# Boost Build System

## Synopsis

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

- A simple target description language
- Build with your choice (or multiple) toolsets from a single command invocation
- Build your choice of basic variants (e.g. debug, release, profile...) and subvariant modifications (e.g. inlining off) from a single command invocation
- ``Feature Normalization'' allows target configurations to be described independently from the toolset used
- Modular toolset description files allow build instructions for different toolsets to be described independently
- Multiple subproject support
- Automatic building of subproject dependencies

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

## Table of Contents

- Synopsis
- Getting Started
  - Installing Jam
  - Initiating a Build
  - Setting Jam Variables
  - An Example Jamfile
  - Support Files
- Basic Design and Terminology
  - Projects and Subprojects
  - Targets
  - Features and Properties
  - Build Variants
  - Subvariants
  - Dependent Targets
- Usage
  - The Command-Line
    - User Targets
    - Global Variables
  - Subproject Jamfiles
    - The `subproject` Rule
    - Describing Main Targets
    - Example
  - Feature Descriptions
  - Variant Descriptions
  - Toolset Description Files
    - Example
- Internals
  - Jam Fundamentals
  - Target Names
  - Global Variables
- Design Criteria
  - Assumptions
  - Requirements
- Footnotes

## Getting Started

### Installing FTJam

- This page describes how and where to get FTJam sources.
- The Jam README contains basic installation instructions.
- If you are installing on Windows NT, please read this note. You may need to use the ```SUBST` hack'' described at the bottom of the message to bootstrap Jam. Also, when defining the `MSVCNT`, `BCCROOT`, etc. environment variables as described in the Jam README, please remember that they are case-sensitive.
- Note that the supplied Makefile may require editing for your platform; see the Jam README for details. The Makefile is used for bootstrapping Jam; it builds Jam into a subdirectory called `bin.`platform.

### Initiating a Build

Currently, the build system must be invoked with the `-fallyourbase-path` option, where allyourbase-path is the path to the `allyourbase.jam` file supplied in this directory. When the system matures, this file may be compiled directly into Jam as the `Jambase` or we will find another way to make sure the `-f` option is no longer needed. The environment variables necessary for bootstrapping Jam are not needed once it has been built.

Here are some sample Boost Jam invocations:

Command Line(s) | Effects
--- | ---
    jam -fallyourbase-path -sTOOLS=gcc my_target
| default (debug) `BUILD` of `my_target`with GCC
    jam -fallyourbase-path -sTOOLS="msvc gcc"
| default-build `all` with msvc and gcc
    set TOOLS=msvc
    jam -fallyourbase-path
| Set an NT environment variable to always build with MSVC default-build `all`.
    jam -fallyourbase-path -sBUILD=release
| release build `all` with default `TOOLS`:
    jam -fallyourbase-path -sBUILD="debug release"
| debug and release build `all`.

### Setting Jam Variables

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

- Jam picks up variable settings from your environment by default, so you can set them there:

  >     > BUILD="debug release"  # assuming Unix
  >     > export BUILD
  >     > jam ...

  This approach can be OK for quick-and-dirty tests, but environment variable settings tend to be unstable and non-uniform across users and machines, so it's best not to rely on the environment much.
- Explicitly on the command-line, with the "`-s`" option.
- Directly in Jam code. A project's Jamrules file is a convenient place to make global settings.

### An Example Jamfile

Here is an example of a simple subproject Jamfile. In this example, it is assumed that the user has set `BOOST_ROOT`, either as an environment variable, on the command-line or in the project's `Jamrules` file:

>     subproject foo/bar/baz ; # path to here from project root

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

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

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

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

### Support Files

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

Filename(s) | Meaning
--- | ---
toolset`-tools.jam` | Feature-to-command-line mapping for toolset.
`features.jam` | Abstract toolset feature descriptions.
`boost-base.jam` | Boost build system-specific rule definitions.
`unit-tests.jam` | Unit tests and assertions for boost Jam code.

