Guidelines for Authors of Boost Libraries Containing Separate Source

###  | # Guidelines for Authors of Boost Libraries Containing Separate Source

These guidelines are designed for the authors of Boost libraries which have separate source that need compiling in order to use the library. Throughout, this guide refers to a fictitious "whatever" library, so replace all occurrences of "whatever" or "WHATEVER" with your own library's name when copying the examples.

## Contents

Changes Affecting Source Code
  Preventing Compiler ABI Clashes
  Supporting Windows Dll's
  Automatic Library Selection and Linking with auto_link.hpp
Changes Affecting the Build System
  Creating the Library Jamfile
  Testing Auto-linking
Copyright

## Changes Affecting Source Code

### Preventing Compiler ABI Clashes

There are some compilers (mostly Microsoft Windows compilers again!), which feature a range of compiler switches that alter the ABI of C++ classes and functions. By way of example, consider Borland's compiler which has the following options:

    -b    (on or off - effects enum sizes).
    -Vx   (on or off - empty members).
    -Ve   (on or off - empty base classes).
    -aX   (alignment - 5 options).
    -pX   (Calling convention - 4 options).
    -VmX  (member pointer size and layout - 5 options).
    -VC   (on or off, changes name mangling).
    -Vl   (on or off, changes struct layout).

These options are provided in addition to those affecting which runtime library is used (more on which later); the total number of combinations of options can be obtained by multiplying together the individual options above, so that gives 2*2*2*5*4*5*2*2 = 3200 combinations!

The problem is that users often expect to be able to build the Boost libraries and then just link to them and have everything just plain work, no matter what their project settings are. Irrespective of whether this is a reasonable expectation or not, without some means of managing this issue, the user may well find that their program will experience strange and hard to track down crashes at runtime unless the library they link to was built with the same options as their project (changes to the default alignment setting are a prime culprit). One way to manage this is with "prefix and suffix" headers: these headers invoke compiler specific #pragma directives to instruct the compiler that whatever code follows was built (or is to be built) with a specific set of compiler ABI settings.

Boost.config provides the macro BOOST_HAS_ABI_HEADERS which is set whenever there are prefix and suffix headers available for the compiler in use, typical usage in a header like this:

    #ifndef BOOST_WHATEVER_HPP
    #define BOOST_WHATEVER_HPP

    #include <boost/config.hpp>

    // this must occur after all of the includes and before any code appears:
    #ifdef BOOST_HAS_ABI_HEADERS
    #  include BOOST_ABI_PREFIX
    #endif
    //
    // this header declares one class, and one function by way of examples:
    //
    class whatever
    {
       // details.
    };

    whatever get_whatever();

    // the suffix header occurs after all of our code:
    #ifdef BOOST_HAS_ABI_HEADERS
    #  include BOOST_ABI_SUFFIX
    #endif

    #endif

You can include this code in your source files as well if you want - although you probably shouldn't need to - these headers fix the ABI to the default used by the compiler, and if the user attempts to compile the source with any other setting then they will get compiler errors if there are any conflicts.

#### Rationale:

Without some means of managing this issue, users often report bugs along the line of "Your silly library always crashes when I try and call it" and so on. These issues can be extremely difficult and time consuming to track down, only to discover in the end that it's a compiler setting that's changed the ABI of the class and/or function types of the program compared to those in the pre-compiled library. The use of prefix/suffix headers can minimize this problem, although probably not remove it completely.

##### Counter Argument #1:

Trust the user, if they want 13-byte alignment (!) let them have it.

##### Counter Argument #2:

Prefix/suffix headers have a tendency to "spread" to other boost libraries - for example if boost::shared_ptr<> forms part of your class's ABI, then including prefix/suffix headers in your code will be of no use unless shared_ptr.hpp also uses them. Authors of header-only boost libraries may not be so keen on this solution - with some justification - since they don't face the same problem.

