[[b2.tasks]]
= Common tasks

This section describes main targets types that B2 supports
out-of-the-box. Unless otherwise noted, all mentioned main target rules
have the common signature, described in
link:#b2.overview.targets[the section called “Declaring Targets”].

[[b2.tasks.programs]]
== Programs

Programs are created using the `exe` rule, which follows the
link:#b2.main-target-rule-syntax[common syntax]. For example:

[source]
----
exe hello
    : hello.cpp some_library.lib /some_project//library
    : <threading>multi
    ;
----

This will create an executable file from the sources--in this case,
one {CPP} file, one library file present in the same directory, and
another library that is created by B2. Generally, sources can
include C and {CPP} files, object files and libraries. B2 will
automatically try to convert targets of other types.

WARNING: On Windows, if an application uses shared libraries, and both the
application and the libraries are built using B2, it is not
possible to immediately run the application, because the `PATH` environment
variable should include the path to the libraries. It means you have to either
add the paths manually, or have the build place the application and the
libraries into the same directory. See
link:#b2.tasks.installing[the section called “Installing”].

[[b2.tasks.libraries]]
== Libraries

Library targets are created using the `lib` rule, which follows the
link:#b2.main-target-rule-syntax[common syntax]. For example:

[source]
----
lib helpers : helpers.cpp ;
----

This will define a library target named `helpers` built from the
`helpers.cpp` source file. It can be either a static library or a shared
library, depending on the value of the
link:#b2.builtin.features.link[`<link>`] feature.

Library targets can represent:

* Libraries that should be built from source, as in the example above.
* Prebuilt libraries which already exist on the system. Such libraries
can be searched for by the tools using them (typically with the linker's
`-l` option) or their paths can be known in advance by the build system.

The syntax for prebuilt libraries is given below:

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

The `name` property specifies the name of the library without the
standard prefixes and suffixes. For example, depending on the system,
`z` could refer to a file called `z.so`, `libz.a`, or `z.lib`, etc. The
`search` feature specifies paths in which to search for the library in
addition to the default compiler paths. `search` can be specified
several times or it can be omitted, in which case only the default
compiler paths will be searched. The `file` property specifies the file
location.

The difference between using the `file` feature and using a combination
of the `name` and `search` features is that `file` is more precise.

[TIP]
====
The value of the `search` feature is just added to the linker search
path. When linking to multiple libraries, the paths specified by
`search` are combined without regard to which `lib` target each path
came from. Thus, given

[source]
----
lib a : : <name>a <search>/pool/release ;
lib b : : <name>b <search>/pool/debug ;
----

If /pool/release/a.so, /pool/release/b.so, /pool/debug/a.so, and
/pool/release/b.so all exist, the linker will probably take both `a` and
`b` from the same directory, instead of finding `a` in /pool/release and
`b` in /pool/debug. If you need to distinguish between multiple
libraries with the same name, it's safer to use `file`.
====

For convenience, the following syntax is allowed:

[source]
----
lib z ;
lib gui db aux ;
----

which has exactly the same effect as:

[source]
----
lib z : : <name>z ;
lib gui : : <name>gui ;
lib db : : <name>db ;
lib aux : : <name>aux ;
----

When a library references another library you should put that other
library in its list of sources. This will do the right thing in all
cases. For portability, you should specify library dependencies even for
searched and prebuilt libraries, otherwise, static linking on Unix will
not work. For example:

[source]
----
lib z ;
lib png : z : <name>png ;
----

[TIP]
====
When a library has a shared library as a source, or a static library has
another static library as a source then any target linking to the first
library with automatically link to its source library as well.

On the other hand, when a shared library has a static library as a
source then the first library will be built so that it completely
includes the second one.

If you do not want a shared library to include all the libraries
specified in its sources (especially statically linked ones), you would
need to use the following:

[source]
----
lib b : a.cpp ;
lib a : a.cpp : <use>b : : <library>b ;
----

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

link:#b2.overview.targets[Usage requirements] are often very useful
for defining library targets. For example, imagine that you want you
build a `helpers` library and its interface is described in its
`helpers.hpp` header file located in the same directory as the
`helpers.cpp` source file. Then you could add the following to the
Jamfile located in that same directory:

[source]
----
lib helpers : helpers.cpp : : : <include>. ;
----

which would automatically add the directory where the target has been
defined (and where the library's header file is located) to the
compiler's include path for all targets using the `helpers` library.
This feature greatly simplifies Jamfiles.

