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## Prebuilt targets

To link to libraries whose build instructions aren't given in a Jamfile, you need to create `lib` targets with an appropriate `file` property. Target alternatives can be used to associate multiple library files with a single conceptual target. For example:

    # util/lib2/Jamfile
    lib lib2
        :
        : <file>lib2_release.a <variant>release
        ;

    lib lib2
        :
        : <file>lib2_debug.a <variant>debug
        ;

This example defines two alternatives for `lib2`, and for each one names a prebuilt file. Naturally, there are no sources. Instead, the `<file>` feature is used to specify the file name.

Once a prebuilt target has been declared, it can be used just like any other target:

    exe app : app.cpp ../util/lib2//lib2 ;

As with any target, the alternative selected depends on the properties propagated from `lib2`'s dependents. If we build the the release and debug versions of `app` will be linked with `lib2_release.a` and `lib2_debug.a`, respectively.

System libraries—those that are automatically found by the toolset by searching through some set of predetermined paths—should be declared almost like regular ones:

    lib pythonlib : : <name>python22 ;

We again don't specify any sources, but give a `name` that should be passed to the compiler. If the gcc toolset were used to link an executable target to `pythonlib`, `-lpython22` would appear in the command line (other compilers may use different options).

We can also specify where the toolset should look for the library:

    lib pythonlib : : <name>python22 <search>/opt/lib ;

And, of course, target alternatives can be used in the usual way:

    lib pythonlib : : <name>python22 <variant>release ;
    lib pythonlib : : <name>python22_d <variant>debug ;

A more advanced use of prebuilt targets is described in the section called “Targets in site-config.jam”.