1 <sect1 id="appendix.porting.abi" xreflabel="abi">
2 <?dbhtml filename="abi.html"?>
24 <title>ABI Policy and Guidelines</title>
29 <sect2 id="abi.cxx_interface" xreflabel="abi.cxx_interface">
30 <title>The C++ Interface</title>
33 C++ applications often dependent on specific language support
34 routines, say for throwing exceptions, or catching exceptions, and
35 perhaps also dependent on features in the C++ Standard Library.
39 The C++ Standard Library has many include files, types defined in
40 those include files, specific named functions, and other
41 behavior. The text of these behaviors, as written in source include
42 files, is called the Application Programing Interface, or API.
46 Furthermore, C++ source that is compiled into object files is
47 transformed by the compiler: it arranges objects with specific
48 alignment and in a particular layout, mangling names according to a
49 well-defined algorithm, has specific arrangements for the support of
50 virtual functions, etc. These details are defined as the compiler
51 Application Binary Interface, or ABI. The GNU C++ compiler uses an
52 industry-standard C++ ABI starting with version 3. Details can be
54 url="http://www.codesourcery.com/cxx-abi/abi.html"> ABI
55 specification</ulink>.
59 The GNU C++ compiler, g++, has a compiler command line option to
60 switch between various different C++ ABIs. This explicit version
61 switch is the flag <code>-fabi-version</code>. In addition, some
62 g++ command line options may change the ABI as a side-effect of
63 use. Such flags include <code>-fpack-struct</code> and
64 <code>-fno-exceptions</code>, but include others: see the complete
65 list in the GCC manual under the heading <ulink url="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code%20Gen%20Options">Options
66 for Code Generation Conventions</ulink>.
70 The configure options used when building a specific libstdc++
71 version may also impact the resulting library ABI. The available
72 configure options, and their impact on the library ABI, are
74 <ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html">
78 <para> Putting all of these ideas together results in the C++ Standard
79 library ABI, which is the compilation of a given library API by a
80 given compiler ABI. In a nutshell:
85 library API + compiler ABI = library ABI
90 The library ABI is mostly of interest for end-users who have
91 unresolved symbols and are linking dynamically to the C++ Standard
92 library, and who thus must be careful to compile their application
93 with a compiler that is compatible with the available C++ Standard
94 library binary. In this case, compatible is defined with the equation
95 above: given an application compiled with a given compiler ABI and
96 library API, it will work correctly with a Standard C++ Library
97 created with the same constraints.
101 To use a specific version of the C++ ABI, one must use a
102 corresponding GNU C++ toolchain (i.e., g++ and libstdc++) that
103 implements the C++ ABI in question.
108 <sect2 id="abi.versioning" xreflabel="abi.versioning">
109 <title>Versioning</title>
111 <para> The C++ interface has evolved throughout the history of the GNU
112 C++ toolchain. With each release, various details have been changed so
113 as to give distinct versions to the C++ interface.
116 <sect3 id="abi.versioning.goals" xreflabel="abi.versioning.goals">
119 <para>Extending existing, stable ABIs. Versioning gives subsequent stable
120 releases series libraries the ability to add new symbols and add
121 functionality, all the while retaining backwards compatibility with
122 the previous releases in the series. Note: the reverse is not true. It
123 is not possible to take binaries linked with the latest version of a
124 release series (if symbols have been added) and expect the initial
125 release of the series to remain link compatible.
128 <para>Allows multiple, incompatible ABIs to coexist at the same time.