### Supporting Windows Dll's

On most Unix-like platforms no special annotations of source code are required in order for that source to be compiled as a shared library because all external symbols are exposed. However the majority of Windows compilers require that symbols that are to be imported or exported from a dll, be prefixed with __declspec(dllimport) or __declspec(dllexport). Without this mangling of source code, it is not possible to correctly build shared libraries on Windows (historical note - originally these declaration modifiers were required on 16-bit Windows where the memory layout for exported classes was different from that of "local" classes - although this is no longer an issue, there is still no way to instruct the linker to "export everything", it also remains to be seen whether 64-bit Windows will resurrect the segmented architecture that led to this problem in the first place. Note also that the mangled names of exported symbols are different from non-exported ones, so __declspec(dllimport) is required in order to link to code within a dll).

In order to support the building of shared libraries on MS Windows your code will have to prefix all the symbols that your library exports with a macro (lets call it BOOST_WHATEVER_DECL) that your library will define to expand to either __declspec(dllexport) or __declspec(dllimport) or nothing, depending upon how your library is being built or used. Typical usage would look like this:

    #ifndef BOOST_WHATEVER_HPP
    #define BOOST_WHATEVER_HPP

    #include <boost/config.hpp>

    #ifdef BOOST_HAS_DECLSPEC // defined in config system
    // we need to import/export our code only if the user has specifically
    // asked for it by defining either BOOST_ALL_DYN_LINK if they want all boost
    // libraries to be dynamically linked, or BOOST_WHATEVER_DYN_LINK
    // if they want just this one to be dynamically liked:
    #if defined(BOOST_ALL_DYN_LINK) || defined(BOOST_WHATEVER_DYN_LINK)
    // export if this is our own source, otherwise import:
    #ifdef BOOST_WHATEVER_SOURCE
    # define BOOST_WHATEVER_DECL __declspec(dllexport)
    #else
    # define BOOST_WHATEVER_DECL __declspec(dllimport)
    #endif  // BOOST_WHATEVER_SOURCE
    #endif  // DYN_LINK
    #endif  // BOOST_HAS_DECLSPEC
    //
    // if BOOST_WHATEVER_DECL isn't defined yet define it now:
    #ifndef BOOST_WHATEVER_DECL
    #define BOOST_WHATEVER_DECL
    #endif

    //
    // this header declares one class, and one function by way of examples:
    //
    class BOOST_WHATEVER_DECL whatever
    {
       // details.
    };

    BOOST_WHATEVER_DECL whatever get_whatever();

    #endif

And then in the source code for this library one would use:

    //
    // define BOOST_WHATEVER SOURCE so that our library's
    // setup code knows that we are building the library (possibly exporting code),
    // rather than using it (possibly importing code):
    //
    #define BOOST_WHATEVER_SOURCE
    #include <boost/whatever.hpp>

    // class members don't need any further annotation:
    whatever::whatever() { }
    // but functions do:
    BOOST_WHATEVER_DECL whatever get_whatever()
    {
       return whatever();
    }

#### Importing/exporting dependencies

As well as exporting your main classes and functions (those that are actually documented), Microsoft Visual C++ will warn loudly and often if you try to import/export a class whose dependencies are not also exported. Dependencies include: any base classes, any user defined types used as data members, plus all of the dependencies of your dependencies and so on. This causes particular problems when a dependency is a template class, because although it is technically possible to export these, it is not at all easy, especially if the template itself has dependencies which are implementation-specific details. In most cases it's probably better to simply suppress the warnings using:

    #ifdef BOOST_MSVC
    #  pragma warning(push)
    #  pragma warning(disable : 4251 4231 4660)
    #endif

    // code here

    #ifdef BOOST_MSVC
    #pragma warning(pop)
    #endif

This is safe provided that there are no dependencies that are (template) classes with non-constant static data members, these really do need exporting, otherwise there will be multiple copies of the static data members in the program, and that's really really bad.

