# User documentation

This section will provide the information necessary to create your own projects using Boost.Build. The information provided here is relatively high-level, and as well as the on-line help system must be used to obtain low-level documentation (see ).

Boost.Build actually consists of two parts - Boost.Jam, a build engine with its own interpreted language, and Boost.Build itself, implemented in Boost.Jam's language. The chain of events when you type `bjam` on the command line is:

1. Boost.Jam tries to find Boost.Build and loads the top-level module. The exact process is described in

2. The top-level module loads user-defined configuration files, `user-config.jam` and `site-config.jam`, which define available toolsets.

3. The Jamfile in the current directory is read. That in turn might cause reading of further Jamfiles. As a result, a tree of projects is created, with targets inside projects.

4. Finally, using the build request specified on the command line, Boost.Build decides which targets should be built, and how. That information is passed back to Boost.Jam, which takes care of actually running commands.

So, to be able to successfully use Boost.Build, you need to know only three things:

- How to configure Boost.Build

- How to write Jamfiles

- How the build process works

- Some Basics about the Boost.Jam language. See the Boost.Jam and Classic Jam documentation.

## Configuration

The Boost.Build configuration is specified in the file `user-config.jam`. You can edit the one that comes with Boost.Build, or create a copy in your home directory and edit that. (See the reference for the exact search paths.) The primary function of that file is to declare which compilers and other tools are available. The simplest syntax to configure a tool is:

    using tool-name ;

The using rule is given a name of tool, and will make that tool available to Boost.Build. For example, `using gcc ;` will make the gcc compiler available.

Since nothing but a tool name is specified, Boost.Build will pick some default settings. For example, it will use the `gcc` executable found in the `PATH`, or look in some known installation locations. In most cases, this strategy works automatically. In case you have several versions of a compiler, it's installed in some unusual location, or you need to tweak its configuration, you'll need to pass additional parameters to the using rule. The parameters to using can be different for each tool. You can obtain specific documentation for any tool's configuration parameters by invoking

    bjam --help tool-name.init

That said, for all the compiler toolsets Boost.Build supports out-of-the-box, the list of parameters to using is the same: toolset-name, version, invocation-command, and options.

The version parameter identifies the toolset version, in case you have several installed. It can have any form you like, but it's recommended that you use a numeric identifier like `7.1`.

The invocation-command parameter is the command that must be executed to run the compiler. This parameter can usually be omitted if the compiler executable

- has its “usual name” and is in the `PATH`, or

- was installed in a standard “installation directory”, or

- can be found through a global mechanism like the Windows registry.

For example:

    using msvc : 7.1 ;
    using gcc ;

If the compiler can be found in the `PATH` but only by a nonstandard name, you can just supply that name:

    using gcc : : g++-3.2 ;

Otherwise, it might be necessary to supply the complete path to the compiler executable:

    using msvc : : Z:/Programs/Microsoft Visual Studio/vc98/bin/cl ;

Some Boost.Build toolsets will use that path to take additional actions required before invoking the compiler, such as calling vendor-supplied scripts to set up its required environment variables.

To configure several versions of a toolset, simply invoke the using rule multiple times:

    using gcc : 3.3 ;
    using gcc : 3.4 : g++-3.4 ;
    using gcc : 3.2 : g++-3.2 ;

Note that in the first call to using, the compiler found in the `PATH` will be used, and there's no need to explicitly specify the command.

As shown above, both the version and invocation-command parameters are optional, but there's an important restriction: if you configure the same toolset more than once, you must pass the version parameter every time. For example, the following is not allowed:

    using gcc ;
    using gcc : 3.4 : g++-3.4 ;

because the first using call does not specify a version.

The options parameter is used to fine-tune the configuration. All of Boost.Build's standard compiler toolsets accept properties of the four builtin features `cflags`, `cxxflags`, `compileflags` and `linkflags` as options specifying flags that will be always passed to the corresponding tools. Values of the `cflags` feature are passed directly to the C compiler, values of the `cxxflags` feature are passed directly to the C++ compiler, and values of the `compileflags` feature are passed to both. For example, to configure a `gcc` toolset so that it always generates 64-bit code you could write:

    using gcc : 3.4 : : <compileflags>-m64 <linkflags>-m64 ;

## Writing Jamfiles

### Overview

Jamfiles are the thing that is most important to the user, bacause they declare the targets that should be built. Jamfiles are also used for organizing targets— each Jamfile is a separate project that can be built independently from the other projects.

