2 GCC Frequently Asked Questions
4 The latest version of this document is always available at
5 [1]http://gcc.gnu.org/faq.html.
7 This FAQ tries to answer specific questions concerning GCC. For
8 general information regarding C, C++, resp. Fortran please check the
9 [2]comp.lang.c FAQ, [3]comp.std.c++ FAQ, and the [4]Fortran
12 Other GCC-related FAQs: [5]libstdc++-v3, and [6]GCJ.
13 _________________________________________________________________
17 1. [7]General information
18 1. [8]What is the relationship between GCC and EGCS?
19 2. [9]What is an open development model?
20 3. [10]How do I get a bug fixed or a feature added?
21 4. [11]Does GCC work on my platform?
23 1. [13]How to install multiple versions of GCC
24 2. [14]Dynamic linker is unable to find GCC libraries
25 3. [15]libstdc++/libio tests fail badly with --enable-shared
26 4. [16]GCC can not find GNU as/GNU ld
27 5. [17]cpp: Usage:... Error
28 6. [18]Optimizing the compiler itself
29 7. [19]Why does libiconv get linked into jc1 on Solaris?
30 3. [20]Testsuite problems
31 1. [21]How do I pass flags like -fnew-abi to the testsuite?
32 2. [22]How can I run the test suite with multiple options?
33 4. [23]Older versions of GCC
34 1. [24]Is there a stringstream / sstream for GCC 2.95.2?
36 1. [26]Friend Templates
37 2. [27]dynamic_cast, throw, typeid don't work with shared
39 3. [28]Why do I need autoconf, bison, xgettext, automake, etc?
40 4. [29]Why can't I build a shared library?
41 5. [30]When building C++, the linker says my constructors,
42 destructors or virtual tables are undefined, but I defined
44 6. [31]Will GCC someday include an incremental linker?
45 _________________________________________________________________
49 What is the relationship between GCC and EGCS?
51 In 1990/1991 gcc version 1 had reached a point of stability. For the
52 targets it could support, it worked well. It had limitations inherent
53 in its design that would be difficult to resolve, so a major effort
54 was made to resolve those limitations and gcc version 2 was the
57 When we had gcc2 in a useful state, development efforts on gcc1
58 stopped and we all concentrated on making gcc2 better than gcc1 could
59 ever be. This is the kind of step forward we wanted to make with the
60 EGCS project when it was formed in 1997.
62 In April 1999 the Free Software Foundation officially halted
63 development on the gcc2 compiler and appointed the EGCS project as the
64 official GCC maintainers. The net result was a single project which
65 carries forward GCC development under the ultimate control of the
66 [32]GCC Steering Committee.
67 _________________________________________________________________
69 What is an open development model?
71 We are using a bazaar style [33][1] approach to GCC development: we
72 make snapshots publicly available to anyone who wants to try them; we
73 welcome anyone to join the development mailing list. All of the
74 discussions on the development mailing list are available via the web.
75 We're going to be making releases with a much higher frequency than
76 they have been made in the past.
78 In addition to weekly snapshots of the GCC development sources, we
79 have the sources readable from a CVS server by anyone. Furthermore we
80 are using remote CVS to allow remote maintainers write access to the
83 There have been many potential GCC developers who were not able to
84 participate in GCC development in the past. We want these people to
85 help in any way they can; we ultimately want GCC to be the best
86 compiler in the world.
88 A compiler is a complicated piece of software, there will still be
89 strong central maintainers who will reject patches, who will demand
90 documentation of implementations, and who will keep the level of
91 quality as high as it is today. Code that could use wider testing may
92 be integrated--code that is simply ill-conceived won't be.
94 GCC is not the first piece of software to use this open development
95 process; FreeBSD, the Emacs lisp repository, and the Linux kernel are
96 a few examples of the bazaar style of development.
98 With GCC, we are adding new features and optimizations at a rate that
99 has not been done since the creation of gcc2; these additions
100 inevitably have a temporarily destabilizing effect. With the help of
101 developers working together with this bazaar style development, the
102 resulting stability and quality levels will be better than we've had
105 [1] We've been discussing different development models a lot over
106 the past few months. The paper which started all of this introduced
107 two terms: A cathedral development model versus a bazaar
108 development model. The paper is written by Eric S. Raymond, it is
109 called ``The Cathedral and the Bazaar''. The paper is a useful
110 starting point for discussions.
111 _________________________________________________________________
113 How do I get a bug fixed or a feature added?
115 There are lots of ways to get something fixed. The list below may be
116 incomplete, but it covers many of the common cases. These are listed
117 roughly in order of decreasing difficulty for the average GCC user,
118 meaning someone who is not skilled in the internals of GCC, and where
119 difficulty is measured in terms of the time required to fix the bug.
