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## Common tasks

This section describes main targets types that Boost.Build supports out-of-the-box. Unless otherwise noted, all mentioned main target rules have the common signature, described in the section called “Declaring Targets”.

### Programs

Programs are created using the `exe` rule, which follows the common syntax. For example:

    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 C++ file, one library file present in the same directory, and another library that is created by Boost.Build. Generally, sources can include C and C++ files, object files and libraries. Boost.Build will automatically try to convert targets of other types.

| Tip
On Windows, if an application uses shared libraries, and both the application and the libraries are built using Boost.Build, 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 the section called “Installing”.

### Libraries

Library targets are created using the `lib` rule, which follows the common syntax . For example:

    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> 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:

    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.

| Warning
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
    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:

    lib z ;
    lib gui db aux ;

which has exactly the same effect as:

    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, othewise, static linking on Unix will not work. For example:

    lib z ;
    lib png : z : <name>png ;

| Note
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:
    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`.

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:

    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.

### 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:

    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 use link statically to the Boost Threads library, you can write the following:

    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:

    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:

    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.

### Installing

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

#### Basic install

For installing a built target you should use the `install` rule, which follows the common syntax. For example:

    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:

    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:

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

See also conditional properties and 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:

    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 `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:

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

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

The 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 `alias` rule can be used when targets need to be installed into several directories:

    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 [15] have no effect when used in requirements of the `install` rule. The only two that matter are `dependency` and, on Unix, `dll-path` .

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

### Testing

Boost.Build has convenient support for running unit tests. The simplest way is the `unit-test` rule, which follows the common syntax. For example:

    unit-test helpers_test : helpers_test.cpp helpers ;

The `unit-test` rule behaves like the 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 behaviour ensures that you can not miss a unit test failure.

By default, the executable is run directly. Sometimes, it is desirable to run the executable using some helper command. You should use the `testing.launcher` property to specify the name of the helper command. For example, if you write:

    unit-test helpers_test
       : helpers_test.cpp helpers
       : <testing.launcher>valgrind
       ;

The command used to run the executable will be:

    valgrind bin/$toolset/debug/helpers_test

There are few specialized testing rules, listed below:

    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 applications, which are more powerful than the `unit-test` rule. The `run` rule has the following signature:

    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 bjam 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:

    boost-test(test-type) path : sources

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

### Custom commands

For most main target rules, Boost.Build automatically figures out the commands to run. When you want to use new file types or support new tools, one approach is to extend Boost.Build to support them smoothly, as documented in the section called “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 finaly, the `generate` rule allows you to describe a transformation using Boost.Build'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 Boost.Build:

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

If you run b2 and `file.out` does not exist, Boost.Build will run the in2out command to create that file. For more details on specifying actions, see 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:

    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 Boost.Build will unconditionally run the action.

The `generate` rule is used when you want to express transformations using Boost.Build'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:

    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 Boost.Build distribution illustrates how the `generate` rule can be used.

### 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. Boost.Build 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. Please wrap the header in `#ifdef BOOST_BUILD_PCH_ENABLED`, so that the potentially expensive inclusion of headers is not done when PCH is not enabled. Include the new header at the top of your source files.

2. Declare a new Boost.Build target for the precompiled header and add that precompiled header to the sources of the target whose compilation you want to speed up:

       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 Boost.Build distribution can be used as reference.

Please note the following:

- The inclusion of the precompiled header must be the first thing in a source file, before any code or preprocessor directives.

- 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.

- On the gcc compiler, the name of the header being precompiled must be equal to the name of the `cpp-pch` target. This is a gcc requirement.

- Prior to version 4.2, the gcc compiler did not allow anonymous namespaces in precompiled headers, which limits their utility. See the bug report for details.

### Generated headers

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

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

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

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

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

### Cross-compilation

Boost.Build 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 the section called “Configuration”), for example:

    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 to be used:

    b2 toolset=gcc-arm

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

    # 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 opeating system names, please see the documentation for target-os feature.

When using the msvc compiler, it's only possible to cross-compiler to a 64-bit system on a 32-bit host. Please see the section called “64-bit support” for details.

[15] see the definition of "free" in the section called “Feature Attributes”.