The `boost-base.jam` file is temporary, and will eventually be compiled into our Jam executable.

## Basic Design and Terminology

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

### Projects and Subprojects

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

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

The build system installation directory is a directory containing Jam files describing compilers and build variants. The installation directory can be specified explicitly by setting the variable `BOOST_BUILD_INSTALLATION`. If the installation directory is not specified, it is the same as the project root, and `BOOST_BUILD_INSTALLATION` is set to refer to that directory.

### Targets

Each `Jamfile` describes one or more main targets.

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

- By default, the build directory tree is overlaid with the project directory tree, with targets generated into a subtree rooted at the `bin` subdirectory of each subproject directory (the name of this directory can be customized by changing the `BIN_DIRECTORY` variable.
- If the variable `ALL_LOCATE_TARGET` is set, it specifies an alternate build directory tree whose structure mirrors that of the project. In this case, built targets of a subproject are generated into the corresponding directory of the build directory tree.

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

### Features and Properties

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

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

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

### Build Variants

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

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

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

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

### Subvariants

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

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

   >      bin
   >      +-foo  <--- foo's build root
   >      | +-msvc
   >      | | +-debug
   >      . . . `-runtime-link-dynamic
   >      . . .

   Because the default value of `runtime-link` is `dynamic`, when the `debug` variant is requested, the `runtime-link-dynamic` subvariant of foo is built.
2. It is possible to request (either on the command-line, or as part of a main target description) that particular subvariants be built. For example, it may be desirable to generate builds that link to the runtime both statically and dynamically. In that case, both subvariant directories in the example above would be generated:

   >      bin
   >      +-foo  <--- foo's build root
   >      | +-msvc
   >      | | +-debug
   >      . . . +-runtime-link-dynamic
   >      . . . `-runtime-link-static
   >      . . .

In no case will targets be built directly into `bin/foo/msvc/debug`, since the `debug` variant doesn't include the `runtime-link` feature, which is relevant to MSVC.

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

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

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

### Dependent Targets

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

## Usage

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

### The Command Line

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

#### User Targets

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

>     exe my_target : my_source1.cpp my_source2.c ;

and you invoke Jam with `-sBUILD="debug release" my_target`, you will build both the debug and release versions of `my_target`.

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

#### Global Variables

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

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

### SubProject Jamfiles

This section describes how to write a Jamfile for a subproject.

#### The `subproject` rule

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

>     subproject path-from-top ;

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

>     project-root ;

#### Describing Main Targets

A main target is described using the following syntax:

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

- target-type may be one of `exe`, `lib`, or `dll`. These are actually names of Jam rules. Additional main target rules are possible; see `status/Jamfile` or `libs/python/build/Jamfile` for examples.
- name specifies the name of the main target
- sources is a list of paths to source files and dependency targets. A dependency target path is preceded by `<lib>`, and the final path component specifies the name of a main target in a Jamfile located in the directory given by the inital path components. Paths may be absolute or relative.
- requirements specifies the build properties intrinsic to the target. Requirements are given as sets of optionally-qualified build properties:

  >     [[<compiler>]<variant>]<feature>value

  `<compiler>` and `<variant>`, if supplied, can be used to restrict the applicability of the requirement. Either one may be replaced by `<*>`, which is the same as ommitting it.

  The system checks that simple feature requirements are not violated by explicit subvariant build requests, and will issue a warning otherwise. Free features specified as requirements are simply added to each corresponding build configuration.

- default-BUILD specifies the configurations that should be built if the `BUILD` variable is not otherwise specified. Any elements not beginning with ```<`...`>`'' refer to build variants. Other elements use the same syntax as the requirements described above, except that multiple values may be specified for a simple feature by separating them with a slash, forming (qualified) multi-valued properties:

  >     [[<compiler>]<variant>]<feature>value1[/value2...]

  When multiple values are specified, it causes all the implied configurations to be built by default.