120 No alternative is better than any other; each has its benefits and
122 * Fix it yourself. This alternative will probably bring results, if
123 you work hard enough, but will probably take a lot of time, and,
124 depending on the quality of your work and the perceived benefits
125 of your changes, your code may or may not ever make it into an
126 official release of GCC.
127 * [34]Report the problem to the GCC bug tracking system and hope
128 that someone will be kind enough to fix it for you. While this is
129 certainly possible, and often happens, there is no guarantee that
130 it will. You should not expect the same response from this method
131 that you would see from a commercial support organization since
132 the people who read GCC bug reports, if they choose to help you,
133 will be volunteering their time.
134 * Hire someone to fix it for you. There are various companies and
135 individuals providing support for GCC. This alternative costs
136 money, but is relatively likely to get results.
137 _________________________________________________________________
139 Does GCC work on my platform?
141 The host/target specific installation notes for GCC include
142 information about known problems with installing or using GCC on
143 particular platforms. These are included in the sources for a release
144 in INSTALL/specific.html, and the [35]latest version is always
145 available at the GCC web site. Reports of [36]successful builds for
146 several versions of GCC are also available at the web site.
147 _________________________________________________________________
151 How to install multiple versions of GCC
153 It may be desirable to install multiple versions of the compiler on
154 the same system. This can be done by using different prefix paths at
155 configure time and a few symlinks.
157 Basically, configure the two compilers with different --prefix
158 options, then build and install each compiler. Assume you want "gcc"
159 to be the latest compiler and available in /usr/local/bin; also assume
160 that you want "gcc2" to be the older gcc2 compiler and also available
163 The easiest way to do this is to configure the new GCC with
164 --prefix=/usr/local/gcc and the older gcc2 with
165 --prefix=/usr/local/gcc2. Build and install both compilers. Then make
166 a symlink from /usr/local/bin/gcc to /usr/local/gcc/bin/gcc and from
167 /usr/local/bin/gcc2 to /usr/local/gcc2/bin/gcc. Create similar links
168 for the "g++", "c++" and "g77" compiler drivers.
170 An alternative to using symlinks is to configure with a
171 --program-transform-name option. This option specifies a sed command
172 to process installed program names with. Using it you can, for
173 instance, have all the new GCC programs installed as "new-gcc" and the
174 like. You will still have to specify different --prefix options for
175 new GCC and old GCC, because it is only the executable program names
176 that are transformed. The difference is that you (as administrator) do
177 not have to set up symlinks, but must specify additional directories
178 in your (as a user) PATH. A complication with --program-transform-name
179 is that the sed command invariably contains characters significant to
180 the shell, and these have to be escaped correctly, also it is not
181 possible to use "^" or "$" in the command. Here is the option to
182 prefix "new-" to the new GCC installed programs:
184 --program-transform-name='s,\\\\(.*\\\\),new-\\\\1,'
186 With the above --prefix option, that will install the new GCC programs
187 into /usr/local/gcc/bin with names prefixed by "new-". You can use
188 --program-transform-name if you have multiple versions of GCC, and
189 wish to be sure about which version you are invoking.
191 If you use --prefix, GCC may have difficulty locating a GNU assembler
192 or linker on your system, [37]GCC can not find GNU as/GNU ld explains
193 how to deal with this.
195 Another option that may be easier is to use the --program-prefix= or
196 --program-suffix= options to configure. So if you're installing GCC
197 2.95.2 and don't want to disturb the current version of GCC in
198 /usr/local/bin/, you could do
200 configure --program-suffix=-2.95.2 <other configure options>
202 This should result in GCC being installed as /usr/local/bin/gcc-2.95.2
203 instead of /usr/local/bin/gcc.
204 _________________________________________________________________
206 Dynamic linker is unable to find GCC libraries
208 This problem manifests itself by programs not finding shared libraries
209 they depend on when the programs are started. Note this problem often
210 manifests itself with failures in the libio/libstdc++ tests after
211 configuring with --enable-shared and building GCC.
213 GCC does not specify a runpath so that the dynamic linker can find
214 dynamic libraries at runtime.
216 The short explanation is that if you always pass a -R option to the
217 linker, then your programs become dependent on directories which may
218 be NFS mounted, and programs may hang unnecessarily when an NFS server
221 The problem is not programs that do require the directories; those
222 programs are going to hang no matter what you do. The problem is
223 programs that do not require the directories.
225 SunOS effectively always passed a -R option for every -L option; this
226 was a bad idea, and so it was removed for Solaris. We should not
229 However, if you feel you really need such an option to be passed
230 automatically to the linker, you may add it to the GCC specs file.