[[b2.tasks.alias]]
== Alias

The `alias` rule gives an alternative name to a group of targets. For
example, to give the name `core` to a group of three other targets with
the following code:

[source]
----
alias core : im reader writer ;
----

Using `core` on the command line, or in the source list of any other
target is the same as explicitly using `im`, `reader`, and `writer`.

Another use of the `alias` rule is to change build properties. For
example, if you want to link statically to the Boost Threads
library, you can write the following:

[source]
----
alias threads : /boost/thread//boost_thread : <link>static ;
----

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

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

[source]
----
alias header_only_library : : : :  <include>/usr/include/header_only_library ;
----

then using `header_only_library` in sources will only add an include
path. Also note that when an alias has sources, their usage requirements
are propagated as well. For example:

[source]
----
lib library1 : library1.cpp : : : <include>/library/include1 ;
lib library2 : library2.cpp : : : <include>/library/include2 ;
alias static_libraries : library1 library2 : <link>static ;
exe main : main.cpp static_libraries ;
----

will compile `main.cpp` with additional includes required for using the
specified static libraries.

[[b2.tasks.installing]]
== Installing

This section describes various ways to install built targets and
arbitrary files.

=== Basic install

For installing a built target you should use the `install` rule, which
follows the link:#b2.main-target-rule-syntax[common syntax]. For
example:

[source]
----
install dist : hello helpers ;
----

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

[source]
----
install dist : hello helpers : <location>/usr/bin ;
----

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

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

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

See also link:#b2.reference.variants.propcond[conditional properties]
and link:#b2.faq.envar[environment variables]

=== Installing with all dependencies

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

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

will find all targets that `hello` depends on, and install all of those
which are either executables or libraries. More specifically, for each
target, other targets that were specified as sources or as dependency
properties, will be recursively found. One exception is that targets
referred with the link:#b2.builtin.features.use[`use`] feature are not
considered, as that feature is typically used to refer to header-only
libraries. If the set of target types is specified, only targets of that
type will be installed, otherwise, all found target will be installed.

=== Preserving Directory Hierarchy

By default, the `install` rule will strip paths from its sources. So, if
sources include `a/b/c.hpp`, the `a/b` part will be ignored. To make the
`install` rule preserve the directory hierarchy you need to use the
`<install-source-root>` feature to specify the root of the hierarchy you
are installing. Relative paths from that root will be preserved. For
example, if you write:

[source]
----
install headers
    : a/b/c.h
    : <location>/tmp <install-source-root>a
    ;
----

the a file named `/tmp/b/c.h` will be created.

The link:#b2.reference.glob-tree[`glob-tree`] rule can be used to find
all files below a given directory, making it easy to install an entire
directory tree.

=== Installing into Several Directories

The link:#b2.tasks.alias[`alias`] rule can be used when targets need
to be installed into several directories:

[source]
----
alias install : install-bin install-lib ;
install install-bin : applications : /usr/bin ;
install install-lib : helper : /usr/lib ;
----

Because the `install` rule just copies targets, most free features
footnote:[see the definition of "free" in
link:#b2.reference.features.attributes[the section called “Feature Attributes”].]
have no effect when used in requirements of the `install` rule. The only two
that matter are
link:#b2.builtin.features.dependency[`dependency`] and, on Unix,
link:#b2.builtin.features.dll-path[`dll-path`].

NOTE: (Unix specific) On Unix, executables built using B2 typically
contain the list of paths to all used shared libraries. For installing,
this is not desired, so B2 relinks the executable with an empty
list of paths. You can also specify additional paths for installed
executables using the `dll-path` feature.

