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Boost Build System

Synopsis

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

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

Table of Contents

Getting Started

Installing Boost Jam

Initiating a Build

The easiest way to get going is to set the BOOST_ROOT environment variable to point at the Boost installation directory, though you can also set BOOST_ROOT on the command-line, using -s.... You can use the TOOLS variable to indicate which toolset(s) to build with, and the BUILD variable to describe how you want things built. In most cases it should be sufficient to invoke Jam with no variable settings. The following examples all assume that BOOST_ROOT has been set in the environment.

Here are some sample Boost Jam invocations:
Command Line(s) Effects
jam -sTOOLS=gcc my_target
default (debug) BUILD of my_targetwith GCC
jam -fallyourbase-path -sTOOLS="msvc gcc"
default-build all with msvc and gcc
set TOOLS=msvc
jam
Set an NT environment variable to always build with MSVC
default-build all.
jam -sBUILD=release
release build all with default TOOLS:
jam -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:

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 Jamfiles 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.

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 ] ] ;

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:

The default-BUILD section:

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: 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:

    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.

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.

Core Jam Extensions

A number of enhancements have been made to the core language of Classic Jam. These changes were aimed primarily at making it easier to manage the complexity of a large system such as Boost.Build.

Command-line and Environment Variable Quoting

Classic Jam had an odd behavior with respect to command-line variable ( -s...) and environment variable settings which made it impossible to define an arbitrary variable with spaces in the value. Boost Jam remedies that by treating all such settings as a single string if they are surrounded by double-quotes. Uses of this feature can look interesting, since shells require quotes to keep characters separated by whitespace from being treated as separate arguments:

jam -sMSVCNT="\"C:\Program Files\Microsoft Visual C++\VC98\"" ...

Jambase Replacement

New logic has been added to allow the built-in Jambase to be replaced without recompiling Jam or adding command-line arguments. The user can control the location of the build system by setting any of the JAMBASE, BOOST_ROOT, or BOOST_BUILD_PATH environment variables (the settings of these variables can also be overridden on the command-line using the -sVARIABLE=... option).

The process is controlled by variables (in decreasing precedence):

The rationale for this behavior is as follows:

Rule Indirection

Boost Jam allows you to call a rule whose name is held in a variable or computed as the result of an expression:

x = foo ;
rule foobar { ECHO foobar ; }   # a trivial rule
$(x)bar ;                       # invokes foobar

Furthermore, if the first expression expands to more than one list item, everything after the first item becomes part of the first argument. This allows a crude form of argument binding:
# return the elements of sequence for which predicate returns non-nil
rule filter ( sequence * : predicate + )
{
    local result ;
    for local x in $(sequence)
    {
        if [ $(predicate) $(x) ] { result += $(x); }
    }
    return $(result);
}

# true iff x == y
rule equal ( x y )
{
    if $(x) = $(y) { return true; }
}

# bind 3 to the first argument of equal
ECHO [ filter 1 2 3 4 5 4 3 : equal 3 ] ; # prints "3 3"

Argument lists

You can now describe the arguments accepted by a rule, and refer to them by name within the rule. For example, the following prints ``I'm sorry, Dave'' to the console:

rule report ( pronoun index ? : state : names + )
{
    local he.suffix she.suffix it.suffix = s ;
    local I.suffix = m ;
    local they.suffix you.suffix = re ;

    ECHO $(pronoun)'$($(pronoun).suffix) $(state), $(names[$(index)]) ;
}
report I 2 : sorry : Joe Dave Pete ;
Each name in a list of formal arguments (separated by ``:'' in the rule declaration) is bound to a single element of the corresponding actual argument unless followed by one of these modifiers:
Symbol Semantics of preceding symbol
? optional
* Bind to zero or more unbound elements of the actual argument. When ``*'' appears where an argument name is expected, any number of additional arguments are accepted. This feature can be used to implement "varargs" rules.
+ Bind to one or more unbound elements of the actual argument.

The actual and formal arguments are checked for inconsistencies, which cause Jam to exit with an error code:

### argument error
# rule report ( pronoun index ?  : state  : names + )
# called with: ( I 2 foo  : sorry  : Joe Dave Pete )
# extra argument foo

### argument error
# rule report ( pronoun index ?  : state  : names + )
# called with: ( I 2  : sorry )
# missing argument names

If you omit the list of formal arguments, all checking is bypassed as in ``classic'' Jam. Argument lists drastically improve the reliability and readability of your rules, however, and are strongly recommended for any new Jam code you write.