132 <sect3 id="abi.versioning.history" xreflabel="abi.versioning.history">
133 <title>History</title>
136 How can this complexity be managed? What does C++ versioning mean?
137 Because library and compiler changes often make binaries compiled
138 with one version of the GNU tools incompatible with binaries
139 compiled with other (either newer or older) versions of the same GNU
140 tools, specific techniques are used to make managing this complexity
145 The following techniques are used:
150 <listitem><para>Release versioning on the libgcc_s.so binary. </para>
152 <para>This is implemented via file names and the ELF DT_SONAME
153 mechanism (at least on ELF systems). It is versioned as follows:
157 <listitem><para>gcc-3.0.0: libgcc_s.so.1</para></listitem>
158 <listitem><para>gcc-3.0.1: libgcc_s.so.1</para></listitem>
159 <listitem><para>gcc-3.0.2: libgcc_s.so.1</para></listitem>
160 <listitem><para>gcc-3.0.3: libgcc_s.so.1</para></listitem>
161 <listitem><para>gcc-3.0.4: libgcc_s.so.1</para></listitem>
162 <listitem><para>gcc-3.1.0: libgcc_s.so.1</para></listitem>
163 <listitem><para>gcc-3.1.1: libgcc_s.so.1</para></listitem>
164 <listitem><para>gcc-3.2.0: libgcc_s.so.1</para></listitem>
165 <listitem><para>gcc-3.2.1: libgcc_s.so.1</para></listitem>
166 <listitem><para>gcc-3.2.2: libgcc_s.so.1</para></listitem>
167 <listitem><para>gcc-3.2.3: libgcc_s.so.1</para></listitem>
168 <listitem><para>gcc-3.3.0: libgcc_s.so.1</para></listitem>
169 <listitem><para>gcc-3.3.1: libgcc_s.so.1</para></listitem>
170 <listitem><para>gcc-3.3.2: libgcc_s.so.1</para></listitem>
171 <listitem><para>gcc-3.3.3: libgcc_s.so.1</para></listitem>
172 <listitem><para>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.x: on m68k-linux and
173 hppa-linux this is either libgcc_s.so.1 (when configuring
174 <code>--with-sjlj-exceptions</code>) or libgcc_s.so.2. For all
175 others, this is libgcc_s.so.1. </para>
181 <listitem><para>Symbol versioning on the libgcc_s.so binary.</para>
183 <para>It is versioned with the following labels and version
184 definitions, where the version definition is the maximum for a
185 particular release. Labels are cumulative. If a particular release
186 is not listed, it has the same version labels as the preceding
189 <para>This corresponds to the mapfile: gcc/libgcc-std.ver</para>
191 <listitem><para>gcc-3.0.0: GCC_3.0</para></listitem>
192 <listitem><para>gcc-3.3.0: GCC_3.3</para></listitem>
193 <listitem><para>gcc-3.3.1: GCC_3.3.1</para></listitem>
194 <listitem><para>gcc-3.3.2: GCC_3.3.2</para></listitem>
195 <listitem><para>gcc-3.3.4: GCC_3.3.4</para></listitem>
196 <listitem><para>gcc-3.4.0: GCC_3.4</para></listitem>
197 <listitem><para>gcc-3.4.2: GCC_3.4.2</para></listitem>
198 <listitem><para>gcc-3.4.4: GCC_3.4.4</para></listitem>
199 <listitem><para>gcc-4.0.0: GCC_4.0.0</para></listitem>
200 <listitem><para>gcc-4.1.0: GCC_4.1.0</para></listitem>
201 <listitem><para>gcc-4.2.0: GCC_4.2.0</para></listitem>
205 <listitem><para>Release versioning on the libstdc++.so binary, implemented in the same was as the libgcc_s.so binary, above.</para>
207 <para>It is versioned as follows:
210 <listitem><para>gcc-3.0.0: libstdc++.so.3.0.0</para></listitem>
211 <listitem><para>gcc-3.0.1: libstdc++.so.3.0.1</para></listitem>
212 <listitem><para>gcc-3.0.2: libstdc++.so.3.0.2</para></listitem>
213 <listitem><para>gcc-3.0.3: libstdc++.so.3.0.2 (Error should be libstdc++.so.3.0.3)</para></listitem>
214 <listitem><para>gcc-3.0.4: libstdc++.so.3.0.4</para></listitem>
215 <listitem><para>gcc-3.1.0: libstdc++.so.4.0.0</para></listitem>
216 <listitem><para>gcc-3.1.1: libstdc++.so.4.0.1</para></listitem>
217 <listitem><para>gcc-3.2.0: libstdc++.so.5.0.0</para></listitem>
218 <listitem><para>gcc-3.2.1: libstdc++.so.5.0.1</para></listitem>
219 <listitem><para>gcc-3.2.2: libstdc++.so.5.0.2</para></listitem>
220 <listitem><para>gcc-3.2.3: libstdc++.so.5.0.3 (Not strictly required)</para></listitem>
221 <listitem><para>gcc-3.3.0: libstdc++.so.5.0.4</para></listitem>
222 <listitem><para>gcc-3.3.1: libstdc++.so.5.0.5</para></listitem>
223 <listitem><para>gcc-3.3.2: libstdc++.so.5.0.5</para></listitem>
224 <listitem><para>gcc-3.3.3: libstdc++.so.5.0.5</para></listitem>
225 <listitem><para>gcc-3.4.0: libstdc++.so.6.0.0</para></listitem>
226 <listitem><para>gcc-3.4.1: libstdc++.so.6.0.1</para></listitem>
227 <listitem><para>gcc-3.4.2: libstdc++.so.6.0.2</para></listitem>
228 <listitem><para>gcc-3.4.3: libstdc++.so.6.0.3</para></listitem>
229 <listitem><para>gcc-3.4.4: libstdc++.so.6.0.3</para></listitem>
230 <listitem><para>gcc-3.4.5: libstdc++.so.6.0.3</para></listitem>
231 <listitem><para>gcc-3.4.6: libstdc++.so.6.0.3</para></listitem>
232 <listitem><para>gcc-4.0.0: libstdc++.so.6.0.4</para></listitem>
233 <listitem><para>gcc-4.0.1: libstdc++.so.6.0.5</para></listitem>
234 <listitem><para>gcc-4.0.2: libstdc++.so.6.0.6</para></listitem>
235 <listitem><para>gcc-4.0.3: libstdc++.so.6.0.7</para></listitem>
236 <listitem><para>gcc-4.1.0: libstdc++.so.6.0.7</para></listitem>
237 <listitem><para>gcc-4.1.1: libstdc++.so.6.0.8</para></listitem>
238 <listitem><para>gcc-4.1.2: libstdc++.so.6.0.8</para></listitem>
239 <listitem><para>gcc-4.2.0: libstdc++.so.6.0.9</para></listitem>
243 <listitem><para>Symbol versioning on the libstdc++.so binary.</para>
245 <para>mapfile: libstdc++/config/linker-map.gnu</para>
246 <para>It is versioned with the following labels and version
247 definitions, where the version definition is the maximum for a
248 particular release. Note, only symbol which are newly introduced
249 will use the maximum version definition. Thus, for release series
250 with the same label, but incremented version definitions, the later
251 release has both versions. (An example of this would be the
252 gcc-3.2.1 release, which has GLIBCPP_3.2.1 for new symbols and
253 GLIBCPP_3.2 for symbols that were introduced in the gcc-3.2.0
254 release.) If a particular release is not listed, it has the same
255 version labels as the preceding release.