[[b2.builtins.testing]]
== Testing

B2 has convenient support for running unit tests. The simplest
way is the `unit-test` rule, which follows the
link:#b2.main-target-rule-syntax[common syntax]. For example:

[source]
----
unit-test helpers_test : helpers_test.cpp helpers ;
----

The `unit-test` rule behaves like the link:#b2.tasks.programs[exe]
rule, but after the executable is created it is also run. If the
executable returns an error code, the build system will also return an
error and will try running the executable on the next invocation until
it runs successfully. This behavior ensures that you can not miss a
unit test failure.

There are few specialized testing rules, listed below:

[source]
----
rule compile ( sources : requirements * : target-name ? )
rule compile-fail ( sources : requirements * : target-name ? )
rule link ( sources + : requirements * : target-name ? )
rule link-fail ( sources + : requirements * : target-name ? )
----

They are given a list of sources and requirements. If the target name is
not provided, the name of the first source file is used instead. The
`compile*` tests try to compile the passed source. The `link*` rules try
to compile and link an application from all the passed sources. The
`compile` and `link` rules expect that compilation/linking succeeds. The
`compile-fail` and `link-fail` rules expect that the
compilation/linking fails.

There are two specialized rules for running executables, which are more
powerful than the `unit-test` rule. The `run` rule has the following
signature:

[source]
----
rule run ( sources + : args * : input-files * : requirements * : target-name ?
    : default-build * )
----

The rule builds application from the provided sources and runs it,
passing `args` and `input-files` as command-line arguments. The `args`
parameter is passed verbatim and the values of the `input-files`
parameter are treated as paths relative to containing Jamfile, and are
adjusted if `b2` is invoked from a different directory. The `run-fail`
rule is identical to the `run` rule, except that it expects that the run
fails.

All rules described in this section, if executed successfully, create a
special manifest file to indicate that the test passed. For the
`unit-test` rule the files is named `target-name.passed` and for the other
rules it is called `target-name.test`. The `run*` rules also capture all
output from the program, and store it in a file named `target-name.output`.

If the `preserve-test-targets` feature has the
value `off`, then `run` and the `run-fail` rules will remove the
executable after running it. This somewhat decreases disk space
requirements for continuous testing environments. The default value of
`preserve-test-targets` feature is `on`.

It is possible to print the list of all test targets (except for
`unit-test`) declared in your project, by passing the `--dump-tests`
command-line option. The output will consist of lines of the form:

[source]
----
boost-test(test-type) path : sources
----

It is possible to process the list of tests, B2 output and the
presence/absence of the `*.test` files created when test passes into
human-readable status table of tests. Such processing utilities are not
included in B2.

The following features adjust behavior of the testing metatargets.

`testing.arg`::

Defines an argument to be passed to the target when it is executed
before the list of input files.
+
[source]
----
unit-test helpers_test
    : helpers_test.cpp helpers
    : <testing.arg>"--foo bar"
    ;
----

`testing.input-file`::

Specifies a file to be passed to the executable on the command line
after the arguments. All files must be specified in alphabetical order
due to constraints in the current implementation.

`testing.launcher`::

By default, the executable is run directly. Sometimes, it is desirable
to run the executable using some helper command. You should use this
property to specify the name of the helper command. For example, if
you write:
+
[source]
----
unit-test helpers_test
    : helpers_test.cpp helpers
    : <testing.launcher>valgrind
    ;
----
+
The command used to run the executable will be:
+
[source,shell]
----
valgrind bin/$toolset/debug/helpers_test
----

`test-info`::

A description of the test. This is displayed as part of the
`--dump-tests` command-line option.

[[b2.builtins.raw]]
== Custom commands

For most main target rules, B2 automatically figures out the
commands to run. When you want to use new file types or support new
tools, one approach is to extend B2 to support them smoothly,
as documented in link:#b2.extender[Extender Manual]. However, if the new
tool is only used in a single place, it might be easier just to specify the
commands to run explicitly.

Three main target rules can be used for that. The `make` rule allows you to
construct a single file from any number of source file, by running a command
you specify. The `notfile` rule allows you to run an arbitrary command,
without creating any files. And finally, the `generate` rule allows you to
describe a transformation using B2's virtual targets. This is
higher-level than the file names that the `make` rule operates with and
allows you to create more than one target, create differently named targets
depending on properties, or use more than one tool.

The `make` rule is used when you want to create one file from a number
of sources using some specific command. The `notfile` is used to
unconditionally run a command.

