[[bbv2.jam]]
= Boost.Jam Documentation

____
Jam is a make(1) replacement that makes building simple things simple and building
complicated things manageable.
____

[[jam.building]]
== Building B2

Installing `B2` after building it is simply a matter of copying the
generated executables someplace in your `PATH`. For building the
executables there are a set of `build` bootstrap scripts to accommodate
particular environments. The scripts take one optional argument, the
name of the toolset to build with. When the toolset is not given an
attempt is made to detect an available toolset and use that. The build
scripts accept these arguments:

[source,shell]
----
build [toolset]
----

Running the scripts without arguments will give you the best chance of
success. On Windows platforms from a command console do:

[source,bat]
----
cd jam source location
.\build.bat
----

On Unix type platforms do:

[source,shell]
----
cd jam source location
sh ./build.sh
----

For the Boost.Jam source included with the Boost distribution the _jam
source location_ is `BOOST_ROOT/tools/build/src/engine`.

If the scripts fail to detect an appropriate toolset to build with your
particular toolset may not be auto-detectable. In that case, you can
specify the toolset as the first argument, this assumes that the toolset
is readily available in the `PATH`.

NOTE: The toolset used to build Boost.Jam is independent of the toolsets used
for B2. Only one version of Boost.Jam is needed to use
B2.

The supported toolsets, and whether they are auto-detected, are:

[%autowidth]
.Supported Toolsets
[cols=",,,",options="header",]
|===
|Script |Platform |Toolset |Detection and Notes

|`build.bat`
|Windows NT, 2000, and XP
|http://www.codegear.com/downloads/free/cppbuilder[`borland`],
http://www.borland.com/[Borland] {CPP}Builder (BCC 5.5)
a|
* Common install location: `C:\Borland\BCC55`
* `BCC32.EXE` in `PATH`

| | |http://www.comeaucomputing.com/[`como`], Comeau Computing C/{CPP} |

| | |http://gcc.gnu.org/[`gcc`], GNU GCC |

| | |http://gcc.gnu.org/[`gcc-nocygwin`], GNU GCC |

| |
|http://www.intel.com/software/products/compilers/c60[`intel-win32`],
Intel {CPP} Compiler for Windows
a|
* `ICL.EXE` in `PATH`

| | |http://www.metrowerks.com/[`metrowerks`], MetroWerks CodeWarrior
C/{CPP} 7.x, 8.x, 9.x
a|
* `CWFolder` variable configured
* `MWCC.EXE` in PATH

| | |http://www.mingw.org/[`mingw`], GNU http://gcc.gnu.org/[GCC] as the
http://www.mingw.org/[MinGW] configuration
a|
* Common install location: `C:\MinGW`

| | |http://msdn.microsoft.com/visualc/[`msvc`], Microsoft Visual {CPP}
6.x
a|
* `VCVARS32.BAT` already configured
* `%MSVCDir%` is present in environment
* Common install locations: `%ProgramFiles%\Microsoft Visual Studio`,
`%ProgramFiles%\Microsoft Visual {CPP}`
* `CL.EXE` in `PATH`

| | |http://msdn.microsoft.com/visualc/[`vc7`], Microsoft Visual {CPP} 7.x
a|
* `VCVARS32.BAT` or `VSVARS32.BAT` already configured
* `%VS71COMNTOOLS%` is present in environment
* `%VCINSTALLDIR%` is present in environment
* Common install locations: `%ProgramFiles%\Microsoft Visual Studio .NET`,
  `%ProgramFiles%\Microsoft Visual Studio .NET 2003`
* `CL.EXE` in `PATH`

| | |http://msdn.microsoft.com/visualc/[`vc8` and `vc9`], Microsoft
Visual {CPP} 8.x and 9.x
a|
Detection:

* `VCVARSALL.BAT` already configured
* `%VS90COMNTOOLS%` is present in environment
* Common install location: `%ProgramFiles%\Microsoft Visual Studio 9`
* `%VS80COMNTOOLS%` is present in environment
* Common install location: `%ProgramFiles%\Microsoft Visual Studio 8`
* `CL.EXE` in `PATH`

Notes:

* If `VCVARSALL.BAT`
is called to set up the toolset, it is passed all the extra arguments,
see below for what those arguments are. This can be used to build, for
example, a Win64 specific version of `b2`. Consult the VisualStudio
documentation for what the possible argument values to the `VCVARSALL.BAT`
are.

|`build.sh` |Unix, Linux, Cygwin, etc. |http://www.hp.com/go/c++[`acc`],
HP-UX aCC
a|
* `aCC` in `PATH`
* `uname` is "HP-UX"

| | |http://www.comeaucomputing.com/[`como`], Comeau Computing C/{CPP}
a|
* `como` in `PATH`

| | |http://gcc.gnu.org/[`gcc`], GNU GCC
a|
* `gcc` in `PATH`

| |
|http://www.intel.com/software/products/compilers/c60l/[`intel-linux`],
Intel {CPP} for Linux
a|
* `icc` in `PATH`
* Common install locations: `/opt/intel/cc/9.0`, `/opt/intel_cc_80`,
  `/opt/intel/compiler70`, `/opt/intel/compiler60`, `/opt/intel/compiler50`

| | |`kcc`, Intel KAI {CPP}
a|
* `KCC` in `PATH`

| | |http://www.codegear.com/downloads/free/cppbuilder[`kylix`],
http://www.borland.com/[Borland] {CPP}Builder
a|
* `bc++` in `PATH`

| |
|http://www.sgi.com/developers/devtools/languages/mipspro.html[`mipspro`],
SGI MIPSpro C
a|
* `uname` is "IRIX" or "IRIX64"

| | |`sunpro`, Sun Workshop 6 {CPP}
a|
* Standard install location: `/opt/SUNWspro`

| | |`qcc`, http://www.qnx.com/[QNX Neutrino]
a|
* `uname` is "QNX" and `qcc` in `PATH`

| | |http://www.tru64unix.compaq.com/cplus/[`true64cxx`], Compaq {CPP}
Compiler for True64 UNIX
a|
* `uname` is "OSF1"

| | |http://www.ibm.com/software/awdtools/vacpp/[`vacpp`], IBM VisualAge
{CPP}
a|
* `xlc` in `PATH`

| |MacOS X |http://developer.apple.com/tools/compilers.html[`darwin`],
Apple MacOS X GCC
a|
* `uname` is "Darwin"

| |Windows NT, 2000, and XP |http://www.mingw.org/[`mingw`], GNU
http://gcc.gnu.org/[GCC] as the http://www.mingw.org/[MinGW]
configuration with the MSYS shell
a|
* Common install location: `/mingw`

|===

The built executables are placed in a subdirectory specific to your
platform. For example, in Linux running on an Intel x86 compatible chip,
the executables are placed in: `bin.linuxx86`. The `b2[.exe]`
executable can be used to invoke B2.