Jamfiles mostly contain calls to Boost.Build functions that do all the work, specifically:

- declare main targets

- define project properties

- do various other things

### Main targets

A Main target is a user-defined named entity that can be built, for example an executable file. Declaring a main target is usually done using one of the main target rules described in . The user can also declare custom main target rules as shown in .

Most main target rules in Boost.Build can be invoked with a common syntax:

    rule-name main-target-name
        : sources...
        : requirements...
        : default-build...
        : usage-requirements...
        ;

- main-target-name is the name used to request the target on command line and to use it from other main targets. A main target name may contain alphanumeric characters, dashes (‘`-`’), and underscores (‘`_`’).

- sources is the list of source files and other main targets that must be combined.

- requirements is the list of properties that must always be present when this main target is built.

- default-build is the list of properties that will be used unless some other value of the same feature is already specified, e.g. on the command line or by propogation from a dependent target.

- usage-requirements is the list of properties that will be propagated to all main targets that use this one, i.e. to all its dependents.

Some main target rules have a shorter list of parameters; consult their documentation for details.

Note that the actual requirements, default-build and usage-requirements attributes for a target are obtained by combining the explicitly specified ones with those specified for the project where a target is declared.

The list of sources specifies what should be processed to get the resulting targets. Most of the time, it's just a list of files. Sometimes, you'll want to automatically construct the list of source files rather than having to spell it out manually, in which case you can use the glob rule. Here are two examples:

    exe a : a.cpp ;           # a.cpp is the only source file
    exe b : [ glob *.cpp ] ;  # all .cpp files in this directory are sources

Unless you specify a files with absolute path, the name is considered relative to the source directory -- which is typically the directory where the Jamfile is located, but can be changed as described in .

The list of sources can also refer to other main targets. Targets in the same project can be referred to by name, while targets in other projects need to be qualified with a directory or a symbolic project name. For example:

    lib helper : helper.cpp ;
    exe a : a.cpp helper ;
    exe b : b.cpp ..//utils ;
    exe c : c.cpp /boost/program_options//program_options ;

The first exe uses the library defined in the same project. The second one uses some target (most likely library) defined by Jamfile one level higher. Finally, the third target uses some C++ Boost library, referring to it by its absolute symbolic name. More information about it can be found in and .

Requirements are the properties that should always be present when building a target. Typically, they are includes and defines:

    exe hello : hello.cpp : <include>/opt/boost <define>MY_DEBUG ;

In special circumstances, other properties can be used, for example if a library can only be built statically, or a file can't be compiled with optimization due to a compiler bug, one can use

    lib util : util.cpp : <link>static ;
    obj main : main.cpp : <optimization>off ;

Sometimes requirements are necessary only for a specific compiler or build variant. Conditional properties can be used in that case:

    lib util : util.cpp : <toolset>msvc:<link>static ;

Whenever `<toolset>msvc` property is in build properties, the `<link>static` property will be included as well. Conditional requirements can be “chained”:

    lib util : util.cpp : <toolset>msvc:<link>static
                          <link>static:<define>STATIC_LINK ;

will set of static link and the `STATIC_LINK` define on the `msvc` toolset.

The `default-build` parameter is a set of properties to be used if the build request does not otherwise specify a value for features in the set. For example:

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

would build a multi-threaded target in unless the user explicitly requests a single-threaded version. The difference between requirements and default-build is that requirements cannot be overriden in any way.

A target of the same name can be declared several times, in which case each declaration is called an alternative. When the target is built, one of the alternatives will be selected and used. Alternatives need not be defined by the same main target rule. For example,

    lib helpers : helpers.hpp ;                 # a header-only library
    alias helpers : helpers.lib : <toolset>msvc ; # except on msvc

The actual commands used to build any given main target can differ greatly from platform to platform. For example, you might have different lists of sources for different compilers, or different options for those compilers. Two approaches to this are explained in the tutorial.

