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<title>Guidelines for Authors of Boost Libraries Containing Separate
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<div class="section-title">
<h1>Guidelines for Authors of Boost Libraries Containing
Separate Source</h1>
</div>
<div class="section-body">
<p>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.</p>
<h2>Contents</h2>
<dl class="index">
<dt><a href="#source_changes">Changes Affecting Source
Code</a></dt>
<dd>
<dl class="index">
<dt><a href="#abi">Preventing Compiler ABI
Clashes</a></dt>
<dt><a href="#static_or_dynamic">Static or Dymanic
Libraries</a></dt>
<dt><a href="#dlls">Supporting Windows Dll's</a></dt>
<dt><a href="#auto-link">Automatic Library Selection and
Linking with auto_link.hpp</a></dt>
</dl>
</dd>
<dt><a href="#build_changes">Changes Affecting the Build
System</a></dt>
<dd>
<dl class="index">
<dt><a href="#jamfile">Creating the Library
Jamfile</a></dt>
<dt><a href="#testing">Testing Auto-linking</a></dt>
</dl>
</dd>
<dt><a href="#copyright">Copyright</a></dt>
</dl>
<h2><a name="source_changes" id="source_changes"></a>Changes
Affecting Source Code</h2>
<h3><a name="abi" id="abi"></a>Preventing Compiler ABI
Clashes</h3>
<p>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:</p>
<pre>
-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).
</pre>
<p>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!</p>
<p>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.</p>
<p>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:</p>
<pre>
#ifndef BOOST_WHATEVER_HPP
#define BOOST_WHATEVER_HPP
#include &lt;boost/config.hpp&gt;
// 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
</pre>
<p>You can include this code in your library source files as
well if you want, although you probably shouldn't need to:</p>
<ul>
<li>If you <em>don't</em> use these in the library source
files (but do in your library's headers) and the user
attempts to compile the library source with a non-default ABI
setting, then they will get compiler errors if there are any
conflicts.</li>
<li>If you <em>do</em> include them in both the library's
headers and the library source files, then the code should
always compile no matter what the compiler settings used,
although the result might not match what the user was
expecting: since we've forced the ABI back into default
mode.</li>
</ul>
<h4>Rationale:</h4>
<p>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.</p>
<h5>Counter Argument #1:</h5>
<p>Trust the user, if they want 13-byte alignment (!) let them
have it.</p>
<h5>Counter Argument #2:</h5>
<p>Prefix/suffix headers have a tendency to "spread" to other
boost libraries - for example if boost::shared_ptr&lt;&gt;
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.</p>
<h3><a name="static_or_dynamic" id=
"static_or_dynamic"></a>Static or Dynamic Libraries</h3>
<p>When the users runtime is dynamically linked the Boost
libraries can be built either as dynamic libraries (.so's on
Unix platforms, .dll's on Windows) or as static libraries (.a's
on Unix, .lib's on Windows). So we have a choice as to which is
supported by default:</p>
<ul>
<li>On Unix platforms it typically makes no difference to the
code: the user just selects in their makesfile which library
they prefer to link to.</li>
<li>On Windows platforms, the code has to be specially
annotated to support DLL's, so we need to pick one option as
the default and one as an alternative.</li>
<li>On Windows platforms, we can inject special code to
automatically select which library variant to link against:
so again we need to decide which is to be the default (see
the section on auto-linking below).</li>
</ul>
<p>The recomendation is to pick static linking by default.</p>
<h4>Rationale:</h4>
<p>There is no one policy that fits all here.</p>
<p>The rationale for the current behaviour was inherited from
Boost.Regex (and it's ancestor regex++): this library
originally used dynamic linking by default whenever the runtime
was dynamic. It's actually safer that way should you be using
regex from a dll for example. However, this behavior brought a
persistent stream of user complaints: mainly about deployment,
all asking if static linking could be the default. After regex
changed behavior the complaints stopped, and the author hasn't
had one complaint about static linking by default being the
wrong choice.</p>
<p>Note that other libraries might need to make other choices:
for example libraries that are intended to be used to implement
dll pluggin's would like need to use dynamic linking in almost
all cases.</p>
<h3>Supporting Windows Dll's</h3>
<p>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).</p>
<p>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:</p>
<pre>
#ifndef BOOST_WHATEVER_HPP
#define BOOST_WHATEVER_HPP
#include &lt;boost/config.hpp&gt;
#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
</pre>And then in the source code for this library one would use:
<pre>
//
// 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 &lt;boost/whatever.hpp&gt;
// class members don't need any further annotation:
whatever::whatever() { }
// but functions do:
BOOST_WHATEVER_DECL whatever get_whatever()
{
return whatever();
}
</pre>
<h4>Importing/exporting dependencies</h4>
<p>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:</p>
<pre>
#ifdef BOOST_MSVC
# pragma warning(push)
# pragma warning(disable : 4251 4231 4660)
#endif
// code here
#ifdef BOOST_MSVC
#pragma warning(pop)
#endif
</pre>
<p>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.</p>
<p>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.</p>
<h4>Rationale:</h4>
<p><em>Why bother - doesn't the import/export mechanism take up
more code that the classes themselves?</em></p>
<p>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.</p>
<p><em>Why static linking by default?</em></p>
<p>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).</p>
<h3><a name="auto-link" id="auto-link"></a>Automatic Library
Selection and Linking with <a href=
"/doc/libs/release/boost/config/auto_link.hpp">auto_link.hpp</a></h3>
<p>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:</p>
<pre>
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
</pre>
<p>The difficulty now is selecting which of these the user
should link his or her code to.</p>
<p>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.</p>
<p>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.</p>
<p>Automatic library selection and linking can be enabled for a
Boost library by including the header
&lt;boost/config/auto_link.hpp&gt;, after first defining
BOOST_LIB_NAME and, if applicable, BOOST_DYN_LINK.</p>
<pre>
//
// Automatically link to the correct build variant where possible.
//
#if !defined(BOOST_ALL_NO_LIB) &amp;&amp; !defined(BOOST_WHATEVER_NO_LIB) &amp;&amp; !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 &lt;boost/config/auto_link.hpp&gt;
#endif // auto-linking disabled
</pre>
<p>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:</p>
<p>If for any reason you need to debug this feature, the header
&lt;boost/config/auto_link.hpp&gt; will output some helpful
diagnostic messages if you first define
BOOST_LIB_DIAGNOSTIC.</p>
<h2><a name="build_changes" id="build_changes"></a>Changes
Affecting the Build System</h2>
<h3><a name="build" id="build"></a><a name="jamfile" id=
"jamfile"></a>Creating the library Jamfile</h3>
<p>The Jamfile for building library "whatever" typically lives
in boost-root/libs/whatever/build, the only extra step required
is to add a &lt;define&gt; requirement to the library target so
that your code knows whether it's building a dll or static
library, a typical Jamfile would like like this:</p>
<pre>
lib boost_regex : ../src/whatever.cpp :
&lt;link&gt;shared:&lt;define&gt;BOOST_WHATEVER_DYN_LINK=1 ;
</pre>
<h3><a name="testing" id="testing"></a>Testing
Auto-linking</h3>
<p>Testing the auto-link feature is somewhat convoluted, and
requires access to a compiler that supports the feature: refer
to <a href=
"/doc/libs/release/libs/config/test/link/test/Jamfile.v2">libs/config/test/link/test/Jamfile.v2</a>
for an example.</p>
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<p>Revised $Date: 2007-10-22 22:55:52 +0100 (Mon, 22 Oct 2007) $</p>
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<p>Copyright John Maddock 1998.</p>
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