The build scripts support additional invocation arguments for use by
developers of Boost.Jam and for additional setup of the toolset. The
extra arguments come after the toolset:

* Arguments not in the form of an option, before option arguments, are
used for extra setup to toolset configuration scripts.
* Arguments of the form `--option`, which are passed to the
`build.jam` build script.
* Arguments not in the form of an option, after the options, which are
targets for the `build.jam` script.

----
build [toolset] [setup*] [--option+ target*]
----

The arguments immediately after the toolset are passed directly to the
setup script of the toolset, if available and if it needs to be invoked.
This allows one to configure the toolset ass needed to do non-default
builds of `b2`. For example to build a Win64 version with `vc8`. See the
toolset description above for when particular toolsets support this.

The arguments starting with the `--option` forms are passed to the
`build.jam` script and are used to further customize what gets built.
Options and targets supported by the `build.jam` script:

`---`::
  Empty option when one wants to only specify a target.
`--release`::
  The default, builds the optimized executable.
`--debug`::
  Builds debugging versions of the executable. When built they are
  placed in their own directory `bin./platform/.debug`.
`--grammar`::
  Normally the Jam language grammar parsing files are not regenerated.
  This forces building of the grammar, although it may not force the
  regeneration of the grammar parser. If the parser is out of date it
  will be regenerated and subsequently built.
`--with-python=path`::
  Enables Python integration, given a path to the Python libraries.
`--gc`::
  Enables use of the Boehm Garbage Collector. The build will look for
  the Boehm-GC source in a "boehm_gc" subdirectory from the `b2`
  sources.
`--duma`::
  Enables use of the DUMA (Detect Unintended Memory Access) debugging
  memory allocator. The build expects to find the DUMA source files in a
  "duma" subdirectory from the `b2` sources.
`--toolset-root=path`::
  Indicates where the toolset used to build is located. This option is
  passed in by the bootstrap (`build.bat` or `build.sh`) script.
`--show-locate-target`::
  For information, prints out where it will put the built executable.
`--noassert`::
  Disable debug assertions, even if building the debug version of the
  executable.
`dist`::
  Generate packages (compressed archives) as appropriate for
  distribution in the platform, if possible.
`clean`::
  Remove all the built executables and objects.

[[jam.language]]
== Language

`B2` has an interpreted, procedural language. Statements in `b2` are
rule (procedure) definitions, rule invocations, flow-of-control
structures, variable assignments, and sundry language support.

[[jam.language.lexical]]
=== Lexical Features

`B2` treats its input files as whitespace-separated tokens, with two
exceptions: double quotes (") can enclose whitespace to embed it into a
token, and everything between the matching curly braces (\{}) in the
definition of a rule action is treated as a single string. A backslash
(\) can escape a double quote, or any single whitespace character.

`B2` requires whitespace (blanks, tabs, or newlines) to surround all
tokens, including the colon (:) and semicolon (;) tokens.

`B2` keywords (an mentioned in this document) are reserved and generally
must be quoted with double quotes (") to be used as arbitrary tokens,
such as variable or target names.

Comments start with the `\#` character and extend until the end of line.
And block comments start with `#|` and extend until the next `|#`.

[[jam.language.target]]
=== Targets

The essential `b2` data entity is a target. Build targets are files to
be updated. Source targets are the files used in updating built targets.
Built targets and source targets are collectively referred to as file
targets, and frequently built targets are source targets for other built
targets. Pseudo-targets are symbols representing dependencies on other
targets, but which are not themselves associated with any real file.

A file target's identifier is generally the file's name, which can be
absolutely rooted, relative to the directory of `b2`s invocation, or
simply local (no directory). Most often it is the last case, and the
actual file path is bound using the `$(SEARCH)` and `$(LOCATE)` special
variables. See link:#jam.language.variables.builtins.search[SEARCH and
LOCATE Variables] below. A local filename is optionally qualified with
grist, a string value used to assure uniqueness. A file target with an
identifier of the form _file(member)_ is a library member (usually an
`ar`(1) archive on Unix).

[[jam.language.target.binding_detection]]
==== 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:

[source]
----
rule bind-rule ( target : path )
----

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

[[jam.language.rules]]
=== Rules

The basic `b2` language entity is called a rule. A rule is defined in
two parts: the procedure and the actions. The procedure is a body of jam
statements to be run when the rule is invoked; the actions are the OS
shell commands to execute when updating the built targets of the rule.

Rules can return values, which can be expanded into a list with "[
_rule_ _args_ ... ]". A rule's value is the value of its last statement,
though only the following statements have values: 'if' (value of the leg
chosen), 'switch' (value of the case chosen), set (value of the
resulting variable), and 'return' (value of its arguments).

The `b2` statements for defining and invoking rules are as follows:

Define a rule's procedure, replacing any previous definition.

[source]
----
rule rulename { statements }
----

Define a rule's updating actions, replacing any previous definition.

[source]
----
actions [ modifiers ] rulename { commands }
----

Invoke a rule.

[source]
----
rulename field1 : field2 : ... : fieldN ;
----

Invoke a rule under the influence of target's specific variables..

[source]
----
on target rulename field1 : field2 : ... : fieldN ;
----

Used as an argument, expands to the return value of the rule invoked.

[source]
----
[ rulename field1 : field2 : ... : fieldN ]
[ on target rulename field1 : field2 : ... : fieldN ]
----

A rule is invoked with values in _field1_ through _fieldN_. They may be
referenced in the procedure's statements as `$(1)` through `$(N)` (9
max), and the first two only may be referenced in the action's
_commands_ as `$(1)` and `$(2)`. `$(<)` and `$(>)` are synonymous with
`$(1)` and `$(2)`.

Rules fall into two categories: updating rules (with actions), and pure
procedure rules (without actions). Updating rules treat arguments `$(1)`
and `$(2)` as built targets and sources, respectively, while pure
procedure rules can take arbitrary arguments.

When an updating rule is invoked, its updating actions are added to
those associated with its built targets (`$(1)`) before the rule's
procedure is run. Later, to build the targets in the updating phase,
_commands_ are passed to the OS command shell, with `$(1)` and `$(2)`
replaced by bound versions of the target names. See Binding above.

Rule invocation may be indirected through a variable:

[source]
----
$(var) field1 : field2 : ... : fieldN ;

on target $(var) field1 : field2 : ... : fieldN ;

[ $(var) field1 : field2 : ... : fieldN ]
[ on target $(var) field1 : field2 : ... : fieldN ]
----

The variable's value names the rule (or rules) to be invoked. A rule is
invoked for each element in the list of `$(var)`s values. The fields
`field1 : field2 : ...` are passed as arguments for each invocation For the [
... ] forms, the return value is the concatenation of the return values
for all of the invocations.