Suppose you want to create the file `file.out` from the file `file.in`
by running the command `in2out`. Here is how you would do this in B2:

[source]
----
make file.out : file.in : @in2out ;
actions in2out
{
    in2out $(<) $(>)
}
----

If you run `b2` and `file.out` does not exist, B2 will run the
`in2out` command to create that file. For more details on specifying
actions, see
link:#b2.overview.jam_language.actions[the section called “Boost.Jam Language”].

It could be that you just want to run some command unconditionally, and
that command does not create any specific files. For that you can use
the `notfile` rule. For example:

[source]
----
notfile echo_something : @echo ;
actions echo
{
    echo "something"
}
----

The only difference from the `make` rule is that the name of the target
is not considered a name of a file, so B2 will unconditionally
run the action.

The `generate` rule is used when you want to express transformations
using B2's virtual targets, as opposed to just filenames. The
`generate` rule has the standard main target rule signature, but you are
required to specify the `generating-rule` property. The value of the
property should be in the form `@_rule-name_`, the named rule should have the
following signature:

[source]
----
rule generating-rule ( project name : property-set : sources * )
----

and will be called with an instance of the `project-target` class, the
name of the main target, an instance of the `property-set` class
containing build properties, and the list of instances of the
`virtual-target` class corresponding to sources. The rule must return a
list of `virtual-target` instances. The interface of the
`virtual-target` class can be learned by looking at the
`build/virtual-target.jam` file. The `generate` example contained in the
B2 distribution illustrates how the `generate` rule can be
used.

[[b2.reference.precompiled_headers]]
== Precompiled Headers

Precompiled headers is a mechanism to speed up compilation by creating a
partially processed version of some header files, and then using that
version during compilations rather then repeatedly parsing the original
headers. B2 supports precompiled headers with gcc and msvc
toolsets.

To use precompiled headers, follow the following steps:

1. Create a header that includes headers used by your project that you
want precompiled. It is better to include only headers that are
sufficiently stable -- like headers from the compiler and external
libraries. B2 will include the header automatically and on-demand.

2. Declare a new B2 target for the precompiled header and add
that precompiled header to the sources of the target whose compilation
you want to speed up:
+
[source]
----
cpp-pch pch : pch.hpp ;
exe main : main.cpp pch ;
----
+
You can use the `c-pch` rule if you want to use the precompiled header
in C programs.

The `pch` example in B2 distribution can be used as reference.

Please note the following:

* The build properties used to compile the source files and the
precompiled header must be the same. Consider using project requirements
to assure this.
* Precompiled headers must be used purely as a way to improve
compilation time, not to save the number of `#include` statements. If a
source file needs to include some header, explicitly include it in the
source file, even if the same header is included from the precompiled
header. This makes sure that your project will build even if precompiled
headers are not supported.
* Prior to version 4.2, the gcc compiler did not allow anonymous
namespaces in precompiled headers, which limits their utility. See the
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29085[bug report] for
details.
* Previosuly B2 had not been automatically inluding the header, a user
was required to include the header at the top of every source file
the precompiled header will be used with.

[[b2.reference.generated_headers]]
== Generated headers

Usually, B2 handles implicit dependencies completely
automatically. For example, for {CPP} files, all `#include` statements are
found and handled. The only aspect where user help might be needed is
implicit dependency on generated files.

By default, B2 handles such dependencies within one main
target. For example, assume that main target "app" has two sources,
"app.cpp" and "parser.y". The latter source is converted into "parser.c"
and "parser.h". Then, if "app.cpp" includes "parser.h", B2 will
detect this dependency. Moreover, since "parser.h" will be generated
into a build directory, the path to that directory will automatically be
added to the include path.

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

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

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

[[b2.tasks.crosscompile]]
== Cross-compilation

B2 supports cross compilation with the gcc and msvc toolsets.

