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34 <!--*********************************************************************-->
35 <h1>"libc++" C++ Standard Library</h1>
36 <!--*********************************************************************-->
38 <p>libc++ is a new implementation of the C++ standard library, targeting
41 <p>All of the code in libc++ is <a
42 href="http://llvm.org/docs/DeveloperPolicy.html#license">dual licensed</a>
43 under the MIT license and the UIUC License (a BSD-like license).</p>
45 <!--=====================================================================-->
46 <h2 id="goals">Features and Goals</h2>
47 <!--=====================================================================-->
50 <li>Correctness as defined by the C++11 standard.</li>
51 <li>Fast execution.</li>
52 <li>Minimal memory use.</li>
53 <li>Fast compile times.</li>
54 <li>ABI compatibility with gcc's libstdc++ for some low-level features
55 such as exception objects, rtti and memory allocation.</li>
56 <li>Extensive unit tests.</li>
59 <!--=====================================================================-->
60 <h2 id="why">Why a new C++ Standard Library for C++11?</h2>
61 <!--=====================================================================-->
63 <p>After its initial introduction, many people have asked "why start a new
64 library instead of contributing to an existing library?" (like Apache's
65 libstdcxx, GNU's libstdc++, STLport, etc). There are many contributing
66 reasons, but some of the major ones are:</p>
69 <li><p>From years of experience (including having implemented the standard
70 library before), we've learned many things about implementing
71 the standard containers which require ABI breakage and fundamental changes
72 to how they are implemented. For example, it is generally accepted that
73 building std::string using the "short string optimization" instead of
74 using Copy On Write (COW) is a superior approach for multicore
75 machines (particularly in C++11, which has rvalue references). Breaking
76 ABI compatibility with old versions of the library was
77 determined to be critical to achieving the performance goals of
80 <li><p>Mainline libstdc++ has switched to GPL3, a license which the developers
81 of libc++ cannot use. libstdc++ 4.2 (the last GPL2 version) could be
82 independently extended to support C++11, but this would be a fork of the
83 codebase (which is often seen as worse for a project than starting a new
84 independent one). Another problem with libstdc++ is that it is tightly
85 integrated with G++ development, tending to be tied fairly closely to the
86 matching version of G++.</p>
89 <li><p>STLport and the Apache libstdcxx library are two other popular
90 candidates, but both lack C++11 support. Our experience (and the
91 experience of libstdc++ developers) is that adding support for C++11 (in
92 particular rvalue references and move-only types) requires changes to
93 almost every class and function, essentially amounting to a rewrite.
94 Faced with a rewrite, we decided to start from scratch and evaluate every
95 design decision from first principles based on experience.</p>
97 <p>Further, both projects are apparently abandoned: STLport 5.2.1 was
98 released in Oct'08, and STDCXX 4.2.1 in May'08.</p>
102 <!--=====================================================================-->
103 <h2 id="requirements">Platform Support</h2>
104 <!--=====================================================================-->
106 <p>libc++ is known to work on the following platforms, using g++-4.2 and
107 clang (lack of C++11 language support disables some functionality).</p>
110 <li>Mac OS X i386</li>
111 <li>Mac OS X x86_64</li>
114 <!--=====================================================================-->
115 <h2 id="dir-structure">Current Status</h2>
116 <!--=====================================================================-->
118 <p>libc++ is a 100% complete C++11 implementation on Apple's OS X. </p>
119 <p>LLVM and Clang can self host in C++ and C++11 mode with libc++ on Linux.</p>
120 <p>C++1Y (C++14) implementation is in progress. The current status is
121 <a href="cxx1y_status.html">here</a></p>
124 Ports to other platforms are underway. Here are recent test
125 results for <a href="results.Windows.html">Windows</a>
126 and <a href="results.Linux.html">Linux</a>.
129 <!--=====================================================================-->
130 <h2>Get it and get involved!</h2>
131 <!--=====================================================================-->
133 <p>First please review our
134 <a href="http://llvm.org/docs/DeveloperPolicy.html">Developer's Policy</a>.
136 <p>To check out the code, use:</p>
139 <li><code>svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx</code></li>
143 On Mac OS 10.7 (Lion) and later, the easiest way to get this library is to install
144 Xcode 4.2 or later. However if you want to install tip-of-trunk from here
145 (getting the bleeding edge), read on. However, be warned that Mac OS
146 10.7 will not boot without a valid copy of <code>libc++.1.dylib</code> in
147 <code>/usr/lib</code>.
155 <li><code>cd libcxx/lib</code></li>
156 <li><code>export TRIPLE=-apple-</code></li>
157 <li><code>./buildit</code></li>
158 <li><code>ln -sf libc++.1.dylib libc++.dylib</code></li>
162 That should result in a libc++.1.dylib and libc++.dylib. The safest thing
163 to do is to use it from where your libcxx is installed instead of replacing
164 these in your Mac OS.
168 To use your system-installed libc++ with clang you can:
172 <li><code>clang++ -stdlib=libc++ test.cpp</code></li>
173 <li><code>clang++ -std=c++11 -stdlib=libc++ test.cpp</code></li>
177 To use your tip-of-trunk libc++ on Mac OS with clang you can:
181 <li><code>export DYLD_LIBRARY_PATH=<path-to-libcxx>/lib</code>
182 <li><code>clang++ -std=c++11 -stdlib=libc++ -nostdinc++
183 -I<path-to-libcxx>/include -L<path-to-libcxx>/lib
187 <p>To run the libc++ test suite (recommended):</p>
190 <li><code>cd libcxx/test</code></li>
191 <li><code>./testit</code></li>
193 <li>You can alter the command line options <code>testit</code> uses
194 with <code>export OPTIONS="whatever you need"</code></li>
198 <!--=====================================================================-->
200 <!--=====================================================================-->
203 Building libc++ with <code>-fno-rtti</code> is not supported. However linking
204 against it with <code>-fno-rtti</code> is supported.