Sometimes a main target is really needed only by some other main target. For example, a rule that declares a test-suite uses a main target that represent test, but those main targets are rarely needed by themselves.

It is possible to declare a target inline, i.e. the "sources" parameter may include calls to other main rules. For example:

    exe hello : hello.cpp
        [ obj helpers : helpers.cpp : <optimization>off ] ;

Will cause "helpers.cpp" to be always compiled without optimization. When referring to an inline main target, its declared name must be prefixed by its parent target's name and two dots. In the example above, to build only helpers, one should run `bjam hello..helpers`.

### Projects

As mentioned before, targets are grouped into projects, and each Jamfile is a separate project. Projects are useful because they allow us to group related targets together, define properties common to all those targets, and assign a symbolic name to the project that can be used in referring to its targets.

Projects are named using the project rule, which has the following syntax:

    project id : attributes ;

Here, attributes is a sequence of rule arguments, each of which begins with an attribute-name and is followed by any number of build properties. The list of attribute names along with its handling is also shown in the table below. For example, it is possible to write:

    project tennis
        : requirements <threading>multi
        : default-build release
        ;

The possible attributes are listed below.

Project id is a short way to denote a project, as opposed to the Jamfile's pathname. It is a hierarchical path, unrelated to filesystem, such as "boost/thread". Target references make use of project ids to specify a target.

Source location specifies the directory where sources for the project are located.

Project requirements are requirements that apply to all the targets in the projects as well as all subprojects.

Default build is the build request that should be used when no build request is specified explicitly.

The default values for those attributes are given in the table below.

Attribute | Name for the 'project' rule | Default value | Handling by the 'project' rule
--- | --- | --- | ---
Project id | none | none | Assigned from the first parameter of the 'project' rule. It is assumed to denote absolute project id.
Source location | `source-location` | The location of jamfile for the project | Sets to the passed value
Requirements | `requirements` | The parent's requirements | The parent's requirements are refined with the passed requirement and the result is used as the project requirements.
Default build | `default-build` | none | Sets to the passed value
Build directory | `build-dir` | Empty if the parent has no build directory set. Otherwise, the parent's build directory with with the relative path from parent to the current project appended to it. | Sets to the passed value, interpreted as relative to the project's location.

Besides defining projects and main targets, Jamfiles commonly invoke utility rules such as constant and path-constant, which inject a specified Boost.Jam variable setting into this project's Jamfile module and those of all its subprojects. See for a complete description of these utility rules. Jamfiles are regular Boost.Jam source files and Boost.Build modules, so naturally they can contain any kind of Boost.Jam code, including rule definitions.

Each subproject inherits attributes, constants and rules from its parent project, which is defined by the nearest Jamfile in an ancestor directory above the subproject. The top-level project is declared in a file called `Jamroot` rather than `Jamfile`. When loading a project, Boost.Build looks for either `Jamroot` or `Jamfile`. They are handled indentically, except that if the file is called `Jamroot`, the search for a parent project is not performed.

Even when building in a subproject directory, parent project files are always loaded before those of their subprojects, so that every definition made in a parent project is always available to its children. The loading order of any other projects is unspecified. Even if one project refers to another via use-project, or a target reference, no specific order should be assumed.

Giving the root project the special name “`Jamroot`” ensures that Boost.Build won't misinterpret a directory above it as the project root just because the directory contains a Jamfile.

### Jamfile Utility Rules

The following table describes utility rules that can be used in Jamfiles. Detailed information for any of these rules can be obtained by running:

    bjam --help project.rulename

Rule | Semantics
--- | ---
project | Define this project's symbolic ID or attributes.
use-project | Make another project known so that it can be referred to by symbolic ID.
build-project | Cause another project to be built when this one is built.
explicit | State that a target should be built only by explicit request.
glob | Translate a list of shell-style wildcards into a corresponding list of files.
constant | Injects a variable setting into this project's Jamfile module and those of all its subprojects.
path-constant | Injects a variable set to a path value into this project's Jamfile module and those of all its subprojects. If the value is a relative path it will be adjusted for each subproject so that it refers to the same directory.

## The Build Process

When you've described your targets, you want Boost.Build to run the right tools and create the needed targets. This section will describe two things: how you specify what to build, and how the main targets are actually constructed.