When using gcc, you first need to specify your cross compiler in
`user-config.jam` (see
link:#b2.overview.configuration[the section called “Configuration”]), for
example:

[source]
----
using gcc : arm : arm-none-linux-gnueabi-g++ ;
----

After that, if the host and target os are the same, for example Linux,
you can just request that this compiler version be used:

[source,shell]
----
b2 toolset=gcc-arm
----

If you want to target a different operating system from the host, you
need to additionally specify the value for the `target-os` feature, for
example:

[source,bat]
----
# On windows box
b2 toolset=gcc-arm target-os=linux
# On Linux box
b2 toolset=gcc-mingw target-os=windows
----

For the complete list of allowed operating system names, please see the
documentation for link:#b2.builtin.features.target-os[target-os
feature].

When using the msvc compiler, it's only possible to cross-compile to a
64-bit system on a 32-bit host. Please see
link:#b2.reference.tools.compiler.msvc.64[the section called “64-bit support”]
for details.

[[b2.tasks.packagemanagers]]
== Package Managers

B2 supports automatic, or manual, loading of generated build files
from package managers. For example using the Conan package manager which
generates `conanbuildinfo.jam` files B2 will load that files automatically
when it loads the project at the same location. The included file can
define targets and other project declarations in the context of the
project it's being loaded into. Control over what package manager file
is loaded can be controlled with (in order of priority):

* With the `use-packages` rule.
* Command line argument `--use-package-manager=X`.
* Environment variable `PACKAGE_MANAGER_BUILD_INFO`.
* Built-in detection of the file. Currently this includes: "conan".

**`use-packages` rule:**

[source]
----
rule use-packages ( name-or-glob-pattern ? )
----

The `use-packages` rule allows one to specify in the projects themselves kind
of package definitions to use either as the ones for a built-in package
manager support. For example:

[source]
----
use-packages conan ;
----

Or to specify a `glob` pattern to find the file with the definitions. For
instance:

[source]
----
use-packages "packages.jam" ;
----

**`--use-package-manager` command line option:**

The `--use-package-manager=NAME` command line option allows one to
non-intrusively specify per invocation which of the built-in package manager
types to use.

**`PACKAGE_MANAGER_BUILD_INFO` variable:**

The `PACKAGE_MANAGER_BUILD_INFO` variable, which is taken from the environment
or defined with the `-sX=Y` option, specifies a `glob` pattern to use to find
the package definitions.

**Built-in detection:**

There are a number of built-in `glob` patterns to support popular package
managers. Currently the supported ones are:

* Conan (`conan`): currently supports the
  link:https://docs.conan.io/en/latest/reference/generators/b2.html[`b2 generator`].

:leveloffset: +1
include::../../src/tools/vcpkg.jam[tag=doc]
:leveloffset: -1

[[b2.tasks.projectsearch]]
== Searching For Projects

B2 supports automatic searching for referenced global projects. For example,
if you have references to `/boost/predef` with some minimal configuration B2
can find the B2 project for it and automatically resolve the reference. The
searching supports two modes: finding regular B2 project directories, and
package/config style loading of single jam files.

[[b2.tasks.projectsearch.path]]
=== Search Path

To control which and where projects are found one can use different methods:

* `B2_PROJECT_PATH` environment variable.
* `--project-search` command line argument.
* `rule project-search` project rule.

The search path in both `B2_PROJECT_PATH` and `--project-search` specifies a
key-value list of _project-id_ and _path_. The parts of that key-value list, as
the name indicates, is a path delimiter separated list. For example if we had
these two projects we wanted to find: `/zlib` and `/boost/asio` the search paths
could look like:

[horizontal]
Linux::
`/zlib:/usr/local/share/zlib:/boost/asio:/home/user/external/boost-1.81/libs/asio`
Windows, VxWorks::
`/zlib;C:/Dev/zlib;/boost/asio;C:Dev/boost-1.81/libs/asio`
VMS::
`/zlib,X:external.zlib,/boost/asio,X:external.boost181.libs.asio`

The _project-id_ in the search path specification maps that project root to the
indicated _path_. Which B2 will use to search for any projects and sub-projects
with that _project-id_ root.

[[b2.tasks.projectsearch.process]]
=== Search Process

Regardless of how the search path is specified, how the search happens is the
same. Searching involves either searching for a B2 project directory, i.e.
a directory containing a jamfile, or searching for a specially named `*.jam`
file to include (similar to how the <<Package Managers>> support includes
jam files).