Module Support

Boost Jam introduces support for modules, which provide some rudimentary namespace protection for rules and variables. A new keyword, ``module'' was also introduced. The features described in this section are primitives, meaning that they are meant to provide the operations needed to write Jam rules which provide a more elegant module interface.

Declaration
module  expression {  ... } 

Code within the ... } executes within the module named by evaluating expression. Rule definitions can be found in the module's own namespace, and in the namespace of the global module as module-name.rule-name, so within a module, other rules in that module may always be invoked without qualification:

module my_module
{
    rule salute ( x ) { ECHO $(x), world ; }
    rule greet ( ) { salute hello ; }
    greet ;
}
my_module.salute goodbye ;
When an invoked rule is not found in the current module's namespace, it is looked up in the namespace of the global module, so qualified calls work across modules:
module your_module
{
    rule bedtime ( ) { my_module.salute goodnight ; }
}

Local Variables
module local  expression ;
- or -
module local  expression =  expression2 ;

The variables named by expression are given a distinct value in the module, and can be manipulated by code executing in the module without affecting variable bindings seen by other modules. If the assignment form is used, expression2 is assigned to the variables when the declaration is executed. For example:

module M {
  module local x = a b c ;

  rule f ( )
  {
    {
      local x = 1 2 3 ; # temp override for M's x
      N.g ;             # call into module N, below
    }
    ECHO $(x) ;         # prints "a b c"
  }
}

module N {
  rule g ( )
  {
    x = foo bar baz ; # sets global x
    M.h ;             # call back into M, below
  }
}

module M {
  rule h ( )
  {
    ECHO $(x) ;      # prints "1 2 3"
  }
}

M.f ; 
ECHO $(x) ;          # prints "foo bar baz"
The only way to access another module's local variables is through a rule defined in that module:
module M {
  rule get ( names * )
  {
    return $($(names)) ;
  }
}
ECHO [ M.get x ] ;  # prints "a b c"
Local Rules
local rule rulename...

The rule is declared locally to the current module. It is not entered in the global module with qualification, and its name will not appear in the result of

[ RULENAMES module-name ].
The RULENAMES Rule
rule RULENAMES ( module ? )
Returns a list of the names of all non-local rules in the given module. If module is ommitted, the names of all non-local rules in the global module are returned.
The IMPORT Rule
IMPORT allows rule name aliasing across modules:
rule IMPORT (  source_module ? : source_rules *
                : target_module ? : target_rules * )
The IMPORT rule copies rules from the source_module into the target_module as local rules. If either source_module or target_module is not supplied, it refers to the global module. source_rules specifies which rules from the source_module to import; TARGET_RULES specifies the names to give those rules in target_module. If source_rules contains a name which doesn't correspond to a rule in source_module, or if it contains a different number of items than target_rules, an error is issued. For example,
# import m1.rule1 into m2 as local rule m1-rule1.
IMPORT m1 : rule1 : m2 : m1-rule1 ;

# import all non-local rules from m1 into m2
IMPORT m1 : [ RULENAMES m1 ] : m2 : [ RULENAMES m1 ] ;
The EXPORT Rule
EXPORT allows rule name aliasing across modules:
rule EXPORT ( module ? : rules * )
The EXPORT rule marks rules from the source_module as non-local (and thus exportable). If an element of rules does not name a rule in module, an error is issued. For example,
module X {
  local rule r { ECHO X.r ; }
}
IMPORT X : r : : r ; # error - r is local in X
EXPORT X : r ;
IMPORT X : r : : r ; # OK.
The CALLER_MODULE Rule
rule CALLER_MODULE ( levels ? )
CALLER_MODULE returns the name of the module scope enclosing the call to its caller (if levels is supplied, it is interpreted as an integer number of additional levels of call stack to traverse to locate the module). If the scope belongs to the global module, or if no such module exists, returns the empty list. For example, the following prints "{Y} {X}":
module X {
    rule get-caller { return [ CALLER_MODULE ] ; }
    rule get-caller's-caller { return [ CALLER_MODULE 1 ] ; }

    rule call-Y { return Y.call-X2 ; }

}
module Y {
    rule call-X { return X.get-caller ; }
    rule call-X2 { return X.get-caller's-caller ; }
}

callers = [ X.get-caller ] [ Y.call-X ] [ X.call-Y ] ;
ECHO {$(callers)} ;

