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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<!--*********************************************************************-->
<h1>"libc++" C++ Standard Library</h1>
<!--*********************************************************************-->
<p>libc++ is a new implementation of the C++ standard library, targeting
C++11.</p>
<p>All of the code in libc++ is <a
href="http://llvm.org/docs/DeveloperPolicy.html#license">dual licensed</a>
under the MIT license and the UIUC License (a BSD-like license).</p>
<!--=====================================================================-->
<h2 id="goals">Features and Goals</h2>
<!--=====================================================================-->
<ul>
<li>Correctness as defined by the C++11 standard.</li>
<li>Fast execution.</li>
<li>Minimal memory use.</li>
<li>Fast compile times.</li>
<li>ABI compatibility with gcc's libstdc++ for some low-level features
such as exception objects, rtti and memory allocation.</li>
<li>Extensive unit tests.</li>
</ul>
<!--=====================================================================-->
<h2 id="why">Why a new C++ Standard Library for C++11?</h2>
<!--=====================================================================-->
<p>After its initial introduction, many people have asked "why start a new
library instead of contributing to an existing library?" (like Apache's
libstdcxx, GNU's libstdc++, STLport, etc). There are many contributing
reasons, but some of the major ones are:</p>
<ul>
<li><p>From years of experience (including having implemented the standard
library before), we've learned many things about implementing
the standard containers which require ABI breakage and fundamental changes
to how they are implemented. For example, it is generally accepted that
building std::string using the "short string optimization" instead of
using Copy On Write (COW) is a superior approach for multicore
machines (particularly in C++11, which has rvalue references). Breaking
ABI compatibility with old versions of the library was
determined to be critical to achieving the performance goals of
libc++.</p></li>
<li><p>Mainline libstdc++ has switched to GPL3, a license which the developers
of libc++ cannot use. libstdc++ 4.2 (the last GPL2 version) could be
independently extended to support C++11, but this would be a fork of the
codebase (which is often seen as worse for a project than starting a new
independent one). Another problem with libstdc++ is that it is tightly
integrated with G++ development, tending to be tied fairly closely to the
matching version of G++.</p>
</li>
<li><p>STLport and the Apache libstdcxx library are two other popular
candidates, but both lack C++11 support. Our experience (and the
experience of libstdc++ developers) is that adding support for C++11 (in
particular rvalue references and move-only types) requires changes to
almost every class and function, essentially amounting to a rewrite.
Faced with a rewrite, we decided to start from scratch and evaluate every
design decision from first principles based on experience.</p>
<p>Further, both projects are apparently abandoned: STLport 5.2.1 was
released in Oct'08, and STDCXX 4.2.1 in May'08.</p>
</ul>
<!--=====================================================================-->
<h2 id="requirements">Platform Support</h2>
<!--=====================================================================-->
<p>
libc++ is known to work on the following platforms, using g++-4.2 and
clang (lack of C++11 language support disables some functionality). Note
that functionality provided by <atomic> is only functional with
clang.
</p>
<ul>
<li>Mac OS X i386</li>
<li>Mac OS X x86_64</li>
<li>FreeBSD 10+ i386</li>
<li>FreeBSD 10+ x86_64</li>
<li>FreeBSD 10+ ARM</li>
</ul>
<!--=====================================================================-->
<h2 id="dir-structure">Current Status</h2>
<!--=====================================================================-->
<p>libc++ is a 100% complete C++11 implementation on Apple's OS X. </p>
<p>LLVM and Clang can self host in C++ and C++11 mode with libc++ on Linux.</p>
<p>libc++ is also a 100% complete C++14 implementation. A list of new features and changes for
C++14 can be found <a href="cxx1y_status.html">here</a>.</p>
<p>Implementation of the post-c++14 Technical Specifications is in progress. A list of features and
the current status of these features can be found <a href="ts1z_status.html">here</a>.</p>
<p>
Ports to other platforms are underway. Here are recent test
results for <a href="results.Windows.html">Windows</a>
and <a href="results.Linux.html">Linux</a>.