For a given _project-id_ of the form `/d1/d2/../dn` we search for the following,
in this order:

. The project at `d1/d2/../dn` in any path registered for the `/` root.
. The project at `dn` in any path registered for the `/d1/d2/../dn-1` root.
. The jamfile `dn.jam` in any path registered for the `/d1/d2/../dn-1` root.
. The project at `dn-1_dn` in any path registered for the `/d1/d2/../dn-2` root.
. The jamfile `dn-1_dn.jam` in any path registered for the `/d1/d2/../dn-2`
  root.
. And so on until it searches for the project `d1_d2_.._dn` in any path
  registered for the `/` root.
. And for the jamfile `d1_d2_.._dn.jam` in any path registered for the `/` root.

For example, with this search paths:

* `/boost`: `/usr/share/boost-1.81.0`, `/home/user/boost-dev/libs`
* `/`: `/usr/share/b2/external`

And given the `/boost/core` _project-id_ to resolve, we search for:

. `/usr/share/b2/external/boost/core/<jamfile>`
. `/usr/share/boost-1.81.0/core/<jamfile>`
. `/home/user/boost-dev/libs/core/<jamfile>`
. `/usr/share/boost-1.81.0/core.jam`
. `/home/user/boost-dev/libs/core.jam`
. `/usr/share/boost-1.81.0/boost_core/<jamfile>`
. `/home/user/boost-dev/libs/boost_core/<jamfile>`
. `/usr/share/boost-1.81.0/boost_core.jam`
. `/home/user/boost-dev/libs/boost_core.jam`
. `/usr/share/b2/external/boost_core.jam`

The first project jamfile will be assigned to the _project-id_. Or the first
`*.jam` file found will be loaded.

[[b2.tasks.projectsearch.loading]]
=== Loading Process

Depending on whether a project jamfile or `*.jam` file determines how the
project is loaded.

When loading a project jamfile with a _project-id_ and _path_ it is equivalent
to calling `use-project _project-id_ : _path_ ;` from the context of the project
that has the reference.

When loading a `*.jam` file as the _path_ it is equivalent to calling:
`use-packages _path_ ;` from the context of the project that has the reference.
In this case it means that the file will be loaded as part of the referenced
project and hence any bare targets or information it declares will be part of
the project.

[[b2.tasks.commanddb]]
== Command Database, and IDE Integration

Many IDE programs accept the use of a
https://clang.llvm.org/docs/JSONCompilationDatabase.html[`compile_commands.json`]
file to learn what and how your project builds. B2 supports generating such
files for any build you make. B2 supports this through a generic facility to
extract commands from the actions it executes. There are two options that
control this. The `--command-database=_format_` option indicates to generate the
file for the given _format_. It has a couple of effects when specified:

* It tells B2 to start observing and extracting commands from actions (as
  specified by the toolset).
* It disables execution of actions. I.e. equivalent to adding the `-n` option.
* It enables building all default and specified targets. I.e. the equivalent to
  adding the `-a` option.
* It disables all action execution output. I.e. as if specifying `-d0` option.
* At the end of the main build it writes out the results of what it observed
  to the database file.

Currently on `json` is supported as a format that follows the
https://clang.llvm.org/docs/JSONCompilationDatabase.html[Clang JSON Compilation
Database Format Specification].

The `--command-database-out=_file_` option controls the name, and optionally
location, of the generated file. By default the _file_ is
`compile_commands.json` to follow the ecosystem convention. And it is generated,
by default, in one of the following locations:

* Relative to the `build-dir` of the root project, if it's specified by the
  project. With the default _file_ name or as given.
* At the absolute _file_ path if it is rooted.
* At the _current working directory_.

The following fields are populated in the generated database:

* `directory` - This will always be the current directory as B2 makes all paths
  relative to that (or absolute).
* `file` - The first source of each action recorded.
* `command` - The quoted, full, command as extracted by the toolset.
* `output` - The first target file of each action recorded. As B2 can build
  multiple variants at once this is required to differentiate between multiple
  compilations of the same source file.

WARNING: Only one command database file is generated per `b2` invocation. And each
time it is generated it overwrites any previous such file.