207 <p>Send discussions to the
208 (<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">clang mailing list</a>).</p>
210 <!--=====================================================================-->
211 <h2>Build on Linux using CMake and libsupc++.</h2>
212 <!--=====================================================================-->
215 You will need libstdc++ in order to provide libsupc++.
219 Figure out where the libsupc++ headers are on your system. On Ubuntu this
220 is <code>/usr/include/c++/<version></code> and
221 <code>/usr/include/c++/<version>/<target-triple></code>
225 You can also figure this out by running
227 $ echo | g++ -Wp,-v -x c++ - -fsyntax-only
228 ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
229 ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../x86_64-linux-gnu/include"
230 #include "..." search starts here:
231 #include <...> search starts here:
233 /usr/include/c++/4.7/x86_64-linux-gnu
234 /usr/include/c++/4.7/backward
235 /usr/lib/gcc/x86_64-linux-gnu/4.7/include
237 /usr/lib/gcc/x86_64-linux-gnu/4.7/include-fixed
238 /usr/include/x86_64-linux-gnu
243 Note the first two entries happen to be what we are looking for. This
244 may not be correct on other platforms.
248 We can now run CMake:
250 <li><code>CC=clang CXX=clang++ cmake -G "Unix Makefiles"
251 -DLIBCXX_CXX_ABI=libstdc++
252 -DLIBCXX_LIBSUPCXX_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/"
253 -DCMAKE_BUILD_TYPE=Release
254 -DCMAKE_INSTALL_PREFIX=/usr
255 <libc++-source-dir></code></li>
256 <li>You can also substitute <code>-DLIBCXX_CXX_ABI=libsupc++</code>
257 above, which will cause the library to be linked to libsupc++ instead
258 of libstdc++, but this is only recommended if you know that you will
259 never need to link against libstdc++ in the same executable as libc++.
260 GCC ships libsupc++ separately but only as a static library. If a
261 program also needs to link against libstdc++, it will provide its
262 own copy of libsupc++ and this can lead to subtle problems.
263 <li><code>make</code></li>
264 <li><code>sudo make install</code></li>
267 You can now run clang with -stdlib=libc++.
271 <!--=====================================================================-->
272 <h2>Build on Linux using CMake and libc++abi.</h2>
273 <!--=====================================================================-->
276 You will need to keep the source tree of <a href="http://libcxxabi.llvm.org">libc++abi</a>
277 available on your build machine and your copy of the libc++abi shared library must
278 be placed where your linker will find it.
282 We can now run CMake:
284 <li><code>CC=clang CXX=clang++ cmake -G "Unix Makefiles"
285 -DLIBCXX_CXX_ABI=libcxxabi
286 -DLIBCXX_LIBCXXABI_INCLUDE_PATHS="<libc++abi-source-dir>/include"
287 -DCMAKE_BUILD_TYPE=Release
288 -DCMAKE_INSTALL_PREFIX=/usr
289 <libc++-source-dir></code></li>
290 <li><code>make</code></li>
291 <li><code>sudo make install</code></li>
294 Unfortunately you can't simply run clang with "-stdlib=libc++" at this point, as
295 clang is set up to link for libc++ linked to libsupc++. To get around this
296 you'll have to set up your linker yourself (or patch clang). For example,
298 <li><code>clang++ -stdlib=libc++ helloworld.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc</code></li>
300 Alternately, you could just add libc++abi to your libraries list, which in most
301 situations will give the same result:
303 <li><code>clang++ -stdlib=libc++ helloworld.cpp -lc++abi</code></li>
308 <!--=====================================================================-->
309 <h2>Build on Linux using CMake and libcxxrt.</h2>
310 <!--=====================================================================-->
313 You will need to keep the source tree of
314 <a href="https://github.com/pathscale/libcxxrt/">libcxxrt</a> available
315 on your build machine and your copy of the libcxxrt shared library must
316 be placed where your linker will find it.
320 We can now run CMake:
322 <li><code>CC=clang CXX=clang++ cmake -G "Unix Makefiles"
323 -DLIBCXX_CXX_ABI=libcxxrt
324 -DLIBCXX_LIBCXXRT_INCLUDE_PATHS="<libcxxrt-source-dir>/src"
325 -DCMAKE_BUILD_TYPE=Release
326 -DCMAKE_INSTALL_PREFIX=/usr
327 <libc++-source-dir></code></li>
328 <li><code>make</code></li>
329 <li><code>sudo make install</code></li>
332 Unfortunately you can't simply run clang with "-stdlib=libc++" at this point, as
333 clang is set up to link for libc++ linked to libsupc++. To get around this
334 you'll have to set up your linker yourself (or patch clang). For example,
336 <li><code>clang++ -stdlib=libc++ helloworld.cpp -nodefaultlibs -lc++ -lcxxrt -lm -lc -lgcc_s -lgcc</code></li>
338 Alternately, you could just add libcxxrt to your libraries list, which in most
339 situations will give the same result:
341 <li><code>clang++ -stdlib=libc++ helloworld.cpp -lcxxrt</code></li>
346 <!--=====================================================================-->
347 <h2>Design Documents</h2>
348 <!--=====================================================================-->
351 <li><a href="atomic_design.html"><tt><atomic></tt></a></li>
352 <li><a href="type_traits_design.html"><tt><type_traits></tt></a></li>
353 <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>
354 <li><a href="debug_mode.html">Status of debug mode</a></li>