The most important thing to note is that in Boost.Build, unlike other build tools, the targets you declare do not correspond to specific files. What you declare in a Jamfile is more like a “metatarget.” Depending on the properties you specify on the command line, each metatarget will produce a set of real targets corresponding to the requested properties. It is quite possible that the same metatarget is built several times with different properties, producing different files.

This means that for Boost.Build, you cannot directly obtain a build variant from a Jamfile. There could be several variants requested by the user, and each target can be built with different properties.

### Build request

The command line specifies which targets to build and with which properties. For example:

    bjam app1 lib1//lib1 toolset=gcc variant=debug optimization=full

would build two targets, "app1" and "lib1//lib1" with the specified properties. You can refer to any targets, using target id and specify arbitrary properties. Some of the properties are very common, and for them the name of the property can be omitted. For example, the above can be written as:

    bjam app1 lib1//lib1 gcc debug optimization=full

The complete syntax, which has some additional shortcuts, is described in .

### Building a main target

When you request, directly or indirectly, a build of a main target with specific requirements, the following steps are made. Some brief explanation is provided, and more details are given in .

1. Applying default build. If the default-build property of a target specifies a value of a feature that is not present in the build request, that value is added.

2. Selecting the main target alternative to use. For each alternative we look how many properties are present both in alternative's requirements, and in build request. The alternative with large number of matching properties is selected.

3. Determining "common" properties. The build request is refined with target's requirements. The conditional properties in requirements are handled as well. Finally, default values of features are added.

4. Building targets referred by the sources list and dependency properties. The list of sources and the properties can refer to other target using target references. For each reference, we take all propagated properties, refine them by explicit properties specified in the target reference, and pass the resulting properties as build request to the other target.

5. Adding the usage requirements produced when building dependencies to the "common" properties. When dependencies are built in the previous step, they return both the set of created "real" targets, and usage requirements. The usage requirements are added to the common properties and the resulting property set will be used for building the current target.

6. Building the target using generators. To convert the sources to the desired type, Boost.Build uses "generators" --- objects that correspond to tools like compilers and linkers. Each generator declares what type of targets it can produce and what type of sources it requires. Using this information, Boost.Build determines which generators must be run to produce a specific target from specific sources. When generators are run, they return the "real" targets.

7. Computing the usage requirements to be returned. The conditional properties in usage requirements are expanded and the result is returned.

### Building a project

Often, a user builds a complete project, not just one main target. In fact, invoking `bjam` without arguments builds the project defined in the current directory.

When a project is built, the build request is passed without modification to all main targets in that project. It's is possible to prevent implicit building of a target in a project with the `explicit` rule:

    explicit hello_test ;

would cause the `hello_test` target to be built only if explicitly requested by the user or by some other target.

The Jamfile for a project can include a number of `build-project` rule calls that specify additional projects to be built.

## Builtin target types

### Programs

Programs are created using the `exe` rule, which follows the common syntax. For example:

    exe hello : hello.cpp some_library.lib /some_project//library
              : <threading>multi
              ;

This will create an executable file from the sources -- in this case, one C++ file, one library file present in the same directory, and another library that is created by Boost.Build. Generally, sources can include C and C++ files, object files and libraries. Boost.Build will automatically try to convert targets of other types.

On Windows, if an application uses dynamic libraries, and both the application and the libraries are built by Boost.Build, its not possible to immediately run the application, because the `PATH` environment variable should include the path to the libraries. It means you have to either add the paths manually, or place the application and the libraries to the same directory, for example using the stage rule.

### Libraries

Libraries are created using the `lib` rule, which follows the common syntax. For example:

    lib helpers : helpers.cpp : <include>boost : : <include>. ;

In the most common case, the `lib` creates a library from the specified sources. Depending on the value of <link> feature the library will be either static or shared. There are two other cases. First is when the library is installed somewhere in compiler's search paths, and should be searched by the compiler (typically, using the `-l` option). The second case is where the library is available as a prebuilt file and the full path is known.