[[jam.language.rules.action_modifiers]]
==== Action Modifiers

The following action modifiers are understood:

`actions bind vars`::
  `$(vars)` will be replaced with bound values.
`actions existing`::
  `$(>)` includes only source targets currently existing.
`actions ignore`::
  The return status of the commands is ignored.
`actions piecemeal`::
  commands are repeatedly invoked with a subset of `$(>)` small enough
  to fit in the command buffer on this OS.
`actions quietly`::
  The action is not echoed to the standard output.
`actions together`::
  The `$(>)` from multiple invocations of the same action on the same
  built target are glommed together.
`actions updated`::
  `$(>)` includes only source targets themselves marked for updating.

[[jam.language.rules.argument_lists]]
==== Argument lists

You can 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:

[source]
----
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:

[%autowidth]
[cols=",",options="header",]
|===
|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 `b2` to exit with an error code:

[source]
----
### 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.

[[jam.language.rules.builtins]]
=== Built-in Rules

`B2` has a growing set of built-in rules, all of which are pure
procedure rules without updating actions. They are in three groups: the
first builds the dependency graph; the second modifies it; and the third
are just utility rules.

[[jam.language.rules.builtins.dependency_building]]
==== Dependency Building

[[jam.language.rules.builtins.dependency_building._depends__]]
===== `DEPENDS`

[source]
----
rule DEPENDS ( targets1 * : targets2 * )
----

Builds a direct dependency: makes each of _targets1_ depend on each of
_targets2_. Generally, _targets1_ will be rebuilt if _targets2_ are
themselves rebuilt or are newer than _targets1_.

[[jam.language.rules.builtins.dependency_building._includes__]]
===== `INCLUDES`

[source]
----
rule INCLUDES ( targets1 * : targets2 * )
----

Builds a sibling dependency: makes any target that depends on any of
_targets1_ also depend on each of _targets2_. This reflects the
dependencies that arise when one source file includes another: the
object built from the source file depends both on the original and
included source file, but the two sources files don't depend on each
other. For example:

[source]
----
DEPENDS foo.o : foo.c ;
INCLUDES foo.c : foo.h ;
----

`foo.o` depends on `foo.c` and `foo.h` in this example.

[[jam.language.rules.builtins.modifying_binding]]
==== Modifying Binding

The six rules `ALWAYS`, `LEAVES`, `NOCARE`, `NOTFILE`, `NOUPDATE`, and
`TEMPORARY` modify the dependency graph so that `b2` treats the targets
differently during its target binding phase. See Binding above.
Normally, `b2` updates a target if it is missing, if its filesystem
modification time is older than any of its dependencies (recursively),
or if any of its dependencies are being updated. This basic behavior can
be changed by invoking the following rules:

[[jam.language.rules.builtins.modifying_binding._always__]]
===== `ALWAYS`

[source]
----
rule ALWAYS ( targets * )
----

Causes _targets_ to be rebuilt regardless of whether they are up-to-date
(they must still be in the dependency graph). This is used for the clean
and uninstall targets, as they have no dependencies and would otherwise
appear never to need building. It is best applied to targets that are
also `NOTFILE` targets, but it can also be used to force a real file to
be updated as well.

[[jam.language.rules.builtins.modifying_binding._leaves__]]
===== `LEAVES`

[source]
----
rule LEAVES ( targets * )
----

Makes each of _targets_ depend only on its leaf sources, and not on any
intermediate targets. This makes it immune to its dependencies being
updated, as the "leaf" dependencies are those without their own
dependencies and without updating actions. This allows a target to be
updated only if original source files change.

[[jam.language.rules.builtins.modifying_binding._nocare__]]
===== `NOCARE`

[source]
----
rule NOCARE ( targets * )
----

Causes `b2` to ignore _targets_ that neither can be found nor have
updating actions to build them. Normally for such targets `b2` issues a
warning and then skips other targets that depend on these missing
targets. The `HdrRule` in `Jambase` uses `NOCARE` on the header file
names found during header file scanning, to let `b2` know that the
included files may not exist. For example, if an `#include` is within an
`#ifdef`, the included file may not actually be around.

WARNING: For targets with build actions: if their build actions exit with a
nonzero return code, dependent targets will still be built.

[[jam.language.rules.builtins.modifying_binding._notfile__]]
===== `NOTFILE`

[source]
----
rule NOTFILE ( targets * )
----

Marks _targets_ as pseudo-targets and not real files. No timestamp is
checked, and so the actions on such a target are only executed if the
target's dependencies are updated, or if the target is also marked with
`ALWAYS`. The default `b2` target `all` is a pseudo-target In
`Jambase`, `NOTFILE` is used to define several addition convenient
pseudo-targets

[[jam.language.rules.builtins.modifying_binding._noupdate__]]
===== `NOUPDATE`

[source]
----
rule NOUPDATE ( targets * )
----

Causes the timestamps on _targets_ to be ignored. This has two effects:
first, once the target has been created it will never be updated;
second, manually updating target will not cause other targets to be
updated. In `Jambase`, for example, this rule is applied to directories
by the `MkDir` rule, because `MkDir` only cares that the target
directory exists, not when it has last been updated.

[[jam.language.rules.builtins.modifying_binding._temporary__]]
===== `TEMPORARY`

[source]
----
rule TEMPORARY ( targets * )
----

Marks _targets_ as temporary, allowing them to be removed after other
targets that depend upon them have been updated. If a `TEMPORARY` target
is missing, `b2` uses the timestamp of the target's parent. `Jambase`
uses `TEMPORARY` to mark object files that are archived in a library
after they are built, so that they can be deleted after they are
archived.

[[jam.language.rules.builtins.modifying_binding._fail_expected__]]
===== `FAIL_EXPECTED`

[source]
----
rule FAIL_EXPECTED ( targets * )
----

For handling targets whose build actions are expected to fail (e.g. when
testing that assertions or compile-time type checking work properly),
Boost Jam supplies the `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.

[[jam.language.rules.builtins.modifying_binding._rmold__]]
===== `RMOLD`

[source]
----
rule RMOLD ( targets * )
----

`B2` removes any target files that may exist on disk when the rule used
to build those targets fails. However, targets whose dependencies fail
to build are not removed by default. The `RMOLD` rule causes its
arguments to be removed if any of their dependencies fail to build.