258 <listitem><para>gcc-3.0.0: (Error, not versioned)</para></listitem>
259 <listitem><para>gcc-3.0.1: (Error, not versioned)</para></listitem>
260 <listitem><para>gcc-3.0.2: (Error, not versioned)</para></listitem>
261 <listitem><para>gcc-3.0.3: (Error, not versioned)</para></listitem>
262 <listitem><para>gcc-3.0.4: (Error, not versioned)</para></listitem>
263 <listitem><para>gcc-3.1.0: GLIBCPP_3.1, CXXABI_1</para></listitem>
264 <listitem><para>gcc-3.1.1: GLIBCPP_3.1, CXXABI_1</para></listitem>
265 <listitem><para>gcc-3.2.0: GLIBCPP_3.2, CXXABI_1.2</para></listitem>
266 <listitem><para>gcc-3.2.1: GLIBCPP_3.2.1, CXXABI_1.2</para></listitem>
267 <listitem><para>gcc-3.2.2: GLIBCPP_3.2.2, CXXABI_1.2</para></listitem>
268 <listitem><para>gcc-3.2.3: GLIBCPP_3.2.2, CXXABI_1.2</para></listitem>
269 <listitem><para>gcc-3.3.0: GLIBCPP_3.2.2, CXXABI_1.2.1</para></listitem>
270 <listitem><para>gcc-3.3.1: GLIBCPP_3.2.3, CXXABI_1.2.1</para></listitem>
271 <listitem><para>gcc-3.3.2: GLIBCPP_3.2.3, CXXABI_1.2.1</para></listitem>
272 <listitem><para>gcc-3.3.3: GLIBCPP_3.2.3, CXXABI_1.2.1</para></listitem>
273 <listitem><para>gcc-3.4.0: GLIBCXX_3.4, CXXABI_1.3</para></listitem>
274 <listitem><para>gcc-3.4.1: GLIBCXX_3.4.1, CXXABI_1.3</para></listitem>
275 <listitem><para>gcc-3.4.2: GLIBCXX_3.4.2</para></listitem>
276 <listitem><para>gcc-3.4.3: GLIBCXX_3.4.3</para></listitem>
277 <listitem><para>gcc-4.0.0: GLIBCXX_3.4.4, CXXABI_1.3.1</para></listitem>
278 <listitem><para>gcc-4.0.1: GLIBCXX_3.4.5</para></listitem>
279 <listitem><para>gcc-4.0.2: GLIBCXX_3.4.6</para></listitem>
280 <listitem><para>gcc-4.0.3: GLIBCXX_3.4.7</para></listitem>
281 <listitem><para>gcc-4.1.1: GLIBCXX_3.4.8</para></listitem>
282 <listitem><para>gcc-4.2.0: GLIBCXX_3.4.9</para></listitem>
287 <para>Incremental bumping of a compiler pre-defined macro,
288 __GXX_ABI_VERSION. This macro is defined as the version of the
289 compiler v3 ABI, with g++ 3.0.x being version 100. This macro will
290 be automatically defined whenever g++ is used (the curious can
291 test this by invoking g++ with the '-v' flag.)
295 This macro was defined in the file "lang-specs.h" in the gcc/cp directory.
296 Later versions defined it in "c-common.c" in the gcc directory, and from
297 G++ 3.4 it is defined in c-cppbuiltin.c and its value determined by the
298 '-fabi-version' command line option.
302 It is versioned as follows, where 'n' is given by '-fabi-version=n':
305 <listitem><para>gcc-3.0.x: 100</para></listitem>
306 <listitem><para>gcc-3.1.x: 100 (Error, should be 101)</para></listitem>
307 <listitem><para>gcc-3.2.x: 102</para></listitem>
308 <listitem><para>gcc-3.3.x: 102</para></listitem>
309 <listitem><para>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.x: 102 (when n=1)</para></listitem>
310 <listitem><para>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.x: 1000 + n (when n>1)</para></listitem>
311 <listitem><para>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.x: 999999 (when n=0)</para></listitem>
317 <para>Changes to the default compiler option for
318 <code>-fabi-version</code>.
