# Detailed reference

## General information

### Initialization

bjam's first job upon startup is to load the Jam code which 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 location and directories in BOOST_BUILD_PATH are then searched for a file called bootstrap.jam which is interpreted and is expected to bootstrap the build system. This arrangement allows the build system to work without any command-line or environment variable settings. For example, if the build system files were located in a directory "build-system/" at your project root, you might place a boost-build.jam at the project root containing:

    boost-build build-system ;

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

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

Locations where those files a search are summarized below:

#### Search paths for configuration files

| site-config.jam | user-config.jam
--- | --- | ---
Linux | /etc $HOME $BOOST_BUILD_PATH | $HOME $BOOST_BUILD_PATH
Windows | $SystemRoot $HOME $BOOST_BUILD_PATH | $HOME $BOOST_BUILD_PATH

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

### Command line

The command line may contain:

- Jam options,

- Boost.Build options,

- Command line arguments

#### Command line arguments

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

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

- An argument with either slashes or the "=" symbol specifies a number of build request elements. In the simplest form, it's just a set of properties, separated by slashes, which become a single build request element, for example:

      borland/<runtime-link>static

  More complex form is used to save typing. For example, instead of

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

  one can use

      borland/runtime-link=static,dynamic

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

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

  or, in case of implicit feature

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

  and will be converted into property set

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

For example, the command line

    target1 debug gcc/runtime-link=dynamic,static

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

#### Command line options

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

##### Command line options

Option | Description
--- | ---
`--version` | Prints information on Boost.Build and Boost.Jam versions.
`--help` | Access to the online help system. This prints general information on how to use the help system with additional --help* options.
`--clean` | Removes everything instead of building. Unlike `clean` target in make, it is possible to clean only some targets.
`--debug` | Enables internal checks.
`--dump-projects` | Cause the project structure to be output.
`--no-error-backtrace` | Don't print backtrace on errors. Primary useful for testing.
`--ignore-config` | Do not load `site-config.jam` and `user-config.jam`

## Writing Jamfiles

This section describes specific information about writing Jamfiles.

### Generated headers

Usually, Boost.Build handles implicit dependendies completely automatically. For example, for C++ files, all `#include` statements are found and handled. The only aspect where user help might be needed is implicit dependency on generated files.

By default, Boost.Build handles such dependencies within one main target. For example, assume that main target "app" has two sources, "app.cpp" and "parser.y". The latter source is converted into "parser.c" and "parser.h". Then, if "app.cpp" includes "parser.h", Boost.Build will detect this dependency. Moreover, since "parser.h" will be generated into a build directory, the path to that directory will automatically added to include path.

Making this mechanism work across main target boundaries is possible, but imposes certain overhead. For that reason, if there's implicit dependency on files from other main targets, the `<implicit-dependency>` [ link ] feature must be used, for example:

    lib parser : parser.y ;
    exe app : app.cpp : <implicit-dependency>parser ;

The above example tells the build system that when scanning all sources of "app" for implicit-dependencies, it should consider targets from "parser" as potential dependencies.

## Build process

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

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

1. applying default build,

2. selecting the main target alternative to use,

3. determining "common" properties

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

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

6. building the target using generators

7. computing the usage requirements to be returned

### Alternative selection

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

1. For each alternative condition is defined as the set of base properies in requirements. [Note: it might be better to specify the condition explicitly, as in conditional requirements].

2. An alternative is viable only if all properties in condition are present in build request.

3. If there's one viable alternative, it's choosen. Otherwise, an attempt is made to find one best alternative. An alternative a is better than another alternative b, iff set of properties in b's condition is strict subset of the set of properities of 'a's condition. If there's one viable alternative, which is better than all other, 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 ;

### 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 which 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 which 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 which controls a compiler's warning level is one example of a likely incidental feature.

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

- propagated

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

- free

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

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

- optional

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

- symmetric

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

- path

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

- implicit

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

- composite

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

- dependency

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

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

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

#### Feature Declaration

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

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

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

### Build Variants

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

### Property refinement

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

1. If original properties and required properties are not link-compatible, refinement fails.

2. Each property in the required set is added to the original property set

3. 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 ;

### Target identifiers and references

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

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

This grammar allows some elements to be recognized as either

- project id (at this point, all project ids start with slash).

- name of target declared in current Jamfile (note that target names may include slash).

- a regular file, denoted by absolute name or name relative to project's sources location.

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

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

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

- It emphasises that projects and targets are different things.

- It allows to have main target names with slashes.

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

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

For example,

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

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

## Generators

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

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

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

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

### Selecting and ranking viable generators

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

1. Required properties, which are properties absolutely necessary for the generator to work. For example, generator encapsulating the gcc compiler would have <toolset>gcc as required property.

2. Optional properties, which increase the generators suitability for a particual build.

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

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

### Running generators

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

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

### Selecting dependency graph

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

### Property adjustment

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

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

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

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

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

### Transformations cache

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