[[jam.language.rules.builtins.modifying_binding._isfile__]]
===== `ISFILE`

[source]
----
rule ISFILE ( targets * )
----

`ISFILE` marks targets as required to be files. This changes the way
`b2` searches for the target such that it ignores matches for file
system items that are not files, like directories. This makes it
possible to avoid `#include "exception"` matching if one happens to have
a directory named exception in the header search path.

WARNING: This is currently not fully implemented.

[[jam.language.rules.builtins.utility]]
==== Utility

The two rules `ECHO` and `EXIT` are utility rules, used only in `b2`s
parsing phase.

[[jam.language.rules.builtins.utility._echo__]]
===== `ECHO`

[source]
----
rule ECHO ( args * )
----

Blurts out the message _args_ to stdout.

[[jam.language.rules.builtins.utility._exit__]]
===== `EXIT`

[source]
----
rule EXIT ( message * : result-value ? )
----

Blurts out the _message_ to stdout and then exits with a failure status
if no _result-value_ is given, otherwise it exits with the given
_result-value_.

`Echo`, `echo`, `Exit`, and `exit` are accepted as aliases for
`ECHO` and `EXIT`, since it is hard to tell that these are built-in
rules and not part of the language, like `include`.

[[jam.language.rules.builtins.utility._glob__]]
===== `GLOB`

The `GLOB` rule does filename globing

[source]
----
rule GLOB ( directories * : patterns * : downcase-opt ? )
----

Using the same wildcards as for the patterns in the switch statement. It
is invoked by being used as an argument to a rule invocation inside of
"[ ]". For example: `FILES = [ GLOB dir1 dir2 : *.c *.h ]` sets `FILES` to
the list of C source
and header files in `dir1` and `dir2`. The resulting filenames are the
full pathnames, including the directory, but the pattern is applied only
to the file name without the directory.

If _downcase-opt_ is supplied, filenames are converted to all-lowercase
before matching against the pattern; you can use this to do
case-insensitive matching using lowercase patterns. The paths returned
will still have mixed case if the OS supplies them. On Windows NT and
Cygwin, and OpenVMS, filenames are always down-cased before matching.

[[jam.language.rules.builtins.utility._glob_archive__]]
===== `GLOB_ARCHIVE`

The `GLOB_ARCHIVE` rule does name globing of object archive members.

[source]
----
rule GLOB_ARCHIVE ( archives * : member-patterns * : downcase-opt ? : symbol-patterns ? )
----

Similarly to `GLOB`, this rule is used to match names of member files in
an archive (static object library). List of successfully matched members
is returned or null otherwise. The resulting member names are qualified
with pathname of the containing archive in the form
`archive-path(member-name)`. Member patterns are for matching member
name only; when no wildcards specified -- an exact match is assumed.
Member names generally correspond to object file names and as such are
platform-specific -- use of platform-defined object suffix in the
matching patterns can allow for portability.

If _downcase-opt_ is supplied, the member names are converted to
all-lowercase before matching against the pattern; you can use this to
do case-insensitive matching using lowercase patterns. The paths
returned will still have mixed case if the OS supplies them. On Windows
NT, Cygwin, and OpenVMS, filenames are always down-cased before matching.

Additionally, members can be matched with symbol/function patterns on
supported platforms (currently, OpenVMS only). In this case, members
containing the matching symbols are returned. Member and symbol patterns
are applied as OR conditions, with member patterns taking precedence. On
unsupported platforms, null is returned when any symbol patterns are
specified.

[[jam.language.rules.builtins.utility._match__]]
===== `MATCH`

The `MATCH` rule does pattern matching.

[source]
----
rule MATCH ( regexps + : list * )
----

Matches the `egrep`(1) style regular expressions _regexps_ against the
strings in _list_. The result is a list of matching `()` subexpressions
for each string in _list_, and for each regular expression in _regexps_.

[[jam.language.rules.builtins.utility._backtrace__]]
===== `BACKTRACE`

[source]
----
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.

[[jam.language.rules.builtins.utility._update__]]
===== `UPDATE`

[source]
----
rule UPDATE ( targets * )
----

Classic jam treats any non-option element of command line as a name of
target to be updated. This prevented more sophisticated handling of
command line. This is now enabled again but with additional changes to
the `UPDATE` rule to allow for the flexibility of changing the list of
targets to update. The UPDATE rule has two effects:

1.  It clears the list of targets to update, and
2.  Causes the specified targets to be updated.

If no target was specified with the `UPDATE` rule, no targets will be
updated. To support changing of the update list in more useful ways, the
rule also returns the targets previously in the update list. This makes
it possible to add targets as such:

[source]
----
local previous-updates = [ UPDATE ] ;
UPDATE $(previous-updates) a-new-target ;
----

[[jam.language.rules.builtins.utility._w32_getreg__]]
===== `W32_GETREG`

[source]
----
rule W32_GETREG ( path : data ? )
----

Defined only for win32 platform. It reads the registry of Windows.
'_path_' is the location of the information, and '_data_' is the name of
the value which we want to get. If '_data_' is omitted, the default
value of '_path_' will be returned. The '_path_' value must conform to
MS key path format and must be prefixed with one of the predefined root
keys. As usual,

* `HKLM` is equivalent to `HKEY_LOCAL_MACHINE`.
* `HKCU` is equivalent to `HKEY_CURRENT_USER`.
* `HKCR` is equivalent to `HKEY_CLASSES_ROOT`.

Other predefined root keys are not supported.

Currently supported data types : `REG_DWORD`, `REG_SZ`,
`REG_EXPAND_SZ`, `REG_MULTI_SZ`. The data with `REG_DWORD` type
will be turned into a string, `REG_MULTI_SZ` into a list of strings,
and for those with `REG_EXPAND_SZ` type environment variables in it
will be replaced with their defined values. The data with `REG_SZ`
type and other unsupported types will be put into a string without
modification. If it can't receive the value of the data, it just return
an empty list. For example,

[source]
----
local PSDK-location =
  [ W32_GETREG HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\MicrosoftSDK\\Directories : "Install Dir" ] ;
----

[[jam.language.rules.builtins.utility._w32_getregnames__]]
===== `W32_GETREGNAMES`

[source]
----
rule W32_GETREGNAMES ( path : result-type )
----

Defined only for win32 platform. It reads the registry of Windows.
'_path_' is the location of the information, and '_result-type_' is
either `subkeys` or `values`. For more information on '_path_'
format and constraints, please see `W32_GETREG`.

Depending on '_result-type_', the rule returns one of the following:

`subkeys`::
  Names of all direct sub-keys of '_path_'.
`values`::
  Names of values contained in registry key given by '_path_'. The
  "default" value of the key appears in the returned list only if its
  value has been set in the registry.