</p>
<!--======================================================================-->
<h2 id="dir-structure">Build Bots</h2>
<!--======================================================================-->
<p>These are the libc++ build bots</p>
<ul>
<li><a href="http://lab.llvm.org:8013/builders/libcxx_clang-x86_64-darwin11-RA">
clang-libcxxabi-x86_64-darwin11
</a></li>
<li><a href="http://llvm-amd64.freebsd.your.org/b/builders/libcxx-amd64-freebsd">
clang-libcxxrt-x86_64-freebsd10
</a></li>
<li><a href="http://llvm-amd64.freebsd.your.org/b/builders/libcxx-i386-freebsd">
clang-libcxxrt-i386-freebsd10
</a></li>
<li><a href="http://lab.llvm.org:8011/builders/libcxx-libcxxabi-x86_64-linux-debian">
clang-libcxxabi-x86_64-linux-debian
</a></li>
<li><a href="http://lab.llvm.org:8011/builders/libcxx-libcxxabi-x86_64-linux-ubuntu">
clang-libcxxabi-x86_64-linux-ubuntu
</a></li>
</ul>
<!--=====================================================================-->
<h2>Get it and get involved!</h2>
<!--=====================================================================-->
<p>First please review our
<a href="http://llvm.org/docs/DeveloperPolicy.html">Developer's Policy</a>.
<p>
On Mac OS 10.7 (Lion) and later, the easiest way to get this library is to install
Xcode 4.2 or later. However if you want to install tip-of-trunk from here
(getting the bleeding edge), read on. However, be warned that Mac OS
10.7 will not boot without a valid copy of <code>libc++.1.dylib</code> in
<code>/usr/lib</code>.
</p>
<p>To check out the code, use:</p>
<ul>
<li><code>svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx</code></li>
</ul>
<p>
Note that for an in-tree build, you should check out libcxx to
llvm/projects.
</p>
<p>
The following instructions are for building libc++ on FreeBSD, Linux, or Mac
using <a href="http://libcxxabi.llvm.org/">libc++abi</a> as the C++ ABI
library. On Linux, it is also possible to use
<a href="#libsupcxx">libsupc++</a> or <a href="#libcxxrt">libcxxrt</a>.
</p>
<p>In-tree build:</p>
<ul>
<li>Check out libcxx and <a href="http://libcxxabi.llvm.org/">libcxxabi</a>
into llvm/projects</li>
<li><code>cd llvm</code></li>
<li><code>mkdir build && cd build</code></li>
<li><code>cmake .. # Linux may require -DCMAKE_C_COMPILER=clang
-DCMAKE_CXX_COMPILER=clang++</code></li>
<li><code>make cxx</code></li>
</ul>
<p>Out-of-tree build:</p>
<ul>
<li>Check out libcxx</li>
<li>If not on a Mac, also check out
<a href="http://libcxxabi.llvm.org/">libcxxabi</a></li>
<li><code>cd libcxx</code></li>
<li><code>mkdir build && cd build</code></li>
<li><code>cmake -DLIBCXX_CXX_ABI=libcxxabi
-DLIBCXX_LIBCXXABI_INCLUDE_PATHS=path/to/libcxxabi/include
-DLIT_EXECUTABLE=path/to/llvm/utils/lit/lit.py .. # Linux may require
-DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++</code></li>
<li><code>make</code></li>
</ul>
<p>To run the tests:</p>
<ul>
<li><code>make check-libcxx</code></li>
</ul>
<p>If you wish to run a subset of the test suite:</p>
<ul>
<li><code>cd path/to/libcxx/libcxx</code></li>
<li><code>alias lit='python path/to/llvm/utils/lit/lit.py'</code></li>
<li><code>ln -s path/to/build/dir/projects/libcxx/test/lit.site.cfg
test/lit.site.cfg</code></li>
<li><code>lit -sv test/re/ # or whichever subset of tests you're interested
in</code></li>
</ul>
<p>The above is currently quite inconvenient. Sorry! We're working on it!</p>
<p>More information on using LIT can be found
<a href="http://llvm.org/docs/CommandGuide/lit.html">here</a>. For more
general information about the LLVM testing infrastructure, see the
<a href="http://llvm.org/docs/TestingGuide.html">LLVM Testing Infrastructure
Guide</a>
</p>
<p>
Shared libraries for libc++ should now be present in llvm/build/lib. Note
that it is safest to use this from its current location rather than
replacing your system's libc++ (if it has one, if not, go right ahead).