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

#### Example

This artificially complex example shows how an executable called "foo" might be described in a Jamfile. The executable is composed of the sources `./foo.cpp` and `./src/bar.cpp` (specified relative to the directory in which the Jamfile resides), and the built target which results from building the target `baz` as described in `../bazlib/Jamfile`.

>     exe foo : foo.cpp src/bar.cpp <lib>../bazlib/baz
>         ## Requirements ##
>         : <include>../bazlib/include
>           <define>BUILDING_FOO=1
>           <release><define>FOO_RELEASE
>           <msvc><*><define>FOO_MSVC
>           <msvc><release><define>FOO_MSVC_RELEASE
>           <gcc><*><optimization>off
>           <gcc><release><optimization>space
>           <threading>multi

>         ## default-BUILD ##
>         : debug release
>           <debug><runtime-link>static/dynamic
>         ;

The requirements section:

- Adds `../bazlib/include` to the `#include` path
- Sets the preprocessor symbol `BUILDING_FOO` to `1`
- In the `release` builds, `#define`s `FOO_RELEASE`.
- When built with MSVC, `#define`s `FOO_MSVC`.
- In `release` variants built with MSVC, `#define`s `FOO_MSVC_RELEASE`.
- Most builds under GCC have optimization turned off, but...
- ...GCC `release` builds require optimization for space.
- Requires multithread support on compilers where it's relevant.

The default-BUILD section:

- specifies that `debug` and `release` base variants are built by default.
- on compilers where the feature is relevant, requests both statically- and dynamically-linked subvariants of the debug variant.

### Feature Descriptions

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

### Variant Descriptions

Variants are described with the following syntax:

>     variant name : [<toolset-name>]<feature>value... ;

The `variant` rule specifies the list of properties comprising a variant. Properties may be optionally qualified with a toolset name, which specifies that the property applies only to that toolset. Examples can be found in features.jam.

### Toolset Description Files

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

1. redefine the following rules:
   - `Link-action` - links an executable from objects and libraries
   - `Archive-action` - links a static library from object files
   - `C++-action` - compiles a 'C++' file into an object file
   - `Cc-action` - compiles a 'C' file into an object file
   These rules should simply invoke the action part of a rule whose name is uniquely defined for the toolset. For example,

   >     rule C++-action
   >     {
   >         msvc-C++-action $(<) : $(>) ;
   >     }

   >     actions msvc-C++-action
   >     {
   >         cl -nologo -GX -c -U$(UNDEFS) -D$(DEFINES) $(CFLAGS) $(C++FLAGS) -I$(HDRS) -I$(STDHDRS) -Fo$(<) -Tp$(>)
   >     }

   Note that `Link-action` may require special care: on platforms where the global variable `gEXPORT_SUFFIX(DLL)` is defined (e.g. Windows), the first argument may have two elements when linking a shared library. The first is the shared library target, and the second is the import library target, with suffix given by `$(gEXPORT_SUFFIX(DLL))`. It will always have a third argument which is either ```EXE`'' or ```DLL`''. This can be used to dispatch to different actions for linking DLLs and EXEs if neccessary, but usually it will be easier to take advantage of the special `<target-type>` feature, which will have the same value using the `flags` rule described below.
2. Translate build settings given in the global `gBUILD_PROPERTIES` variable into something that can be used by the toolset. The build system provides the `flags` rule to help translate build properties into elements of global variables which are later attached to targets so that they can affect the build actions. The `flags` rule is used as follows:

   >     flags toolset variable condition [: value...]