If '_result-type_' is not recognized, or requested data cannot be
retrieved, the rule returns an empty list. Example:

[source]
----
local key = "HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\App Paths" ;
local subkeys = [ W32_GETREGNAMES "$(key)" : subkeys ] ;
for local subkey in $(subkeys)
{
    local values = [ W32_GETREGNAMES "$(key)\\$(subkey)" : values ] ;
    for local value in $(values)
    {
        local data = [ W32_GETREG "$(key)\\$(subkey)" : "$(value)" ] ;
        ECHO "Registry path: " $(key)\\$(subkey) ":" $(value) "=" $(data) ;
    }
}
----

[[jam.language.rules.builtins.utility._shell__]]
===== `SHELL`

[source]
----
rule SHELL ( command : * )
----

`SHELL` executes _command_, and then returns the standard output of
_command_. `SHELL` only works on platforms with a `popen()` function in
the C library. On platforms without a working `popen()` function,
`SHELL` is implemented as a no-op. `SHELL` works on Unix, MacOS X, and
most Windows compilers. `SHELL` is a no-op on Metrowerks compilers under
Windows. There is a variable set of allowed options as additional
arguments:

`exit-status`::
  In addition to the output the result status of the executed command is
  returned as a second element of the result.
`no-output`::
  Don't capture the output of the command. Instead an empty ("") string
  value is returned in place of the output.
`strip-eol`::
  Remove trailing end-of-line character from output, if any.

Because the Perforce/Jambase defines a `SHELL` rule which hides the
builtin rule, `COMMAND` can be used as an alias for `SHELL` in such a
case.

[[jam.language.rules.builtins.utility._md5__]]
===== `MD5`

[source]
----
rule MD5 ( string )
----

`MD5` computes the MD5 hash of the string passed as parameter and
returns it.

[[jam.language.rules.builtins.utility._split_by_characters__]]
===== `SPLIT_BY_CHARACTERS`

[source]
----
rule SPLIT_BY_CHARACTERS ( string : delimiters )
----

`SPLIT_BY_CHARACTERS` splits the specified _string_ on any delimiter
character present in _delimiters_ and returns the resulting list.

[[jam.language.rules.builtins.utility._precious__]]
===== `PRECIOUS`

[source]
----
rule PRECIOUS ( targets * )
----

The `PRECIOUS` rule specifies that each of the targets passed as the
arguments should not be removed even if the command updating that target
fails.

[[jam.language.rules.builtins.utility._pad__]]
===== `PAD`

[source]
----
rule PAD ( string : width )
----

If _string_ is shorter than _width_ characters, pads it with whitespace
characters on the right, and returns the result. Otherwise, returns
_string_ unmodified.

[[jam.language.rules.builtins.utility._file_open__]]
===== `FILE_OPEN`

[source]
----
rule FILE_OPEN ( filename : mode )
----

The `FILE_OPEN` rule opens the specified file and returns a file
descriptor. The _mode_ parameter can be either "w" or "r". Note that at
present, only the `UPDATE_NOW` rule can use the resulting file
descriptor number.

[[jam.language.rules.builtins.utility._update_now__]]
===== `UPDATE_NOW`

[source]
----
rule UPDATE_NOW ( targets * : log ? : ignore-minus-n ? )
----

The `UPDATE_NOW` caused the specified targets to be updated immediately.
If update was successful, non-empty string is returned. The _log_
parameter, if present, specifies a descriptor of a file where all output
from building is redirected. If the _ignore-minus-n_ parameter is
specified, the targets are updated even if the `-n` parameter is
specified on the command line.

[[jam.language.flow_of_control]]
=== Flow-of-Control

`B2` has several simple flow-of-control statements:

[source]
----
for var in list { statements }
----

Executes _statements_ for each element in _list_, setting the variable
_var_ to the element value.

[source]
----
if cond { statements }
[ else { statements } ]
----

Does the obvious; the `else` clause is optional. _cond_ is built of:

`a`::
  true if any _a_ element is a non-zero-length string
`a = b`::
  list _a_ matches list _b_ string-for-string
`a != b`::
  list _a_ does not match list _b_
`a < b`::
  _a[i]_ string is less than _b[i]_ string, where _i_ is first
  mismatched element in lists _a_ and _b_
`+a <= b+`::
  every _a_ string is less than or equal to its _b_ counterpart
`a > b`::
  _a[i]_ string is greater than _b[i]_ string, where _i_ is first
  mismatched element
`a >= b`::
  every _a_ string is greater than or equal to its _b_ counterpart
`a in b`::
  true if all elements of _a_ can be found in _b_, or if _a_ has no
  elements
`! cond`::
  condition not true
`cond && cond`::
  conjunction
`cond || cond`::
  disjunction
`( cond )`::
  precedence grouping

[source]
----
include file ;
----

Causes `b2` to read the named _file_. The _file_ is bound like a regular
target (see Binding above) but unlike a regular target the include
_file_ cannot be built.

The include _file_ is inserted into the input stream during the parsing
phase. The primary input file and all the included file(s) are treated
as a single file; that is, `b2` infers no scope boundaries from included
files.

[source]
----
local vars [ = values ] ;
----

Creates new _vars_ inside to the enclosing `{}` block, obscuring any
previous values they might have. The previous values for vars are
restored when the current block ends. Any rule called or file included
will see the local and not the previous value (this is sometimes called
Dynamic Scoping). The local statement may appear anywhere, even outside
of a block (in which case the previous value is restored when the input
ends). The _vars_ are initialized to _values_ if present, or left
uninitialized otherwise.

[source]
----
return values ;
----

Within a rule body, the return statement sets the return value for an
invocation of the rule and returns to the caller.

[source]
----
switch value
{
    case pattern1 : statements ;
    case pattern2 : statements ;
    ...
}
----

The switch statement executes zero or one of the enclosed _statements_,
depending on which, if any, is the first case whose _pattern_ matches
_value_. The _pattern_ values are not variable-expanded. The pattern
values may include the following wildcards:

`?`::
  match any single character
`*`::
  match zero or more characters
`[chars]`::
  match any single character in _chars_
`[^chars]`::
  match any single character not in _chars_
`\x`::
  match _x_ (escapes the other wildcards)

[source]
----
while cond { statements }
----

Repeatedly execute _statements_ while _cond_ remains true upon entry.
(See the description of _cond_ expression syntax under if, above).

[source]
----
break ;
----

Immediately exits the nearest enclosing while or for loop.

[source]
----
continue ;
----

Jumps to the top of the nearest enclosing while or for loop.

[[jam.language.variables]]
=== Variables

`B2` variables are lists of zero or more elements, with each element
being a string value. An undefined variable is indistinguishable from a
variable with an empty list, however, a defined variable may have one
more elements which are null strings. All variables are referenced as
`$(variable)`.