321 It is versioned as follows:
324 <listitem><para>gcc-3.0.x: (Error, not versioned) </para></listitem>
325 <listitem><para>gcc-3.1.x: (Error, not versioned) </para></listitem>
326 <listitem><para>gcc-3.2.x: <code>-fabi-version=1</code></para></listitem>
327 <listitem><para>gcc-3.3.x: <code>-fabi-version=1</code></para></listitem>
328 <listitem><para>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.x: <code>-fabi-version=2</code></para></listitem>
334 <para>Incremental bumping of a library pre-defined macro. For releases
335 before 3.4.0, the macro is __GLIBCPP__. For later releases, it's
336 __GLIBCXX__. (The libstdc++ project generously changed from CPP to
337 CXX throughout its source to allow the "C" pre-processor the CPP
338 macro namespace.) These macros are defined as the date the library
339 was released, in compressed ISO date format, as an unsigned long.
343 This macro is defined in the file "c++config" in the
344 "libstdc++/include/bits" directory. (Up to gcc-4.1.0, it was
345 changed every night by an automated script. Since gcc-4.1.0, it is
346 the same value as gcc/DATESTAMP.)
349 It is versioned as follows:
352 <listitem><para>gcc-3.0.0: 20010615</para></listitem>
353 <listitem><para>gcc-3.0.1: 20010819</para></listitem>
354 <listitem><para>gcc-3.0.2: 20011023</para></listitem>
355 <listitem><para>gcc-3.0.3: 20011220</para></listitem>
356 <listitem><para>gcc-3.0.4: 20020220</para></listitem>
357 <listitem><para>gcc-3.1.0: 20020514</para></listitem>
358 <listitem><para>gcc-3.1.1: 20020725</para></listitem>
359 <listitem><para>gcc-3.2.0: 20020814</para></listitem>
360 <listitem><para>gcc-3.2.1: 20021119</para></listitem>
361 <listitem><para>gcc-3.2.2: 20030205</para></listitem>
362 <listitem><para>gcc-3.2.3: 20030422</para></listitem>
363 <listitem><para>gcc-3.3.0: 20030513</para></listitem>
364 <listitem><para>gcc-3.3.1: 20030804</para></listitem>
365 <listitem><para>gcc-3.3.2: 20031016</para></listitem>
366 <listitem><para>gcc-3.3.3: 20040214</para></listitem>
367 <listitem><para>gcc-3.4.0: 20040419</para></listitem>
368 <listitem><para>gcc-3.4.1: 20040701</para></listitem>
369 <listitem><para>gcc-3.4.2: 20040906</para></listitem>
370 <listitem><para>gcc-3.4.3: 20041105</para></listitem>
371 <listitem><para>gcc-3.4.4: 20050519</para></listitem>
372 <listitem><para>gcc-3.4.5: 20051201</para></listitem>
373 <listitem><para>gcc-3.4.6: 20060306</para></listitem>
374 <listitem><para>gcc-4.0.0: 20050421</para></listitem>
375 <listitem><para>gcc-4.0.1: 20050707</para></listitem>
376 <listitem><para>gcc-4.0.2: 20050921</para></listitem>
377 <listitem><para>gcc-4.0.3: 20060309</para></listitem>
378 <listitem><para>gcc-4.1.0: 20060228</para></listitem>
379 <listitem><para>gcc-4.1.1: 20060524</para></listitem>
380 <listitem><para>gcc-4.1.2: 20070214</para></listitem>
381 <listitem><para>gcc-4.2.0: 20070514</para></listitem>
388 Incremental bumping of a library pre-defined macro,
389 _GLIBCPP_VERSION. This macro is defined as the released version of
390 the library, as a string literal. This is only implemented in
391 gcc-3.1.0 releases and higher, and is deprecated in 3.4 (where it
392 is called _GLIBCXX_VERSION).