   The parameters are:
   - toolset - the name of the toolset
   - variable - the name of a global variable which can be used to carry information to a command-line
   - condition - one one or more elements in the following forms:
     1. a property-set of the form: `<feature>value`[`/<feature>value`...]
     2. `<feature>`
   - values - anything

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

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

   #### Example

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

   >     flags msvc CFLAGS <optimization>off : /Od ;

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

   Similarly, in the following example,

   >     flags msvc CFLAGS <runtime-build>release/<runtime-link>dynamic : /MD ;

   we add `/MD` to the CFLAGS variable when both of the specified conditions are satisfied. We could grab all of the values of the free feature `<include>` in the `HDRS` variable as follows:

   >     flags msvc HDRS <include> ;

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

   ## Internals

   ### Jam Fundamentals

   This section is derived from the official Jam documentation and from my experience using it and reading the Jambase rules. I repeat the information here mostly because it is essential to understanding and using Jam, but is not consolidated in a single place. Some of it is missing from the official documentation altogether. I hope it will be useful to anyone wishing to become familiar with Jam and the Boost build system.

   - Jam ``rules'' are actually simple procedural entities. Think of them as functions. Arguments are separated by colons.
   - A Jam target is an abstract entity identified by an arbitrary string. The build-in `DEPENDS` rule creates a link in the dependency graph between the named targets.
   - Note that the documentation for the built-in `INCLUDES` rule is incorrect: `INCLUDES targets1 : targets2` causes everything that depends on a member of targets1 to depend on all members of targets2.
   - When a rule is invoked, if there are `actions` declared with the same name as the rule, the `actions` are added to the updating actions for the target identified by the rule's first argument. It is actually possible to invoke an undeclared rule if corresponding actions are declared: the rule is treated as empty.
   - Targets (other than `NOTFILE` targets) are associated with paths in the file system through a process called binding.
   - In addition to local and global variables, jam allows you to set a variable `on` a target. Target-specific variable values can usually not be read, and take effect only in the following contexts:
     - In updating `actions`, variable values are first looked up `on` the target named by the first argument (the target being updated). Because Jam builds its entire dependency tree before executing `actions`, Jam rules make target-specific variable settings as a way of supplying parameters to the corresponding `actions`.
     - Binding is controlled entirely by the target-specific setting of the `SEARCH` and `LOCATE` variables, as described here.
     - In the special rule used for header file scanning, variable values are first looked up `on` the target named by the rule's first argument (the source file being scanned).
   - The ``bound value'' of a variable is the path associated with the target named by the variable. In build `actions`, the first two arguments are automatically replaced with their bound values. Target-specific variables can be selectively replaced by their bound values using the bind action modifier.
   - Note that the term ``binding'' as used in the Jam documentation indicates a phase of processing that includes three sub-phases: binding (yes!), update determination, and header file scanning. The repetition of the term ``binding'' can lead to some confusion. In particular, the Modifying Binding section in the Jam documentation should probably be titled ``Modifying Update Determination''.

   - In the Jam language, a name can be divided into two parts. If the name starts with a '`<`' symbol and contains a '`>`' symbol, the characters between the '`<`' and the first '`>`', inclusive, is known as grist (if the name doesn't match this format, the grist is empty). The rest of the name is known as the ungristed part. In Jam, Grist is added to user-specified target names from different subprojects to distinguish them from one another in case the same target name is used twice. The Boost build system also takes full advantage of Jam's ability to divide strings on grist boundaries, sometimes concatenating multiple gristed elements at the beginning of a string. ``Grist'' is just a string prefix of the form `<`characters`>`. It is used in Jam to create unique target names based on simpler names. For example, the file name ```test.exe`'' may be used by targets in separate subprojects, or for the debug and release variants of the ``same'' abstract target. Grist is used instead of identifying targets with absolute paths for two reasons:

     1. The location of header files can not be derived solely from what the user puts in a Jamfile, but depend also on the binding process. Some mechanism to identify headers with the same name is still needed.
     2. Grist allows us to use a uniform abstract identifier for each built target, regardless of target file location (as allowed by setting `ALL_LOCATE_TARGET`
     When grist is extracted from a name with `$(`var`:G)`, the result includes the leading and trailing angle brackets. When grist is added to a name with `$(`var`:G=`expr`)`, existing grist is first stripped. Then, if expr is non-empty, as few leading `<`s and `>`s as neccessary to form an expression of the form `<`expr2`>` are added; `<`expr2`>` is then prepended. Grist prefixes are stripped from target names when binding them to filesystem paths.