Variables are either global or target-specific. In the latter case, the
variable takes on the given value only during the updating of the
specific target.

A variable is defined with:

[source]
----
variable = elements ;
variable += elements ;
variable on targets = elements ;
variable on targets += elements ;
variable default = elements ;
variable ?= elements ;
----

The first two forms set _variable_ globally. The third and forth forms
set a target-specific variable. The `=` operator replaces any previous
elements of _variable_ with _elements_; the `+=` operation adds
_elements_ to _variable_'s list of elements. The final two forms are
synonymous: they set _variable_ globally, but only if it was previously
unset.

Variables referenced in updating commands will be replaced with their
values; target-specific values take precedence over global values.
Variables passed as arguments (`$(1)` and `$(2)`) to actions are
replaced with their bound values; the `bind` modifier can be used on
actions to cause other variables to be replaced with bound values. See
Action Modifiers above.

`B2` variables are not re-exported to the environment of the shell that
executes the updating actions, but the updating actions can reference
`b2` variables with `$(variable)`.

[[jam.language.variables.expansion]]
==== Variable Expansion

During parsing, `b2` performs variable expansion on each token that is
not a keyword or rule name. Such tokens with embedded variable
references are replaced with zero or more tokens. Variable references
are of the form `$(v)` or `$(vm)`, where _v_ is the variable name, and
_m_ are optional modifiers.

Variable expansion in a rule's actions is similar to variable expansion
in statements, except that the action string is tokenized at whitespace
regardless of quoting.

The result of a token after variable expansion is the _product_ of the
components of the token, where each component is a literal substring or
a list substituting a variable reference. For example:

----
$(X) -> a b c
t$(X) -> ta tb tc
$(X)z -> az bz cz
$(X)-$(X) -> a-a a-b a-c b-a b-b b-c c-a c-b c-c
----

The variable name and modifiers can themselves contain a variable
reference, and this partakes of the product as well:

----
$(X) -> a b c
$(Y) -> 1 2
$(Z) -> X Y
$($(Z)) -> a b c 1 2
----

Because of this product expansion, if any variable reference in a token
is undefined, the result of the expansion is an empty list. If any
variable element is a null string, the result propagates the non-null
elements:

----
$(X) -> a ""
$(Y) -> "" 1
$(Z) ->
-$(X)$(Y)- -> -a- -a1- -- -1-
-$(X)$(Z)- ->
----

A variable element's string value can be parsed into grist and
filename-related components. Modifiers to a variable are used to select
elements, select components, and replace components. The modifiers are:

`[n]`::
+
Select element number _n_ (starting at 1). If the variable contains
fewer than _n_ elements, the result is a zero-element list. _n_ can be
negative in which case the element number _n_ from the last leftward
is returned.

`[n-m]`::
+
Select elements number _n_ through _m_. _n_ and _m_ can be negative in
which case they refer to elements counting from the last leftward.

`[n-]`::
+
Select elements number _n_ through the last. _n_ can be negative in
which case it refers to the element counting from the last leftward.

`:B`::
+
Select filename base -- a basename without extension.

`:S`::
+
Select file extension -- a (last) filename suffix.

`:M`::
+
Select archive member name.
`:D`::
+
Select directory path.

`:P`::
+
Select parent directory.

`:G`::
+
Select grist.

`:U`::
+
Replace lowercase characters with uppercase.

`:L`::
+
Replace uppercase characters with lowercase.

`:T`::
+
Converts all back-slashes ("\") to forward slashes ("/"). For example
+
----
x = "C:\\Program Files\\Borland" ; ECHO $(x:T) ;
----
+
prints `C:/Program Files/Borland`

`:W`::
+
When invoking Windows-based tools from http://www.cygwin.com/[Cygwin]
it can be important to pass them true windows-style paths. The `:W`
modifier, *under Cygwin only*, turns a cygwin path into a Win32 path
using the
http://www.cygwin.com/cygwin-api/func-cygwin-conv-to-win32-path.html[`cygwin_conv_to_win32_path`]
function. For example
+
----
x = "/cygdrive/c/Program Files/Borland" ; ECHO $(x:W) ;
----
+
prints `C:\Program Files\Borland` on Cygwin
+
Similarly, when used on OpenVMS, the `:W` modifier translates a
POSIX-style path into native VMS-style format using `decc$to_vms` CRTL
function. This modifier is generally used inside action blocks to
properly specify file paths in VMS-specific commands. For example
+
----
x = "subdir/filename.c" ; ECHO $(x:W) ;
----
+
prints `[.subdir]filename.c` on OpenVMS
+
On other platforms, the string is unchanged.

`:chars`::
Select the components listed in _chars_.
+
For example, `:BS` selects filename (basename and extension).

`:G=grist`::
Replace grist with _grist_.

`:D=path`::
Replace directory with _path_.

`:B=base`::
Replace the base part of file name with _base_.

`:S=suf`::
Replace the suffix of file name with _suf_.

`:M=mem`::
Replace the archive member name with _mem_.

`:R=root`::
Prepend _root_ to the whole file name, if not already rooted.

`:E=value`::
Assign _value_ to the variable if it is unset.

`:J=joinval`::
Concatenate list elements into single element, separated by
_joinval_.

On VMS, `$(var:P)` is the parent directory of `$(var:D)`.

[[jam.language.variables.local_for_loop_variables]]
==== Local For Loop Variables

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

[source]
----
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"
----

[[jam.language.variables.atfile]]
==== Generated File Expansion

During expansion of expressions `b2` also looks for subexpressions of
the form `@(filename:E=filecontents)` and replaces the expression with
`filename` after creating the given file with the contents set to
`filecontents`. This is useful for creating compiler response files, and
other "internal" files. The expansion works both during parsing and
action execution. Hence it is possible to create files during any of the
three build phases.

[[jam.language.variables.builtins]]
==== Built-in Variables

This section discusses variables that have special meaning to `b2`. All
of these must be defined or used in the global module -- using those
variables inside a named module will not have the desired effect. See
link:#jam.language.modules[Modules].

[[jam.language.variables.builtins.search]]
===== SEARCH and LOCATE

These two variables control the binding of file target names to
locations in the file system. Generally, `$(SEARCH)` is used to find
existing sources while `$(LOCATE)` is used to fix the location for built
targets.

Rooted (absolute path) file targets are bound as is. Unrooted file
target names are also normally bound as is, and thus relative to the
current directory, but the settings of `$(LOCATE)` and `$(SEARCH)` alter
this:

* If `$(LOCATE)` is set then the target is bound relative to the first
directory in `$(LOCATE)`. Only the first element is used for binding.
* If `$(SEARCH)` is set then the target is bound to the first directory
in `$(SEARCH)` where the target file already exists.
* If the `$(SEARCH)` search fails, the target is bound relative to the
current directory anyhow.