396 This macro is defined in the file "c++config" in the
397 "libstdc++/include/bits" directory and is generated
398 automatically by autoconf as part of the configure-time generation
403 It is versioned as follows:
406 <listitem><para>gcc-3.0.0: "3.0.0"</para></listitem>
407 <listitem><para>gcc-3.0.1: "3.0.0" (Error, should be "3.0.1")</para></listitem>
408 <listitem><para>gcc-3.0.2: "3.0.0" (Error, should be "3.0.2")</para></listitem>
409 <listitem><para>gcc-3.0.3: "3.0.0" (Error, should be "3.0.3")</para></listitem>
410 <listitem><para>gcc-3.0.4: "3.0.0" (Error, should be "3.0.4")</para></listitem>
411 <listitem><para>gcc-3.1.0: "3.1.0"</para></listitem>
412 <listitem><para>gcc-3.1.1: "3.1.1"</para></listitem>
413 <listitem><para>gcc-3.2.0: "3.2"</para></listitem>
414 <listitem><para>gcc-3.2.1: "3.2.1"</para></listitem>
415 <listitem><para>gcc-3.2.2: "3.2.2"</para></listitem>
416 <listitem><para>gcc-3.2.3: "3.2.3"</para></listitem>
417 <listitem><para>gcc-3.3.0: "3.3"</para></listitem>
418 <listitem><para>gcc-3.3.1: "3.3.1"</para></listitem>
419 <listitem><para>gcc-3.3.2: "3.3.2"</para></listitem>
420 <listitem><para>gcc-3.3.3: "3.3.3"</para></listitem>
421 <listitem><para>gcc-3.4.x: "version-unused"</para></listitem>
422 <listitem><para>gcc-4.0.x: "version-unused"</para></listitem>
423 <listitem><para>gcc-4.1.x: "version-unused"</para></listitem>
424 <listitem><para>gcc-4.2.x: "version-unused"</para></listitem>
431 Matching each specific C++ compiler release to a specific set of
432 C++ include files. This is only implemented in gcc-3.1.1 releases
436 All C++ includes are installed in include/c++, then nest in a
437 directory hierarchy corresponding to the C++ compiler's released
438 version. This version corresponds to the variable "gcc_version" in
439 "libstdc++/acinclude.m4," and more details can be found in that
440 file's macro GLIBCXX_CONFIGURE (GLIBCPP_CONFIGURE before gcc-3.4.0).
443 C++ includes are versioned as follows:
446 <listitem><para>gcc-3.0.0: include/g++-v3</para></listitem>
447 <listitem><para>gcc-3.0.1: include/g++-v3</para></listitem>
448 <listitem><para>gcc-3.0.2: include/g++-v3</para></listitem>
449 <listitem><para>gcc-3.0.3: include/g++-v3</para></listitem>
450 <listitem><para>gcc-3.0.4: include/g++-v3</para></listitem>
451 <listitem><para>gcc-3.1.0: include/g++-v3</para></listitem>
452 <listitem><para>gcc-3.1.1: include/c++/3.1.1</para></listitem>
453 <listitem><para>gcc-3.2.0: include/c++/3.2</para></listitem>
454 <listitem><para>gcc-3.2.1: include/c++/3.2.1</para></listitem>
455 <listitem><para>gcc-3.2.2: include/c++/3.2.2</para></listitem>
456 <listitem><para>gcc-3.2.3: include/c++/3.2.3</para></listitem>
457 <listitem><para>gcc-3.3.0: include/c++/3.3</para></listitem>
458 <listitem><para>gcc-3.3.1: include/c++/3.3.1</para></listitem>
459 <listitem><para>gcc-3.3.2: include/c++/3.3.2</para></listitem>
460 <listitem><para>gcc-3.3.3: include/c++/3.3.3</para></listitem>
461 <listitem><para>gcc-3.4.0: include/c++/3.4.0</para></listitem>
462 <listitem><para>gcc-3.4.1: include/c++/3.4.1</para></listitem>
463 <listitem><para>gcc-3.4.2: include/c++/3.4.2</para></listitem>
464 <listitem><para>gcc-3.4.3: include/c++/3.4.3</para></listitem>
465 <listitem><para>gcc-3.4.4: include/c++/3.4.4</para></listitem>
466 <listitem><para>gcc-3.4.5: include/c++/3.4.5</para></listitem>
467 <listitem><para>gcc-3.4.6: include/c++/3.4.6</para></listitem>
468 <listitem><para>gcc-4.0.0: include/c++/4.0.0</para></listitem>
469 <listitem><para>gcc-4.0.1: include/c++/4.0.1</para></listitem>
470 <listitem><para>gcc-4.0.2: include/c++/4.0.2</para></listitem>
471 <listitem><para>gcc-4.0.3: include/c++/4.0.3</para></listitem>
472 <listitem><para>gcc-4.1.0: include/c++/4.1.0</para></listitem>
473 <listitem><para>gcc-4.1.1: include/c++/4.1.1</para></listitem>
474 <listitem><para>gcc-4.1.2: include/c++/4.1.2</para></listitem>
475 <listitem><para>gcc-4.2.0: include/c++/4.2.0</para></listitem>
482 Taken together, these techniques can accurately specify interface
483 and implementation changes in the GNU C++ tools themselves. Used
484 properly, they allow both the GNU C++ tools implementation, and
485 programs using them, an evolving yet controlled development that
486 maintains backward compatibility.
492 <sect3 id="abi.versioning.prereq" xreflabel="abi.versioning.prereq">
493 <title>Prerequisites</title>
495 Minimum environment that supports a versioned ABI: A supported
496 dynamic linker, a GNU linker of sufficient vintage to understand
497 demangled C++ name globbing (ld), a shared executable compiled
498 with g++, and shared libraries (libgcc_s, libstdc++) compiled by
499 a compiler (g++) with a compatible ABI. Phew.