The syntax for these case is given below:

    lib z : : <name>z <search>/home/ghost ;
    lib compress : : <file>/opt/libs/compress.a ;

The `name` property specifies the name that should be passed to the `-l` option, and the `file` property specifies the file location. The `search` feature specifies paths in which to search for the library. That feature can be specified several times, or it can be omitted, in which case only default compiler paths will be searched.

The difference between using the `file` feature as opposed to the `name` feature together with the `search` feature is that `file` is more precise. A specific file will be used. On the other hand, the `search` feature only adds a library path, and the `name` feature gives the basic name of the library. The search rules are specific to the linker. For example, given these definition:

    lib a : : <variant>release <file>/pool/release/a.so ;
    lib a : : <variant>debug <file>/pool/debug/a.so ;
    lib b : : <variant>release <file>/pool/release/b.so ;
    lib b : : <variant>debug <file>/pool/debug/b.so ;

It's possible to use release version of `a` and debug version of `b`. Had we used the `name` and `search` features, the linker would always pick either release or debug versions.

For convenience, the following syntax is allowed:

    lib z ;
    lib gui db aux ;

and is does exactly the same as:

    lib z : : <name>z ;
    lib gui : : <name>gui ;
    lib db : : <name>db ;
    lib aux : : <name>aux ;

When a library uses another library you should put that another library in the list of sources. This will do the right thing in all cases. For portability, you should specify library dependencies even for searched and prebuilt libraries, othewise, static linking on Unix won't work. For example:

    lib z ;
    lib png : z : <name>png ;

When a library (say, `a`), that has another library, (say, `b`) is linked dynamically, the `b` library will be incorporated in `a`. (If `b` is dynamic library as well, then `a` will only refer to it, and not include any extra code.) When the `a` library is linked statically, Boost.Build will assure that all executables that link to `a` will also link to `b`.

One feature of Boost.Build that is very important for libraries is usage requirements. For example, if you write:

    lib helpers : helpers.cpp : : : <include>. ;

then the compiler include path for all targets that use `helpers` will contain the directory where the target is defined.path to "helpers.cpp". The user only needs to add `helpers` to the list of sources, and needn't consider the requirements its use imposes on a dependent target. This feature greatly simplifies Jamfiles.

If you don't want shared libraries to include all libraries that are specified in sources (especially statically linked ones), you'd need to use the following:

    lib b : a.cpp ;
    lib a : a.cpp : <use>b : : <library>b ;

This specifies that `a` uses `b`, and causes all executables that link to `a` also link to `b`. In this case, even for shared linking, the `a` library won't even refer to `b`.

### Alias

The `alias` rule follows the common syntax. For example:

    alias core : im reader writer ;

will build the sources and return the generated source targets without modification.

The alias rule is a convenience tool. If you often build the same group of targets at the same time, you can define an alias to save typing.

Another use of the `alias` rule is to change build properties. For example, if you always want static linking for a specific C++ Boost library, you can write the following:

    alias threads : /boost/thread//boost_thread : <link>static ;

and use only the `threads` alias in your Jamfiles.

You can also specify usage requirements for the `alias` target. If you write the following:

    alias header_only_library : : : :  <include>/usr/include/header_only_library ;

then using `header_only_library` in sources will only add an include path. Also note that when there are some sources, their usage requirements are propagated, too. For example:

    lib lib : lib.cpp : : : <include>. ;
    alias lib_alias ;
    exe main : main.cpp lib_alias ;

will compile `main.cpp` with the additional include.

### Installing

For installing a built target you should use the `install` rule, which follows the common syntax. For example:

    install dist : hello helpers ;

will cause the targets `hello` and `helpers` to be moved to the `dist` directory, relative to Jamfile's directory. The directory can be changed with the `location` property:

    install dist : hello helpers : <location>/usr/bin ;

While you can achieve the same effect by changing the target name to `/usr/bin`, using the `location` property is better, because it allows you to use a memnonic target name.