Both `$(SEARCH)` and `$(LOCATE)` should be set target-specific and not
globally. If they were set globally, `b2` would use the same paths for
all file binding, which is not likely to produce sane results. When
writing your own rules, especially ones not built upon those in Jambase,
you may need to set `$(SEARCH)` or `$(LOCATE)` directly. Almost all of
the rules defined in Jambase set `$(SEARCH)` and `$(LOCATE)` to sensible
values for sources they are looking for and targets they create,
respectively.

[[jam.language.variables.builtins.hdrscan]]
===== HDRSCAN and HDRRULE

These two variables control header file scanning. `$(HDRSCAN)` is an
`egrep(1)` pattern, with ()'s surrounding the file name, used to find
file inclusion statements in source files. `Jambase` uses
`$(HDRPATTERN)` as the pattern for `$(HDRSCAN)`. `$(HDRRULE)` is the
name of a rule to invoke with the results of the scan: the scanned file
is the target, the found files are the sources. This is the only place
where `b2` invokes a rule through a variable setting.

Both `$(HDRSCAN)` and `$(HDRRULE)` must be set for header file scanning
to take place, and they should be set target-specific and not globally.
If they were set globally, all files, including executables and
libraries, would be scanned for header file include statements.

The scanning for header file inclusions is not exact, but it is at least
dynamic, so there is no need to run something like `makedepend(GNU)` to
create a static dependency file. The scanning mechanism errs on the side
of inclusion (i.e., it is more likely to return filenames that are not
actually used by the compiler than to miss include files) because it
can't tell if `#include` lines are inside `#ifdefs` or other conditional
logic. In `Jambase`, `HdrRule` applies the `NOCARE` rule to each header
file found during scanning so that if the file isn't present yet doesn't
cause the compilation to fail, `b2` won't care.

Also, scanning for regular expressions only works where the included
file name is literally in the source file. It can't handle languages
that allow including files using variable names (as the `Jam` language
itself does).

[[jam.language.variables.builtins.semaphores]]
===== Semaphores

It is sometimes desirable to disallow parallel execution of some
actions. For example:

* Old versions of yacc use files with fixed names. So, running two yacc
actions is dangerous.
* One might want to perform parallel compiling, but not do parallel
linking, because linking is i/o bound and only gets slower.

Craig McPeeters has extended Perforce Jam to solve such problems, and
that extension was integrated in Boost.Jam.

Any target can be assigned a _semaphore_, by setting a variable called
`SEMAPHORE` on that target. The value of the variable is the semaphore
name. It must be different from names of any declared target, but is
arbitrary otherwise.

The semantic of semaphores is that in a group of targets which have the
same semaphore, only one can be updated at the moment, regardless of
`-j` option.

[[jam.language.variables.builtins.platform_identifier]]
===== Platform Identifier

A number of Jam built-in variables can be used to identify runtime
platform:

`OS`::
  OS identifier string
`OSPLAT`::
  Underlying architecture, when applicable
`MAC`::
  true on MAC platform
`NT`::
  true on NT platform
`OS2`::
  true on OS2 platform
`UNIX`::
  true on Unix platforms
`VMS`::
  true on VMS platform

[[jam.language.variables.builtins.jam_version]]
==== Jam Version

`JAMDATE`::
  Time and date at `b2` start-up as an ISO-8601 UTC value.
`JAMUNAME`::
  Output of uname(1) command (Unix only)
`JAMVERSION`::
  `b2` version, as a sematic triplet "X.Y.Z".
`JAM_VERSION`::
  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`.

[[jam.language.variables.builtins.jamshell]]
===== JAMSHELL

When `b2` executes a rule's action block, it forks and execs a shell,
passing the action block as an argument to the shell. The invocation of
the shell can be controlled by `$(JAMSHELL)`. The default on Unix is,
for example:

[source]
----
JAMSHELL = /bin/sh -c % ;
----

The `%` is replaced with the text of the action block.

`B2` does not directly support building in parallel across multiple
hosts, since that is heavily dependent on the local environment. To
build in parallel across multiple hosts, you need to write your own
shell that provides access to the multiple hosts. You then reset
`$(JAMSHELL)` to reference it.

Just as `b2` expands a `%` to be the text of the rule's action block, it
expands a `!` to be the multi-process slot number. The slot number
varies between 1 and the number of concurrent jobs permitted by the `-j`
flag given on the command line. Armed with this, it is possible to write
a multiple host shell. For example:

[source,bash]
----
#!/bin/sh

# This sample JAMSHELL uses the SunOS on(1) command to execute a
# command string with an identical environment on another host.

# Set JAMSHELL = jamshell ! %
#
# where jamshell is the name of this shell file.
#
# This version handles up to -j6; after that they get executed
# locally.

case $1 in
1|4) on winken sh -c "$2";;
2|5) on blinken sh -c "$2";;
3|6) on nod sh -c "$2";;
*) eval "$2";;
esac
----

[[jam.language.variables.builtins.actionrule]]
===== `+__TIMING_RULE__+` and `+__ACTION_RULE__+`

The `+__TIMING_RULE__+` and `+__ACTION_RULE__+` can be set to the name of a
rule for `b2` to call *after* an action completes for a target. They
both give diagnostic information about the action that completed. For
`+__TIMING_RULE__+` the rule is called as:

[source]
----
rule timing-rule ( args * : target : start end user system )
----

And `+__ACTION_RULE__+` is called as:

[source]
----
rule action-rule ( args * : target : command status start end user system : output ? )
----

The arguments for both are:

`args`::
  Any values following the rule name in the `+__TIMING_RULE__+` or
  `+__ACTION_RULE__+` are passed along here.
`target`::
  The `b2` target that was built.
`command`::
  The text of the executed command in the action body.
`status`::
  The integer result of the executed command.
`start`::
  The starting timestamp of the executed command as a ISO-8601 UTC
  value.
`end`::
  The completion timestamp of the executed command as a ISO-8601 UTC
  value.
`user`::
  The number of user CPU seconds the executed command spent as a
  floating point value.
`system`::
  The number of system CPU seconds the executed command spent as a
  floating point value.
`output`::
  The output of the command as a single string. The content of the
  output reflects the use of the `-pX` option.

NOTE: If both variables are set for a target both are called, first
`+__TIMING_RULE__+` then `+__ACTION_RULE__+`.

[[jam.language.modules]]
=== Modules

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.