503 On top of all that, an additional constraint: libstdc++ did not
504 attempt to version symbols (or age gracefully, really) until
509 Most modern Linux and BSD versions, particularly ones using
510 gcc-3.1.x tools and more recent vintages, will meet the
515 <sect3 id="abi.versioning.config" xreflabel="abi.versioning.config">
516 <title>Configuring</title>
519 It turns out that most of the configure options that change
520 default behavior will impact the mangled names of exported
521 symbols, and thus impact versioning and compatibility.
525 For more information on configure options, including ABI
527 http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html
531 There is one flag that explicitly deals with symbol versioning:
536 In particular, libstdc++/acinclude.m4 has a macro called
537 GLIBCXX_ENABLE_SYMVERS that defaults to yes (or the argument
538 passed in via --enable-symvers=foo). At that point, the macro
539 attempts to make sure that all the requirement for symbol
540 versioning are in place. For more information, please consult
545 <sect3 id="abi.versioning.active" xreflabel="abi.versioning.active">
546 <title>Checking Active</title>
549 When the GNU C++ library is being built with symbol versioning
550 on, you should see the following at configure time for
556 checking versioning on shared library symbols... gnu
561 If you don't see this line in the configure output, or if this line
562 appears but the last word is 'no', then you are out of luck.
566 If the compiler is pre-installed, a quick way to test is to compile
567 the following (or any) simple C++ file and link it to the shared
572 #include <iostream>
575 { std::cout << "hello" << std::endl; return 0; }
577 %g++ hello.cc -o hello.out
580 libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x00764000)
581 libm.so.6 => /lib/tls/libm.so.6 (0x004a8000)
582 libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x40016000)
583 libc.so.6 => /lib/tls/libc.so.6 (0x0036d000)
584 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
590 If you see symbols in the resulting output with "GLIBCXX_3" as part
591 of the name, then the executable is versioned. Here's an example:
595 <code>U _ZNSt8ios_base4InitC1Ev@@GLIBCXX_3.4</code>
601 <sect2 id="abi.changes_allowed" xreflabel="abi.changes_allowed">
602 <title>Allowed Changes</title>
605 The following will cause the library minor version number to
606 increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.3.0.5".
609 <listitem><para>Adding an exported global or static data member</para></listitem>
610 <listitem><para>Adding an exported function, static or non-virtual member function</para></listitem>
611 <listitem><para>Adding an exported symbol or symbols by additional instantiations</para></listitem>
614 Other allowed changes are possible.
619 <sect2 id="abi.changes_no" xreflabel="abi.changes_no">
620 <title>Prohibited Changes</title>
623 The following non-exhaustive list will cause the library major version
624 number to increase, say from "libstdc++.so.3.0.4" to
625 "libstdc++.so.4.0.0".
629 <listitem><para>Changes in the gcc/g++ compiler ABI</para></listitem>
630 <listitem><para>Changing size of an exported symbol</para></listitem>
631 <listitem><para>Changing alignment of an exported symbol</para></listitem>
632 <listitem><para>Changing the layout of an exported symbol</para></listitem>
633 <listitem><para>Changing mangling on an exported symbol</para></listitem>
634 <listitem><para>Deleting an exported symbol</para></listitem>
635 <listitem><para>Changing the inheritance properties of a type by adding or removing
636 base classes</para></listitem>
638 Changing the size, alignment, or layout of types
639 specified in the C++ standard. These may not necessarily be
640 instantiated or otherwise exported in the library binary, and
641 include all the required locale facets, as well as things like
642 std::basic_streambuf, et al.
645 <listitem><para> Adding an explicit copy constructor or destructor to a
646 class that would otherwise have implicit versions. This will change
647 the way the compiler deals with this class in by-value return
648 statements or parameters: instead of being passing instances of this
649 class in registers, the compiler will be forced to use memory. See <ulink url="http://www.codesourcery.com/cxx-abi/abi.html#calls"> this part</ulink>
650 of the C++ ABI documentation for further details.
659 <sect2 id="abi.impl" xreflabel="abi.impl">
660 <title>Implementation</title>
665 Separation of interface and implementation
668 This is accomplished by two techniques that separate the API from
669 the ABI: forcing undefined references to link against a library
670 binary for definitions.
675 <term>Include files have declarations, source files have defines</term>
679 For non-templatized types, such as much of <code>class
680 locale</code>, the appropriate standard C++ include, say
681 <code>locale</code>, can contain full declarations, while
682 various source files (say <code> locale.cc, locale_init.cc,
683 localename.cc</code>) contain definitions.
689 <term>Extern template on required types</term>
693 For parts of the standard that have an explicit list of
694 required instantiations, the GNU extension syntax <code> extern
695 template </code> can be used to control where template
696 definitions reside. By marking required instantiations as
697 <code> extern template </code> in include files, and providing
698 explicit instantiations in the appropriate instantiation files,
699 non-inlined template functions can be versioned. This technique
700 is mostly used on parts of the standard that require <code>
701 char</code> and <code> wchar_t</code> instantiations, and
702 includes <code> basic_string</code>, the locale facets, and the
703 types in <code> iostreams</code>.