Historical note: on 16-bit Windows you really did have to export all dependencies or the code wouldn't work, however since the latest Visual Studio .NET supports the import/export of individual member functions, it's a reasonably safe bet that Windows compilers won't do anything nasty - like changing the class's ABI - when importing/exporting a class.

#### Rationale:

Why bother - doesn't the import/export mechanism take up more code that the classes themselves?

A good point, and probably true, however there are some circumstances where library code must be placed in a shared library - for example when the application consists of multiple dll's as well as the executable, and more than one those dll's link to the same Boost library - in this case if the library isn't dynamically linked and it contains any global data (even if that data is private to the internals of the library) then really bad things can happen - even without global data, we will still get a code bloating effect. Incidentally, for larger applications, splitting the application into multiple dll's can be highly advantageous - by using Microsoft's "delay load" feature the application will load only those parts it really needs at any one time, giving the impression of a much more responsive and faster-loading application.

Why static linking by default?

In the worked example above, the code assumes that the library will be statically linked unless the user asks otherwise. Most users seem to prefer this (there are no separate dll's to distribute, and the overall distribution size is often significantly smaller this way as well: i.e. you pay for what you use and no more), but this is a subjective call, and some libraries may even only be available in dynamic versions (Boost.threads for example).

### Automatic Library Selection and Linking with auto_link.hpp

Many Windows compilers ship with multiple runtime libraries - for example Microsoft Visual Studio .NET comes with 6 versions of the C and C++ runtime. It is essential that the Boost library that the user links to is built against the same C runtime as the program is built against. If that is not the case, then the user will experience linker errors at best, and runtime crashes at worst. The Boost build system manages this by providing different build variants, each of which is build against a different runtime, and gets a slightly different mangled name depending upon which runtime it is built against. For example the regex libraries get named as follows when built with Visual Studio .NET 2003:

    boost_regex-vc71-mt-1_31.lib
    boost_regex-vc71-mt-gd-1_31.lib
    libboost_regex-vc71-mt-1_31.lib
    libboost_regex-vc71-mt-gd-1_31.lib
    libboost_regex-vc71-mt-s-1_31.lib
    libboost_regex-vc71-mt-sgd-1_31.lib
    libboost_regex-vc71-s-1_31.lib
    libboost_regex-vc71-sgd-1_31.lib

The difficulty now is selecting which of these the user should link his or her code to.

In contrast, most Unix compilers typically only have one runtime (or sometimes two if there is a separate thread safe option). For these systems the only choice in selecting the right library variant is whether they want debugging info, and possibly thread safety.

Historically Microsoft Windows compilers have managed this issue by providing a #pragma option that allows the header for a library to automatically select the library to link to. This makes everything automatic and extremely easy for the end user: as soon as they include a header file that has separate source code, the name of the right library build variant gets embedded in the object file, and as long as that library is in the linker search path, it will get pulled in by the linker without any user intervention.

Automatic library selection and linking can be enabled for a Boost library by including the header <boost/config/auto_link.hpp>, after first defining BOOST_LIB_NAME and, if applicable, BOOST_DYN_LINK.

    //
    // Automatically link to the correct build variant where possible.
    //
    #if !defined(BOOST_ALL_NO_LIB) && !defined(BOOST_WHATEVER_NO_LIB) && !defined(BOOST_WHATEVER_SOURCE)
    //
    // Set the name of our library, this will get undef'ed by auto_link.hpp
    // once it's done with it:
    //
    #define BOOST_LIB_NAME boost_whatever
    //
    // If we're importing code from a dll, then tell auto_link.hpp about it:
    //
    #if defined(BOOST_ALL_DYN_LINK) || defined(BOOST_WHATEVER_DYN_LINK)
    #  define BOOST_DYN_LINK
    #endif
    //
    // And include the header that does the work:
    //
    #include <boost/config/auto_link.hpp>
    #endif  // auto-linking disabled

The library's user documentation should note that the feature can be disabled by defining either BOOST_ALL_NO_LIB or BOOST_WHATEVER_NO_LIB:

If for any reason you need to debug this feature, the header <boost/config/auto_link.hpp> will output some helpful diagnostic messages if you first define BOOST_LIB_DIAGNOSTIC.