Local For Loop Variables

Boost Jam allows you to declare a local for loop control variable right in the loop:

x = 1 2 3 ;
y = 4 5 6 ;
for local y in $(x)
{
    ECHO $(y) ; # prints "1", "2", or "3"
}
ECHO $(y) ;     # prints "4 5 6"

While Loops

In classic Jam, some constructs are only possible using recursion:
# returns the part of $(list) following the first occurrence of $(symbol)
rule after-symbol ( symbol : list * )
{
    if ! $(list) || ( $(symbol) = $(list[1]) )
    {
        return $(list[2-]) ;
    }
    else
    {
        return [ after-symbol $(symbol) : $(list[2-]) ] ;
    }
}
The addition of while loops allows a simpler formulation for this and many other rules:
rule after-symbol ( symbol : list * )
{
    while $(list) && $(list[1]) != $(symbol)
    {
        list = $(list[2-]) ;
    }
    return $(list) ;
}

Negative Indexing

Classic Jam supplies 1-based list indexing, and slicing on a closed (inclusive) range:
x = 1 2 3 4 5 ;
ECHO $(x[3]) ;   # prints "3"
ECHO $(x[2-4]) ; # prints "2 3 4"
ECHO $(x[2-]) ;  # prints "2 3 4 5"
Boost Jam adds Python-style negative indexing to access locations relative to the end of the list.
ECHO $(x[-1]) $(x[-3]) ; # prints "5 3"
ECHO $(x[-3--1]) ;       # prints "3 4 5"
ECHO $(x[-3-4]) ;        # prints "3 4"
ECHO $(x[2--2]) ;        # prints "2 3 4"       
Consistency with the 1-based, inclusive indexing of Classic Jam and the use of ``-'' as the range separator make this feature a bit clumsier than it would otherwise need to be, but it does work.

Target Binding Detection

Whenever a target is bound to a location in the filesystem, Boost Jam will look for a variable called BINDRULE (first ``on'' the target being bound, then in the global module). If non-empty, $(BINDRULE[1]) names a rule which is called with the name of the target and the path it is being bound to. The signature of the rule named by $(BINDRULE[1]) should match the following:

rule bind-rule ( target : path )
This facility is useful for correct header file scanning, since many compilers will search for #included files first in the directory containing the file doing the #include directive. $(BINDRULE) can be used to make a record of that directory.

Return Code Inversion

For handling targets whose build actions are expected to fail (e.g. when testing that assertions or compile-time type checkin work properly), Boost Jam supplies a FAIL_EXPECTED rule in the same style as NOCARE, et. al. During target updating, the return code of the build actions for arguments to FAIL_EXPECTED is inverted: if it fails, building of dependent targets continues as though it succeeded. If it succeeds, dependent targets are skipped.

Ignoring Return Codes

Perforce Jam suppplied a NOCARE rule which is typically used for header files to indicate that if they are not found, the dependent targets should be built anyway. Boost Jam extends NOCARE to apply to targets with build actions: if their build actions exit with a nonzero return code, dependent targets will still be built.

The SUBST Rule

The behavior of the SUBST rule for regular-expression matching and replacement (originally added in FTJam) has been modified:

The JAM_VERSION global variable

A predefined global variable with two elements indicates the version number of Boost Jam. Boost Jam versions start at "03" "00". Earlier versions of Jam do not automatically define JAM_VERSION.

Debugging Support

The BACKTRACE rule
rule BACKTRACE ( )
Returns a list of quadruples: filename line module rulename..., describing each shallower level of the call stack. This rule can be used to generate useful diagnostic messages from Jam rules.

The -d command-line option admits new arguments:

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:

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:

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

Grist Variables

Design Criteria

Assumptions

The requirements are driven by several basic assumptions:

Requirements

This build system was designed to satisfy the following requirements:

Footnotes

[1] Boost Jam is actually descended directly from FTJam, which was itself a variant of Jam/MR. It is hoped that crucial features we rely on will eventually be incorportated back into the Jam/MR 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 11 November, 2001