The `location` property is especially handy when the location is not fixed, but depends on build variant or environment variables:

    install dist : hello helpers : <variant>release:<location>dist/release
                                 <variant>debug:<location>dist/debug ;
    install dist2 : hello helpers : <location>$(DIST) ;

See also conditional properties and environment variables

Specifying the names of all libraries to install can be boring. The `install` allows you to specify only the top-level executable targets to install, and automatically install all dependencies:

    install dist : hello
               : <install-dependencies>on <install-type>EXE
                 <install-type>LIB
               ;

will find all targets that `hello` depends on, and install all of the which are either executables or libraries. More specifically, for each target, other targets that were specified as sources or as dependency properties, will be recursively found. One exception is that targets referred with the `use` feature are not considered, because that feature is typically used to refer to header-only libraries. If the set of target types is specified, only targets of that type will be installed, otherwise, all found target will be installed.

The `alias` rule can be used when targets must be installed into several directories:

    install install : install-bin install-lib ;
    install install-bin : applications : /usr/bin ;
    install install-lib : helper : /usr/lib ;

Because the `install` rule just copies targets, most free features

see the definition of "free" in .

have no effect when used in requirements of the `install`. The only two which matter are `dependency` and, on Unix, `dll-path`.

(Unix specific). On Unix, executables built with Boost.Build typically contain the list of paths to all used dynamic libraries. For installing, this is not desired, so Boost.Build relinks the executable with an empty list of paths. You can also specify additional paths for installed executables with the `dll-path` feature.

### Testing

Boost.Build has convenient support for running unit tests. The simplest way is the `unit-test` rule, which follows the common syntax. For example:

    unit-test helpers_test : helpers_test.cpp helpers ;

The unit-test rule behaves like the exe rule, but after the executable is created it is run. If the executable returns an error code, the build system will also return an error and will try running the executable on the next invocation until it runs successfully. This behaviour ensures that you can't miss a unit test failure.

There are rules for more elaborate testing: `compile`, `compile-fail`, `run` and `run-fail`. They are more suitable for automated testing, and are not covered here.

## 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

  Rationale: Runtime debugging is on in debug builds to suit the expectations of people used to various IDEs. It's assumed other folks don't have any specific expectation in this point.

`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. The feature is sometimes more convenient: you can put `<source>X` in the requirements for a project and `X` will be included as a source in all of the project's main targets.

`library`

  This feature is equivalent to the <source> feature, and exists for backward compatibility reasons.

`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), 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.

`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 in 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 C and C++ compilers, for `cxxflags` that's C++ compiler and for `linkflags` that's linker. The features are handy when you're trying to do something special that cannot be achieved by higher-level feature in Boost.Build.

## Differences to Boost.Build V1

While Boost.Build V2 is based on the same ideas as Boost.Build V1, some of the syntax was changed, and some new important features were added. This chapter describes most of the changes.

### Configuration

In V1, there were two methods to configure a toolset. One was to set some environment variable, or use the `-s` command line option to set a variable inside BJam. Another method was to create a new toolset module that would set the variables and then invoke the base toolset. Neither method is necessary now: the using rule provides a consistent way to initialize a toolset, including several versions. See for details.

### Writing Jamfiles

Probably one of the most important differences in V2 Jamfiles is the use of project requirements. In V1, if several targets had the same requirements (for example, a common `#include` path), it was necessary to manually write the requirements or use a helper rule or template target. In V2, the common properties can be specified with the `requirements` project attribute, as documented in .

Usage requirements also help to simplify Jamfiles. If a library requires all clients to use specific `#include` paths or macros when compiling code that depends on the library, that information can be cleanly represented.

The difference between `lib` and `dll` targets in V1 is completely eliminated in V2. There's only one library target type, `lib`, which can create either static or shared libraries depending on the value of the `<link>` feature. If your target should be only built in one way, you can add `<link>shared` or `<link>static` to its requirements.

The syntax for referring to other targets was changed a bit. While in V1 one would use:

    exe a : a.cpp <lib>../foo/bar ;

the V2 syntax is:

    exe a : a.cpp ../foo//bar ;

Note that you don't need to specify the type of other target, but the last element should be separated from the others by a double slash to indicate that you're referring to target `bar` in project `../foo`, and not to project `../foo/bar`.

### Build process

The command line syntax in V2 is completely different. For example

    bjam -sTOOLS=msvc -sBUILD=release some_target

now becomes:

    bjam toolset=msvc variant=release some_target

or, using implicit features, just:

    bjam msvc release some_target

See the reference for a complete description of the syntax.