711 In addition, these techniques have the additional benefit that they
712 reduce binary size, which can increase runtime performance.
718 Namespaces linking symbol definitions to export mapfiles
721 All symbols in the shared library binary are processed by a
722 linker script at build time that either allows or disallows
723 external linkage. Because of this, some symbols, regardless of
724 normal C/C++ linkage, are not visible. Symbols that are internal
725 have several appealing characteristics: by not exporting the
726 symbols, there are no relocations when the shared library is
727 started and thus this makes for faster runtime loading
728 performance by the underlying dynamic loading mechanism. In
729 addition, they have the possibility of changing without impacting
733 <para>The following namespaces are transformed by the mapfile:</para>
738 <term><code>namespace std</code></term>
739 <listitem><para> Defaults to exporting all symbols in label
740 <code>GLIBCXX</code> that do not begin with an underscore, i.e.,
741 <code>__test_func</code> would not be exported by default. Select
742 exceptional symbols are allowed to be visible.</para></listitem>
746 <term><code>namespace __gnu_cxx</code></term>
747 <listitem><para> Defaults to not exporting any symbols in label
748 <code>GLIBCXX</code>, select items are allowed to be visible.</para></listitem>
752 <term><code>namespace __gnu_internal</code></term>
753 <listitem><para> Defaults to not exported, no items are allowed to be visible.</para></listitem>
757 <term><code>namespace __cxxabiv1</code>, aliased to <code> namespace abi</code></term>
758 <listitem><para> Defaults to not exporting any symbols in label
759 <code>CXXABI</code>, select items are allowed to be visible.</para></listitem>
767 <listitem><para>Freezing the API</para>
768 <para>Disallowed changes, as above, are not made on a stable release
769 branch. Enforcement tends to be less strict with GNU extensions that
770 standard includes.</para>
776 <sect2 id="abi.testing" xreflabel="abi.testing">
777 <title>Testing</title>
779 <sect3 id="abi.testing.single" xreflabel="abi.testing.single">
780 <title>Single ABI Testing</title>
783 Testing for GNU C++ ABI changes is composed of two distinct
784 areas: testing the C++ compiler (g++) for compiler changes, and
785 testing the C++ library (libstdc++) for library changes.
789 Testing the C++ compiler ABI can be done various ways.
793 One. Intel ABI checker. More information can be obtained <ulink
794 url="http://developer.intel.com/software/products/opensource/">here.</ulink>
799 The second is yet unreleased, but has been announced on the gcc
800 mailing list. It is yet unspecified if these tools will be freely
801 available, and able to be included in a GNU project. Please contact
802 Mark Mitchell (mark@codesourcery.com) for more details, and current
808 Involves using the vlad.consistency test framework. This has also been
809 discussed on the gcc mailing lists.
813 Testing the C++ library ABI can also be done various ways.
818 (Brendan Kehoe, Jeff Law suggestion to run 'make check-c++' two ways,
819 one with a new compiler and an old library, and the other with an old
820 compiler and a new library, and look for testsuite regressions)
824 Details on how to set this kind of test up can be found here:
825 http://gcc.gnu.org/ml/gcc/2002-08/msg00142.html
830 Use the 'make check-abi' rule in the libstdc++ Makefile.
834 This is a proactive check the library ABI. Currently, exported symbol
835 names that are either weak or defined are checked against a last known
836 good baseline. Currently, this baseline is keyed off of 3.4.0
837 binaries, as this was the last time the .so number was incremented. In
838 addition, all exported names are demangled, and the exported objects
839 are checked to make sure they are the same size as the same object in
842 Notice that each baseline is relative to a <emphasis>default</emphasis>
843 configured library and compiler: in particular, if options such as
844 --enable-clocale, or --with-cpu, in case of multilibs, are used at
845 configure time, the check may fail, either because of substantive
846 differences or because of limitations of the current checking
851 This dataset is insufficient, yet a start. Also needed is a
852 comprehensive check for all user-visible types part of the standard
853 library for sizeof() and alignof() changes.
857 Verifying compatible layouts of objects is not even attempted. It
858 should be possible to use sizeof, alignof, and offsetof to compute
859 offsets for each structure and type in the standard library, saving to
860 another datafile. Then, compute this in a similar way for new
861 binaries, and look for differences.
865 Another approach might be to use the -fdump-class-hierarchy flag to
866 get information. However, currently this approach gives insufficient
867 data for use in library testing, as class data members, their offsets,
868 and other detailed data is not displayed with this flag.
869 (See g++/7470 on how this was used to find bugs.)
873 Perhaps there are other C++ ABI checkers. If so, please notify
874 us. We'd like to know about them!