</p>
<p>
Mac users, remember to be careful when replacing the system's libc++.
<strong>Your system will not be able to boot without a funcioning
libc++.</strong>
</p>
<!--=====================================================================-->
<h3>Notes</h3>
<!--=====================================================================-->
<p>
Building libc++ with <code>-fno-rtti</code> is not supported. However
linking against it with <code>-fno-rtti</code> is supported.
</p>
<p>Send discussions to the
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">clang mailing list</a>.</p>
<!--=====================================================================-->
<h2>Using libc++ in your programs</h2>
<!--=====================================================================-->
<!--=====================================================================-->
<h3>FreeBSD and Mac OS X</h3>
<!--=====================================================================-->
<p>
To use your system-installed libc++ with clang you can:
</p>
<ul>
<li><code>clang++ -stdlib=libc++ test.cpp</code></li>
<li><code>clang++ -std=c++11 -stdlib=libc++ test.cpp</code></li>
</ul>
<p>
To use your tip-of-trunk libc++ on Mac OS with clang you can:
</p>
<ul>
<li><code>export DYLD_LIBRARY_PATH=path/to/build/lib</code>
<li><code>clang++ -std=c++11 -stdlib=libc++ -nostdinc++
-I<path-to-libcxx>/include -L<path-to-libcxx>/lib
test.cpp</code></li>
</ul>
<!--=====================================================================-->
<h3>Linux</h3>
<!--=====================================================================-->
<p>
You will need to keep the source tree of
<a href="http://libcxxabi.llvm.org">libc++abi</a> available on your build
machine and your copy of the libc++abi shared library must be placed where
your linker will find it.
</p>
<p>
Unfortunately you can't simply run clang with "-stdlib=libc++" at this
point, as clang is set up to link for libc++ linked to libsupc++. To get
around this you'll have to set up your linker yourself (or patch clang).
For example:
</p>
<ul>
<li><code>clang++ -stdlib=libc++ helloworld.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc</code></li>
</ul>
<p>
Alternately, you could just add libc++abi to your libraries list, which in
most situations will give the same result:
</p>
<ul>
<li><code>clang++ -stdlib=libc++ helloworld.cpp -lc++abi</code></li>
</ul>
<!--=====================================================================-->
<h2>Bug reports and patches</h2>
<!--=====================================================================-->
<p>
If you think you've found a bug in libc++, please report it using
the <a href="http://llvm.org/bugs">LLVM Bugzilla</a>. If you're not sure, you
can post a message to the <a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">cfe-dev</a>
mailing list or on IRC. Please include "libc++" in your subject.
</p>
<p>
If you want to contribute a patch to libc++, the best place for that is the
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits">cfe-commits</a>
mailing list. Please include "libc++" and "PATCH" in your subject.
</p>
<!--=====================================================================-->
<h2 id="libsupcxx">Build on Linux using CMake and libsupc++.</h2>
<!--=====================================================================-->
<p>
You will need libstdc++ in order to provide libsupc++.
</p>
<p>
Figure out where the libsupc++ headers are on your system. On Ubuntu this
is <code>/usr/include/c++/<version></code> and
<code>/usr/include/c++/<version>/<target-triple></code>
</p>
<p>
You can also figure this out by running
<pre>
$ echo | g++ -Wp,-v -x c++ - -fsyntax-only
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/4.7
/usr/include/c++/4.7/x86_64-linux-gnu
/usr/include/c++/4.7/backward
/usr/lib/gcc/x86_64-linux-gnu/4.7/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/4.7/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
</pre>
Note the first two entries happen to be what we are looking for. This
may not be correct on other platforms.
