` prefix on unix.

15. `rule change-generated-target-prefix ( type : properties * : prefix )`

    Change the prefix previously registered for this type/properties combination. If prefix is not yet specified, sets it.

16. `rule generated-target-prefix ( type : property-set )`

    Returns the prefix used when generating a file of `type` with the given properties.

17. `rule type ( filename )`

    Returns file type given its name. If there are several dots in filename, tries each suffix. E.g. for name of "file.so.1.2" suffixes "2", "1", and "so" will be tried.

### Builtin classes

#### Class abstract-target

Base class for all abstract targets.

    class abstract-target {
        rule __init__ ( name : project )
        rule name ( )
        rule project ( )
        rule location ( )
        rule full-name ( )
        rule generate ( property-set )
    }

Classes derived from abstract-target:

- project-target
- main-target
- basic-target

1. `rule __init__ ( name : project )`

   `name`

     The name of the target in the Jamfile.

   `project`

     The project to which this target belongs.

2. `rule name ( )`

   Returns the name of this target.

3. `rule project ( )`

   Returns the project for this target.

4. `rule location ( )`

   Returns the location where the target was declared.

5. `rule full-name ( )`

   Returns a user-readable name for this target.

6. `rule generate ( property-set )`

   Generates virtual targets for this abstract target using the specified properties, unless a different value of some feature is required by the target. This is an abstract method which must be overriden by derived classes.

   On success, returns:

   - a property-set with the usage requirements to be applied to dependents
   - a list of produced virtual targets, which may be empty.

   If `property-set` is empty, performs the default build of this target, in a way specific to the derived class.

#### Class project-target

    class project-target : abstract-target {
        rule generate ( property-set )
        rule build-dir ( )
        rule main-target ( name )
        rule has-main-target ( name )
        rule find ( id : no-error ? )

        # Methods inherited from abstract-target
        rule name ( )
        rule project ( )
        rule location ( )
        rule full-name ( )
    }

This class has the following responsibilities:

- Maintaining a list of main targets in this project and building them.

1. `rule generate ( property-set )`

   Overrides abstract-target.generate. Generates virtual targets for all the targets contained in this project.

   On success, returns:

   - a property-set with the usage requirements to be applied to dependents
   - a list of produced virtual targets, which may be empty.

2. `rule build-dir ( )`

   Returns the root build directory of the project.

3. `rule main-target ( name )`

   Returns a main-target class instance corresponding to `name`. Can only be called after the project has been fully loaded.

4. `rule has-main-target ( name )`

   Returns whether a main-target with the specified name exists. Can only be called after the project has been fully loaded.

5. `rule find ( id : no-error ? )`

   Find and return the target with the specified id, treated relative to self. Id may specify either a target or a file name with the target taking priority. May report an error or return nothing if the target is not found depending on the `no-error` parameter.

#### Class main-target

    class main-target : abstract-target {
        rule generate ( property-set )

        # Methods inherited from abstract-target
        rule name ( )
        rule project ( )
        rule location ( )
        rule full-name ( )
    }

A main-target represents a named top-level target in a Jamfile.

1. `rule generate ( property-set )`

   Overrides abstract-target.generate. Select an alternative for this main target, by finding all alternatives whose requirements are satisfied by `property-set` and picking the one with the longest requirements set. Returns the result of calling generate on that alternative.

   On success, returns:

   - a property-set with the usage requirements to be applied to dependents
   - a list of produced virtual targets, which may be empty.

#### Class basic-target

    class basic-target : abstract-target {
        rule __init__ ( name : project : sources * : requirements * : default-build * : usage-requirements * )
        rule generate ( property-set )
        rule construct ( name : source-targets * : property-set )

        # Methods inherited from abstract-target
        rule name ( )
        rule project ( )
        rule location ( )
        rule full-name ( )
    }

Implements the most standard way of constructing main target alternative from sources. Allows sources to be either files or other main targets and handles generation of those dependency targets.

1. `rule __init__ ( name : project : sources * : requirements * : default-build * : usage-requirements * )`

   `name`

     The name of the target

   `project`

     The project in which the target is declared.

2. `rule generate ( property-set )`

   Overrides abstract-target.generate. Determines final build properties, generates sources, and calls construct. This method should not be overridden.