878 <sect3 id="abi.testing.multi" xreflabel="abi.testing.multi">
879 <title>Multiple ABI Testing</title>
881 A "C" application, dynamically linked to two shared libraries, liba,
882 libb. The dependent library liba is C++ shared library compiled with
883 gcc-3.3.x, and uses io, exceptions, locale, etc. The dependent library
884 libb is a C++ shared library compiled with gcc-3.4.x, and also uses io,
885 exceptions, locale, etc.
888 <para> As above, libone is constructed as follows: </para>
890 %$bld/H-x86-gcc-3.4.0/bin/g++ -fPIC -DPIC -c a.cc
892 %$bld/H-x86-gcc-3.4.0/bin/g++ -shared -Wl,-soname -Wl,libone.so.1 -Wl,-O1 -Wl,-z,defs a.o -o libone.so.1.0.0
894 %ln -s libone.so.1.0.0 libone.so
896 %$bld/H-x86-gcc-3.4.0/bin/g++ -c a.cc
901 <para> And, libtwo is constructed as follows: </para>
904 %$bld/H-x86-gcc-3.3.3/bin/g++ -fPIC -DPIC -c b.cc
906 %$bld/H-x86-gcc-3.3.3/bin/g++ -shared -Wl,-soname -Wl,libtwo.so.1 -Wl,-O1 -Wl,-z,defs b.o -o libtwo.so.1.0.0
908 %ln -s libtwo.so.1.0.0 libtwo.so
910 %$bld/H-x86-gcc-3.3.3/bin/g++ -c b.cc
915 <para> ...with the resulting libraries looking like </para>
920 libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x40016000)
921 libm.so.6 => /lib/tls/libm.so.6 (0x400fa000)
922 libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x4011c000)
923 libc.so.6 => /lib/tls/libc.so.6 (0x40125000)
924 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
927 libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x40027000)
928 libm.so.6 => /lib/tls/libm.so.6 (0x400e1000)
929 libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x40103000)
930 libc.so.6 => /lib/tls/libc.so.6 (0x4010c000)
931 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
936 Then, the "C" compiler is used to compile a source file that uses
937 functions from each library.
940 gcc test.c -g -O2 -L. -lone -ltwo /usr/lib/libstdc++.so.5 /usr/lib/libstdc++.so.6
944 Which gives the expected:
950 libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x00764000)
951 libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x40015000)
952 libc.so.6 => /lib/tls/libc.so.6 (0x0036d000)
953 libm.so.6 => /lib/tls/libm.so.6 (0x004a8000)
954 libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x400e5000)
955 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
960 This resulting binary, when executed, will be able to safely use
961 code from both liba, and the dependent libstdc++.so.6, and libb,
962 with the dependent libstdc++.so.5.
967 <sect2 id="abi.issues" xreflabel="abi.issues">
968 <title>Outstanding Issues</title>
971 Some features in the C++ language make versioning especially
972 difficult. In particular, compiler generated constructs such as
973 implicit instantiations for templates, typeinfo information, and
974 virtual tables all may cause ABI leakage across shared library
975 boundaries. Because of this, mixing C++ ABIs is not recommended at
980 For more background on this issue, see these bugzilla entries:
984 <ulink url="http://gcc.gnu.org/PR24660">24660: versioning weak symbols in libstdc++</ulink>
988 <ulink url="http://gcc.gnu.org/PR19664">19664: libstdc++ headers should have pop/push of the visibility around the declarations</ulink>
993 <bibliography id="abi.biblio" xreflabel="abi.biblio">
994 <title>Bibliography</title>
998 ABIcheck, a vague idea of checking ABI compatibility
1002 <ulink url="http://abicheck.sourceforge.net/">
1013 <ulink url="http://www.codesourcery.com/cxx-abi">
1020 IntelĀ® Compilers for Linux* -Compatibility with the GNU Compilers
1024 <ulink url="http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm">
1031 IntelĀ® Compilers for Linux* -Compatibility with the GNU Compilers
1035 <ulink url="http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm">
1042 Sun Solaris 2.9 : Linker and Libraries Guide (document 816-1386)
1046 <ulink url="http://docs.sun.com/?p=/doc/816-1386&a=load">
1054 Sun Solaris 2.9 : C++ Migration Guide (document 816-2459)
1058 <ulink url="http://docs.sun.com/db/prod/solaris.9">
1065 ELF Symbol Versioning
1069 <firstname>Ulrich</firstname>
1070 <surname>Drepper</surname>
1074 <ulink url="http://people.redhat.com/drepper/symbol-versioning">
1081 C++ ABI for the ARM Architecture
1085 <ulink url="http://www.arm.com/miscPDFs/8033.pdf">
1092 Dynamic Shared Objects: Survey and Issues
1099 <firstname>Benjamin</firstname>
1100 <surname>Kosnik</surname>
1104 <ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1976.html">
1111 Versioning With Namespaces
1118 <firstname>Benjamin</firstname>
1119 <surname>Kosnik</surname>
1123 <ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2013.html">