</p>
<p>
We can now run CMake:
<ul>
<li><code>CC=clang CXX=clang++ cmake -G "Unix Makefiles"
-DLIBCXX_CXX_ABI=libstdc++
-DLIBCXX_LIBSUPCXX_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/"
-DCMAKE_BUILD_TYPE=Release
-DCMAKE_INSTALL_PREFIX=/usr
<libc++-source-dir></code></li>
<li>You can also substitute <code>-DLIBCXX_CXX_ABI=libsupc++</code>
above, which will cause the library to be linked to libsupc++ instead
of libstdc++, but this is only recommended if you know that you will
never need to link against libstdc++ in the same executable as libc++.
GCC ships libsupc++ separately but only as a static library. If a
program also needs to link against libstdc++, it will provide its
own copy of libsupc++ and this can lead to subtle problems.
<li><code>make</code></li>
<li><code>sudo make install</code></li>
</ul>
<p>
You can now run clang with -stdlib=libc++.
</p>
</p>
<!--=====================================================================-->
<h2 id="libcxxrt">Build on Linux using CMake and libcxxrt.</h2>
<!--=====================================================================-->
<p>
You will need to keep the source tree of
<a href="https://github.com/pathscale/libcxxrt/">libcxxrt</a> available
on your build machine and your copy of the libcxxrt shared library must
be placed where your linker will find it.
</p>
<p>
We can now run CMake:
<ul>
<li><code>CC=clang CXX=clang++ cmake -G "Unix Makefiles"
-DLIBCXX_CXX_ABI=libcxxrt
-DLIBCXX_LIBCXXRT_INCLUDE_PATHS="<libcxxrt-source-dir>/src"
-DCMAKE_BUILD_TYPE=Release
-DCMAKE_INSTALL_PREFIX=/usr
<libc++-source-dir></code></li>
<li><code>make</code></li>
<li><code>sudo make install</code></li>
</ul>
<p>
Unfortunately you can't simply run clang with "-stdlib=libc++" at this point, as
clang is set up to link for libc++ linked to libsupc++. To get around this
you'll have to set up your linker yourself (or patch clang). For example,
<ul>
<li><code>clang++ -stdlib=libc++ helloworld.cpp -nodefaultlibs -lc++ -lcxxrt -lm -lc -lgcc_s -lgcc</code></li>
</ul>
Alternately, you could just add libcxxrt to your libraries list, which in most
situations will give the same result:
<ul>
<li><code>clang++ -stdlib=libc++ helloworld.cpp -lcxxrt</code></li>
</ul>
</p>
</p>
<!--=====================================================================-->
<h2 id="local-abi">Using a local ABI library</h2>
<!--=====================================================================-->
<p>
<strong>Note: This is not recommended in almost all cases.</strong><br>
Generally these instructions should only be used when you can't install
your ABI library.
</p>
<p>
Normally you must link libc++ against a ABI shared library that the
linker can find. If you want to build and test libc++ against an ABI
library not in the linker's path you need to set
<code>-DCMAKE_LIBRARY_PATH=/path/to/abi/lib</code> when configuring CMake.
</p>
<p>
An example build using libc++abi would look like:
<ul>
<li><code>CC=clang CXX=clang++ cmake
-DLIBCXX_CXX_ABI=libc++abi
-DLIBCXX_LIBCXXABI_INCLUDE_PATHS="/path/to/libcxxabi/include"
-DCMAKE_LIBRARY_PATH="/path/to/libcxxabi-build/lib"
path/to/libcxx</code></li>
<li><code>make</code></li>
</ul>
</p>
<p>
When testing libc++ LIT will automatically link against the proper ABI
library.
</p>
<!--=====================================================================-->
<h2>Design Documents</h2>
<!--=====================================================================-->
<ul>
<li><a href="atomic_design.html"><tt><atomic></tt></a></li>
<li><a href="type_traits_design.html"><tt><type_traits></tt></a></li>
<li><a href="http://cplusplusmusings.wordpress.com/2012/07/05/clang-and-standard-libraries-on-mac-os-x/">Excellent notes by Marshall Clow</a></li>
<li><a href="debug_mode.html">Status of debug mode</a></li>
</ul>
</div>
</body>
</html>
|
