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## Frequently Asked Questions

### How do I get the current value of feature in Jamfile?

This is not possible, since Jamfile does not have "current" value of any feature, be it toolset, build variant or anything else. For a single run of Boost.Build, any given main target can be built with several property sets. For example, user can request two build variants on the command line. Or one library is built as shared when used from one application, and as static when used from another. Each Jamfile is read only once so generally there is no single value of a feature you can access in Jamfile.

A feature has a specific value only when building a target, and there are two ways you can use that value:

- Use conditional requirements or indirect conditional requirements. See the section called “Requirements”.
- Define a custom generator and a custom main target type. The custom generator can do arbitrary processing or properties. See the the section called “Extender Manual”.

### I am getting a "Duplicate name of actual target" error. What does that mean?

The most likely case is that you are trying to compile the same file twice, with almost the same, but differing properties. For example:

    exe a : a.cpp : <include>/usr/local/include ;
    exe b : a.cpp ;

The above snippet requires two different compilations of `a.cpp`, which differ only in their `include` property. Since the `include` feature is declared as `free` Boost.Build does not create a separate build directory for each of its values and those two builds would both produce object files generated in the same build directory. Ignoring this and compiling the file only once would be dangerous as different includes could potentially cause completely different code to be compiled.

To solve this issue, you need to decide if the file should be compiled once or twice.

1. To compile the file only once, make sure that properties are the same for both target requests:

       exe a : a.cpp : <include>/usr/local/include ;
       exe b : a.cpp : <include>/usr/local/include ;

   or:

       alias a-with-include : a.cpp : <include>/usr/local/include ;
       exe a : a-with-include ;
       exe b : a-with-include ;

   or if you want the `includes` property not to affect how any other sources added for the built `a` and `b` executables would be compiled:

       obj a-obj : a.cpp : <include>/usr/local/include ;
       exe a : a-obj ;
       exe b : a-obj ;

   Note that in both of these cases the `include` property will be applied only for building these object files and not any other sources that might be added for targets `a` and `b`.

2. To compile the file twice, you can tell Boost.Build to compile it to two separate object files like so:

             obj a_obj : a.cpp : <include>/usr/local/include ;
             obj b_obj : a.cpp ;
             exe a : a_obj ;
             exe b : b_obj ;

   or you can make the object file targets local to the main target:

             exe a : [ obj a_obj : a.cpp : <include>/usr/local/include ] ;
             exe b : [ obj a_obj : a.cpp ] ;

   which will cause Boost.Build to actually change the generated object file names a bit for you and thus avoid any conflicts.

   Note that in both of these cases the `include` property will be applied only for building these object files and not any other sources that might be added for targets `a` and `b`.

A good question is why Boost.Build can not use some of the above approaches automatically. The problem is that such magic would only help in half of the cases, while in the other half it would be silently doing the wrong thing. It is simpler and safer to ask the user to clarify his intention in such cases.

### Accessing environment variables

Many users would like to use environment variables in Jamfiles, for example, to control the location of external libraries. In many cases it is better to declare those external libraries in the site-config.jam file, as documented in the recipes section. However, if the users already have the environment variables set up, it may not be convenient for them to set up their site-config.jam files as well and using the environment variables might be reasonable.

Boost.Jam automatically imports all environment variables into its built-in .ENVIRON module so user can read them from there directly or by using the helper os.environ rule. For example:

    import os ;
    local unga-unga = [ os.environ UNGA_UNGA ] ;
    ECHO $(unga-unga) ;

or a bit more realistic:

    import os ;
    local SOME_LIBRARY_PATH = [ os.environ SOME_LIBRARY_PATH ] ;
    exe a : a.cpp : <include>$(SOME_LIBRARY_PATH) ;

### How to control properties order?

For internal reasons, Boost.Build sorts all the properties alphabetically. This means that if you write:

    exe a : a.cpp : <include>b <include>a ;

then the command line with first mention the `a` include directory, and then `b`, even though they are specified in the opposite order. In most cases, the user does not care. But sometimes the order of includes, or other properties, is important. For such cases, a special syntax is provided:

    exe a : a.cpp : <include>a&&b ;

The `&&` symbols separate property values and specify that their order should be preserved. You are advised to use this feature only when the order of properties really matters and not as a convenient shortcut. Using it everywhere might negatively affect performance.

### How to control the library linking order on Unix?

On Unix-like operating systems, the order in which static libraries are specified when invoking the linker is important, because by default, the linker uses one pass though the libraries list. Passing the libraries in the incorrect order will lead to a link error. Further, this behaviour is often used to make one library override symbols from another. So, sometimes it is necessary to force specific library linking order.