   On success, returns:

   - a property-set with the usage requirements to be applied to dependents
   - a list of produced virtual targets, which may be empty.

3. `rule construct ( name : source-targets * : property-set )`

   Constructs virtual targets for this abstract target. Returns a usage-requirements property-set and a list of virtual targets. Should be overriden in derived classes.

#### Class typed-target

    class typed-target : basic-target {
        rule __init__ ( name : project : type : sources * : requirements * : default-build * : usage-requirements * )
        rule type ( )
        rule construct ( name : source-targets * : property-set )

        # Methods inherited from abstract-target
        rule name ( )
        rule project ( )
        rule location ( )
        rule full-name ( )

        # Methods inherited from basic-target
        rule generate ( property-set )
      }

typed-target is the most common kind of target alternative. Rules for creating typed targets are defined automatically for each type.

1. `rule __init__ ( name : project : type : sources * : requirements * : default-build * : usage-requirements * )`

   `name`

     The name of the target

   `project`

     The project in which the target is declared.

   `type`

     The type of the target.

2. `rule type ( )`

   Returns the type of the target.

3. `rule construct ( name : source-targets * : property-set )`

   Implements basic-target.construct. Attempts to create a target of the correct type using generators appropriate for the given property-set. Returns a property-set containing the usage requirements and a list of virtual targets.

   | Note
   This function is invoked automatically by basic-target.generate and should not be called directly by users.

#### Class property-set

Class for storing a set of properties.

    class property-set {
        rule raw ( )
        rule str ( )
        rule propagated ( )
        rule add ( ps )
        rule add-raw ( properties * )
        rule refine ( ps )
        rule get ( feature )
    }

There is 1<->1 correspondence between identity and value. No two instances of the class are equal. To maintain this property, the 'property-set.create' rule should be used to create new instances. Instances are immutable.

1. `rule raw ( )`

   Returns a Jam list of the stored properties.

2. `rule str ( )`

   Returns the string repesentation of the stored properties.

3. `rule propagated ( )`

   Returns a property-set containing all the propagated properties in this property-set.

4. `rule add ( ps )`

   Returns a new property-set containing the union of the properties in this property-set and in `ps`.

   | Note
   If `ps` contains non-free properties that should override the values in this object, use refine instead.

5. `rule add-raw ( properties * )`

   Link add, except that it takes a list of properties instead of a property-set.

6. `rule refine ( ps )`

   Refines properties by overriding any non-free and non-conditional properties for which a different value is specified in `ps`. Returns the resulting property-set.

7. `rule get ( feature )`

   Returns all the values of `feature`.

### 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 properties 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 ;

#### Target Paths

Several factors determine the location of a concrete file target. All files in a project are built under the directory bin unless this is overriden by the build-dir project attribute. Under bin is a path that depends on the properties used to build each target. This path is uniquely determined by all non-free, non-incidental properties. For example, given a property set containing: `<toolset>gcc <toolset-gcc:version>4.6.1 <variant>debug <warnings>all <define>_DEBUG <include>/usr/local/include <link>static`, the path will be gcc-4.6.1/debug/link-static. <warnings> is an incidental feature and <define> and <include> are free features, so they do not affect the path.

Sometimes the paths produced by Boost.Build can become excessively long. There are a couple of command line options that can help with this. --abbreviate-paths reduces each element to no more than five characters. For example, link-static becomes lnk-sttc. The --hash option reduces the path to a single directory using an MD5 hash.

There are two features that affect the build directory. The <location> feature completely overrides the default build directory. For example,

    exe a : a.cpp : <location>. ;

builds all the files produced by `a` in the directory of the Jamfile. This is generally discouraged, as it precludes variant builds.

The <location-prefix> feature adds a prefix to the path, under the project's build directory. For example,

    exe a : a.cpp : <location-prefix>subdir ;

will create the files for `a` in bin/subdir/gcc-4.6.1/debug

### 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 the section called “Target Paths”.

- 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

  Normally a feature only generates a subvariant directory when its value differs from its default value, leading to an assymmetric subvariant directory structure for certain values of the feature. A symmetric feature 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 a dependency feature is 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.

#### 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.