## Changes Affecting the Build System

### Creating the library Jamfile

The Jamfile for building library "whatever" typically lives in boost-root/libs/whatever/build, start by defining the project root for the Jamfile:

    subproject libs/whatever/build ;

Then add the static library build target (if supported):

    lib
       boost_whatever
          : # list all the sources for this
          library:
       ../src/whatever.cpp
          : # all build requirements go
          here. # the "common-variant-tag" rule ensures that the library will
          # be named according to the rules used by the install
          # and auto-link features:
          common-variant-tag
          # set include path for Boost headers:
          <sysinclude>$(BOOST_ROOT)
       :
          # list default build variants here
          debug release
       ;

Then add the dll build target (if supported). In this case the build requirements section get an extra define: so that our sources know to export their own symbols (and import those from any other boost libs on which we may be dependent). We also restict shared library builds to dynamic-runtime build variants, if we don't do this then dll's linked against static runtimes are unlikely to function correctly (the dll will have a separate runtime from the executable using it, this generally causing problems with new and delete, as well as exception handling runtimes).

    dll
       boost_whatever
          : # list all the sources for this
          library:
       ../src/whatever.cpp
          : # all build requirements go
          here. # the "common-variant-tag" rule ensures that the library will
          # be named according to the rules used by the install
          # and auto-link features:
          common-variant-tag
          # tell our source that we're building (and maybe using) dll's:
          <define>BOOST_ALL_DYN_LINK=1
          # only build this for dynamic runtimes:
          <runtime-link>dynamic
          # set include path for Boost headers:
          <sysinclude>$(BOOST_ROOT)
       :
          # list default build variants here
          debug release
       ;

Now add an install target so that Boost.Install can find this library to install:

    install whatever lib
        : <dll>boost_whatever <lib>boost_whatever
        ;

Finally add a stage target that will copy the built libraries to a common sub-directory (boost-root/stage/lib):

    stage stage/lib : <lib>boost_whatever <dll>boost_whatever
        :
            # copy to a path rooted at BOOST_ROOT:
            <locate>$(BOOST_ROOT)
            # make sure the names of the libraries are correctly named:
            common-variant-tag
            # add this target to the "stage" and "all" psuedo-targets:
            <target>stage
            <target>all
        :
            debug release
        ;

### Testing Auto-linking

Testing the auto-link feature reasonable straightforward using the Boost.build system: we need to build the "whatever" library's test files without explicitly specifying the library to link to in the Jamfile, for example:

    subproject libs/whatever/test/auto-link-test ;

    # bring in the rules for testing
    import testing ;

    # start with a static linking version:

    run
       # sources
          ../whatever_test.cpp
       :
       :	# input files
       : 	# requirements
          <library-path>../../../../stage/lib
          <define>BOOST_LIB_DIAGNOSTIC=1
       : 	# program name
          whatever_test
       ;

       # and then a dll linking version:
       run
       # sources
          ../whatever_test.cpp
       :
       :	# input files
       : 	# requirements
          <library-path>../../../../stage/lib
          <define>BOOST_LIB_DIAGNOSTIC=1
          <define>BOOST_ALL_DYN_LINK=1
          <runtime-link>dynamic
       : 	# program name
          whatever_test_dll
       ;

Please note however that this Jamfile will only build with compilers that do actually support auto-linking, so it should not be added to the regular regression tests. The Jamfile should also be built for all possible build variants, for the Microsoft / Borland compilers that means doing a:

    bjam -sBUILD="release debug <threading>multi/single <runtime-link>static/dynamic" test

Revised 26 November, 2003

© Copyright John Maddock 1998- 2003

Use, modification and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt).

The use of code snippets from this article does not require the reproduction of this copyright notice and license declaration; if you wish to provide attribution then please provide a link to this article.