Boost.Build tries to automatically compute the right order. The primary rule is that if library `a` "uses" library `b`, then library `a` will appear on the command line before library `b`. Library `a` is considered to use `b` if `b` is present either in the `a` library's sources or its usage is listed in its requirements. To explicitly specify the `use` relationship one can use the `<use>` feature. For example, both of the following lines will cause `a` to appear before `b` on the command line:

    lib a : a.cpp b ;
    lib a : a.cpp : <use>b ;

The same approach works for searched libraries as well:

    lib z ;
    lib png : : <use>z ;
    exe viewer : viewer png z ;

### Can I get capture external program output using a Boost.Jam variable?

The `SHELL` builtin rule may be used for this purpose:

    local gtk_includes = [ SHELL "gtk-config --cflags" ] ;

### How to get the project root (a.k.a. Jamroot) location?

You might want to use your project's root location in your Jamfiles. To access it just declare a path constant in your Jamroot.jam file using:

    path-constant TOP : . ;

After that, the `TOP` variable can be used in every Jamfile.

### How to change compilation flags for one file?

If one file must be compiled with special options, you need to explicitly declare an `obj` target for that file and then use that target in your `exe` or `lib` target:

    exe a : a.cpp b ;
    obj b : b.cpp : <optimization>off ;

Of course you can use other properties, for example to specify specific C/C++ compiler options:

    exe a : a.cpp b ;
    obj b : b.cpp : <cflags>-g ;

You can also use conditional properties for finer control:

    exe a : a.cpp b ;
    obj b : b.cpp : <variant>release:<optimization>off ;

### Why are the `dll-path` and `hardcode-dll-paths` properties useful?

| Note
This entry is specific to Unix systems.

Before answering the questions, let us recall a few points about shared libraries. Shared libraries can be used by several applications, or other libraries, without physically including the library in the application which can greatly decrease the total application size. It is also possible to upgrade a shared library when the application is already installed.

However, in order for application depending on shared libraries to be started the OS may need to find the shared library when the application is started. The dynamic linker will search in a system-defined list of paths, load the library and resolve the symbols. Which means that you should either change the system-defined list, given by the `LD_LIBRARY_PATH` environment variable, or install the libraries to a system location. This can be inconvenient when developing, since the libraries are not yet ready to be installed, and cluttering system paths may be undesirable. Luckily, on Unix there is another way.

An executable can include a list of additional library paths, which will be searched before system paths. This is excellent for development because the build system knows the paths to all libraries and can include them in the executables. That is done when the `hardcode-dll-paths` feature has the `true` value, which is the default. When the executables should be installed, the story is different.

Obviously, installed executable should not contain hardcoded paths to your development tree. (The `install` rule explicitly disables the `hardcode-dll-paths` feature for that reason.) However, you can use the `dll-path` feature to add explicit paths manually. For example:

    install installed : application : <dll-path>/usr/lib/snake
                                      <location>/usr/bin ;

will allow the application to find libraries placed in the `/usr/lib/snake` directory.

If you install libraries to a nonstandard location and add an explicit path, you get more control over libraries which will be used. A library of the same name in a system location will not be inadvertently used. If you install libraries to a system location and do not add any paths, the system administrator will have more control. Each library can be individually upgraded, and all applications will use the new library.

Which approach is best depends on your situation. If the libraries are relatively standalone and can be used by third party applications, they should be installed in the system location. If you have lots of libraries which can be used only by your application, it makes sense to install them to a nonstandard directory and add an explicit path, like the example above shows. Please also note that guidelines for different systems differ in this respect. For example, the Debian GNU guidelines prohibit any additional search paths while Solaris guidelines suggest that they should always be used.

### Targets in site-config.jam

It is desirable to declare standard libraries available on a given system. Putting target declaration in a specific project's Jamfile is not really good, since locations of the libraries can vary between different development machines and then such declarations would need to be duplicated in different projects. The solution is to declare the targets in Boost.Build's `site-config.jam` configuration file:

    project site-config ;
    lib zlib : : <name>z ;

Recall that both `site-config.jam` and `user-config.jam` are projects, and everything you can do in a Jamfile you can do in those files as well. So, you declare a project id and a target. Now, one can write:

    exe hello : hello.cpp /site-config//zlib ;

in any Jamfile.

### Header-only libraries

In modern C++, libraries often consist of just header files, without any source files to compile. To use such libraries, you need to add proper includes and possibly defines to your project. But with a large number of external libraries it becomes problematic to remember which libraries are header only, and which ones you have to link to. However, with Boost.Build a header-only library can be declared as Boost.Build target and all dependents can use such library without having to remeber whether it is a header-only library or not.

Header-only libraries may be declared using the `alias` rule, specifying their include path as a part of its usage requirements, for example:

    alias my-lib
        : # no sources
        : # no build requirements
        : # no default build
        : <include>whatever ;

The includes specified in usage requirements of `my-lib` are automatically added to all of its dependants' build properties. The dependants need not care if `my-lib` is a header-only or not, and it is possible to later make `my-lib` into a regular compiled library without having to that its dependants' declarations.

If you already have proper usage requirements declared for a project where a header-only library is defined, you do not need to duplicate them for the `alias` target:

    project my : usage-requirements <include>whatever ;
    alias mylib ;