[[jam.language.modules.declaration]]
==== Declaration

[source]
----
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:

[source]
----
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:

[source]
----
module your_module
{
    rule bedtime ( ) { my_module.salute goodnight ; }
}
----

[[jam.language.modules.variable_scope]]
==== Variable Scope

Each module has its own set of dynamically nested variable scopes. When
execution passes from module A to module B, all the variable bindings
from A become unavailable, and are replaced by the bindings that belong
to B. This applies equally to local and global variables:

[source]
----
module A
{
    x = 1 ;
    rule f ( )
    {
        local y = 999 ; # becomes visible again when B.f calls A.g
        B.f ;
    }
    rule g ( )
    {
        ECHO $(y) ;     # prints "999"
    }
}
module B
{
    y = 2 ;
    rule f ( )
    {
        ECHO $(y) ; # always prints "2"
        A.g ;
    }
}
----

The only way to access another module's variables is by entering that
module:

[source]
----
rule peek ( module-name ? : variables + )
{
    module $(module-name)
    {
        return $($(>)) ;
    }
}
----

Note that because existing variable bindings change whenever a new
module scope is entered, argument bindings become unavailable. That
explains the use of `$(>)` in the peek rule above.

[[jam.language.modules.local_rules]]
==== Local Rules

[source]
----
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:

[source]
----
[ RULENAMES module-name ]
----

[[jam.language.modules.the__rulenames__rule]]
==== The `RULENAMES` Rule

[source]
----
rule RULENAMES ( module ? )
----

Returns a list of the names of all non-local rules in the given module.
If _module_ is omitted, the names of all non-local rules in the global
module are returned.

[[jam.language.modules.the__varnames__rule]]
==== The `VARNAMES` Rule

[source]
----
rule VARNAMES ( module ? )
----

Returns a list of the names of all variable bindings in the given
module. If _module_ is omitted, the names of all variable bindings in
the global module are returned.

NOTE: This includes any local variables in rules from the call stack which
have not returned at the time of the `VARNAMES` invocation.

[[jam.language.modules.the__import__rule]]
==== The `IMPORT` Rule

`IMPORT` allows rule name aliasing across modules:

[source]
----
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,

[source]
----
# 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 ] ;
----

[[jam.language.modules.the__export__rule]]
==== The `EXPORT` Rule

`EXPORT` allows rule name aliasing across modules:

[source]
----
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,

[source]
----
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.
----

[[jam.language.modules.the__caller_module__rule]]
==== The `CALLER_MODULE` Rule

[source]
----
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}":

[source]
----
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)} ;
----

[[jam.language.modules.the__delete_module__rule]]
==== The `DELETE_MODULE` Rule

[source]
----
rule DELETE_MODULE ( module ? )
----

`DELETE_MODULE` removes all of the variable bindings and
otherwise-unreferenced rules from the given module (or the global
module, if no module is supplied), and returns their memory to the
system.

NOTE: Though it won't affect rules that are currently executing until they
complete, `DELETE_MODULE` should be used with extreme care because it
will wipe out any others and all variable (including locals in that
module) immediately. Because of the way dynamic binding works, variables
which are shadowed by locals will not be destroyed, so the results can
be really unpredictable.

[[jam.miscellaneous]]
== Miscellaneous

[[jam.miscellaneous.diagnostics]]
=== Diagnostics

In addition to generic error messages, `b2` may emit one of the
following:

----
warning: unknown rule X
----

A rule was invoked that has not been defined with an `actions` or
`rule` statement.

----
using N temp target(s)
----

Targets marked as being temporary (but nonetheless present) have been
found.

----
updating N target(s)
----

Targets are out-of-date and will be updated.

----
can't find N target(s)
----

Source files can't be found and there are no actions to create them.

----
can't make N target(s)
----

Due to sources not being found, other targets cannot be made.

----
warning: X depends on itself
----

A target depends on itself either directly or through its sources.

----
don't know how to make X
----

A target is not present and no actions have been defined to create it.

----
X skipped for lack of Y
----

A source failed to build, and thus a target cannot be built.

----
warning: using independent target X
----

A target that is not a dependency of any other target is being
referenced with `$(<)` or `$(>)`.

----
X removed
----

`B2` removed a partially built target after being interrupted.

[[jam.miscellaneous.bugs__limitations]]
=== Bugs, Limitations

For parallel building to be successful, the dependencies among files
must be properly spelled out, as targets tend to get built in a
quickest-first ordering. Also, beware of un-parallelizable commands that
drop fixed-named files into the current directory, like `yacc(1)` does.

A poorly set `$(JAMSHELL)` is likely to result in silent failure.

[[jam.miscellaneous.fundamentals]]
=== Fundamentals

This section is derived from the official Jam documentation and from
experience using it and reading the Jambase rules. We 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. We 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 built-in `DEPENDS` rule creates a link in the dependency
graph between the named targets.
* Note that the original Jam 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_. It does this in an
odd way, by tacking _targets2_ onto a special tail section in the
dependency list of everything in _targets1_. It seems to be OK to create
circular dependencies this way; in fact, it appears to be the "right
thing to do" when a single build action produces both _targets1_ and
_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. Binding is a process
of searching for a file with the same name as the target (sans grist),
based on the settings of the target-specific `SEARCH` and `LOCATE`
variables.
* 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".
* "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
sub-projects, or for the debug and release variants of the "same"
abstract target. Each distinct target bound to a file called "test.exe"
has its own unique grist prefix. 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 used instead of identifying targets with absolute paths
for two reasons:
1.  The location of targets cannot always be derived solely from what
the user puts in a Jamfile, but sometimes depends also on the binding
process. Some mechanism to distinctly identify targets 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, leading <s and trailing >s are added if necessary to form an
expression of the form <expr2>; <expr2> is then prepended.
* When Jam is invoked it imports all environment variable settings into
corresponding Jam variables, followed by all command-line (-s...)
variable settings. Variables whose name ends in PATH, Path, or path are
split into string lists on OS-specific path-list separator boundaries
(e.g. ":" for UNIX and ";" for Windows). All other variables are split
on space (" ") boundaries. Boost Jam modifies that behavior by allowing
variables to be quoted.
* A variable whose value is an empty list or which consists entirely of
empty strings has a negative logical value. Thus, for example, code like
the following allows a sensible non-empty default which can easily be
overridden by the user:
+
----
MESSAGE ?\= starting jam... ;
if $(MESSAGE) { ECHO The message is: $(MESSAGE) ; }
----
+
If the user wants a specific message, he invokes jam with
`-sMESSAGE=message
            text`. If he wants no message, he invokes jam with
`-sMESSAGE=` and nothing at all is printed.
* The parsing of command line options in Jam can be rather unintuitive,
with regards to how other Unix programs accept options. There are two
variants accepted as valid for an option:
1.  `-xvalue`, and
2.  `-x value`.