   Please also read The Jam language reference for the additional details, and the Jam release notes for a brief description of recent, but fundamental changes to the Jam language without which you will probably not understand any of the build system code. In particular, note that the `return` statement does not affect control flow.

   ### Target Names

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

   - A main target has all the same dependencies as a user target (i.e. building it updates all requested subvariants). Its name is the same except for the addition of `$(SOURCE_GRIST)`, which identifies the subproject. The identification looks like the names of the path components from the project root to the subproject, separated by exclamation points. Thus, if the project is rooted at `foo`, in the subproject at `foo/bar/baz` the target `my_target` is identified by `<bar!baz>my_target`.
   - A subvariant target has additional grist identifying its main target and subvariant. This grist is joined to `$(SOURCE_GRIST)` with the platform's directory separator. Thus, on Unix, a subvariant target of `my_target` above might be identified as `<bar!baz/my_target/optimization-space/runtime-link-static>my_source.o`. Note that the part of the grist following the first slash, known as the subvariant id, also corresponds to a fragment of the subvariant directory path where the corresponding target is generated. Most built targets will be identified this way.

   ### Global Variables

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

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

   - Global variables intended to be set by the user on the command-line or in the environment use `ALL_UPPER_CASE` names.
   - Internal global variables begin with a lower-case "g" and continue in upper-case: `gSOME_GLOBAL`
   - Global variables of the form: `gBASE_NAME(`arguments`)`, where arguments is a comma-separated argument list, are used internally to achieve a kind of indirection by concatenating variable values:

     >         ECHO $(gFUBAR($(x),$(y))) ;

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

   ### Variables Associated with SubProject Identity

   - `SUBDIR_TOKENS` - a list of the path elements relative to the project root of the current subproject.
   - `SUBDIR` - the path from the invocation directory to the current subproject directory.

   ### Grist Variables

   - `TARGET_GRIST` takes the form `subproject!id/target/toolset/variant/subvariant-path`.

   ## Design Criteria

   ### Assumptions

   The requirements are driven by several basic assumptions:

   - There is no single Boost developer or test facility with access to or knowledge of all the platforms and compilers Boost libraries are used with.
   - Boost libraries are used across such a wide range of platforms and compilers that almost no other assumptions can be made.

   ### Requirements

   This build system was designed to satisfy the following requirements:

   - A developer adding a new library or test program must only have to add simple entries naming the source files to a text file, and not have to know anything about platform specific files. The developer should not have to supply header dependency information.
   - There should be a very high likelihood of builds succeeding on all platforms if a build succeeds on any platform. In other words, a developer must not be required to have access to many platforms or compilers to ensure correct builds
   - A user or developer adding support for a new platform or compiler should only have to add to a single file describing how to do the build for that platform or compiler, and shouldn't have to identify the files that will need to be built.
   - The build should rely only on tools native to the platform and compiler, or supplied via the boost download.
   - The details of how the build is done for a particular platform or compiler should be appropriate for that platform.
   - It should be possible to build multiple variants (e.g. debug/release) of a single target.
   - It should be possible to build multiple variants of multiple targets with multiple compilers from a single build command.
   - The build tools must be able to handle Boost growth issues such as identified in Directory Structure proposals and discussion.
   - Support for dynamic and static linking should be included.
   - It should be relatively straightforward to add support for a new compiler. In most cases, no modification of files used to describe existing targets should be required.
   - Support for compiler- and variant-specific configuration for each target
   - It should be possible to build targets into a directory unrelated to the source directories (they may be read-only)

   ## Footnotes

   [1] FTJam is a variant of Jam/MR. It is hoped that crucial features we rely on from FTJam will eventually be incorportated into the official Jam release.

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

   © Copyright David Abrahams 2001. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.

   Revised 29 July, 2001