231 This file can be found in the same directory that contains cc1 (run
232 gcc -print-prog-name=cc1 to find it). You may add linker flags such as
233 -R or -rpath, depending on platform and linker, to the *link or *lib
236 Another alternative is to install a wrapper script around gcc, g++ or
237 ld that adds the appropriate directory to the environment variable
238 LD_RUN_PATH or equivalent (again, it's platform-dependent).
240 Yet another option, that works on a few platforms, is to hard-code the
241 full pathname of the library into its soname. This can only be
242 accomplished by modifying the appropriate .ml file within
243 libstdc++/config (and also libg++/config, if you are building libg++),
244 so that $(libdir)/ appears just before the library name in -soname or
246 _________________________________________________________________
248 GCC can not find GNU as/GNU ld
250 GCC searches the PATH for an assembler and a loader, but it only does
251 so after searching a directory list hard-coded in the GCC executables.
252 Since, on most platforms, the hard-coded list includes directories in
253 which the system assembler and loader can be found, you may have to
254 take one of the following actions to arrange that GCC uses the GNU
255 versions of those programs.
257 To ensure that GCC finds the GNU assembler (the GNU loader), which are
258 required by [38]some configurations, you should configure these with
259 the same --prefix option as you used for GCC. Then build & install GNU
260 as (GNU ld) and proceed with building GCC.
262 Another alternative is to create links to GNU as and ld in any of the
263 directories printed by the command `gcc -print-search-dirs | grep
264 '^programs:''. The link to `ld' should be named `real-ld' if `ld'
265 already exists. If such links do not exist while you're compiling GCC,
266 you may have to create them in the build directories too, within the
267 gcc directory and in all the gcc/stage* subdirectories.
269 GCC 2.95 allows you to specify the full pathname of the assembler and
270 the linker to use. The configure flags are `--with-as=/path/to/as' and
271 `--with-ld=/path/to/ld'. GCC will try to use these pathnames before
272 looking for `as' or `(real-)ld' in the standard search dirs. If, at
273 configure-time, the specified programs are found to be GNU utilities,
274 `--with-gnu-as' and `--with-gnu-ld' need not be used; these flags will
275 be auto-detected. One drawback of this option is that it won't allow
276 you to override the search path for assembler and linker with
277 command-line options -B/path/ if the specified filenames exist.
278 _________________________________________________________________
282 If you get an error like this when building GCC (particularly when
283 building __mulsi3), then you likely have a problem with your
284 environment variables.
285 cpp: Usage: /usr/lib/gcc-lib/i586-unknown-linux-gnulibc1/2.7.2.3/cpp
286 [switches] input output
288 First look for an explicit '.' in either LIBRARY_PATH or
289 GCC_EXEC_PREFIX from your environment. If you do not find an explicit
290 '.', look for an empty pathname in those variables. Note that ':' at
291 either the start or end of these variables is an implicit '.' and will
294 Also note '::' in these paths will also cause similar problems.
295 _________________________________________________________________
297 Optimizing the compiler itself
299 If you want to test a particular optimization option, it's useful to
300 try bootstrapping the compiler with that option turned on. For
301 example, to test the -fssa option, you could bootstrap like this:
302 make BOOT_CFLAGS="-O2 -fssa" bootstrap
303 _________________________________________________________________
305 Why does libiconv get linked into jc1 on Solaris?
307 The Java front end requires iconv. If the compiler used to bootstrap
308 GCC finds libiconv (because the GNU version of libiconv has been
309 installed in the same prefix as the bootstrap compiler), but the newly
310 built GCC does not find the library (because it will be installed with
311 a different prefix), then a link-time error will occur when building
312 jc1. This problem does not show up so often on platforms that have
313 libiconv in a default location (like /usr/lib) because then both
314 compilers can find a library named libiconv, even though it is a
317 Using --disable-nls at configure-time does not prevent this problem
318 because jc1 uses iconv even in that case. Solutions include
319 temporarily removing the GNU libiconv, copying it to a default
320 location such as /usr/lib/, and using --enable-languages at
321 configure-time to disable Java.
322 _________________________________________________________________
326 How do I pass flags like -fnew-abi to the testsuite?
328 If you invoke runtest directly, you can use the --tool_opts option,
330 runtest --tool_opts "-fnew-abi -fno-honor-std" <other options>
332 Or, if you use make check you can use the make variable RUNTESTFLAGS,
334 make RUNTESTFLAGS="--tool_opts '-fnew-abi -fno-honor-std'" check-g++
335 _________________________________________________________________
337 How can I run the test suite with multiple options?
339 If you invoke runtest directly, you can use the --target_board option,
341 runtest --target_board "unix{-fPIC,-fpic,}" <other options>
343 Or, if you use make check you can use the make variable RUNTESTFLAGS,
345 make RUNTESTFLAGS="--target_board 'unix{-fPIC,-fpic,}'" check-gcc
347 Either of these examples will run the tests three times. Once with
348 -fPIC, once with -fpic, and once with no additional flags.
350 This technique is particularly useful on multilibbed targets.
351 _________________________________________________________________
353 Older versions of GCC and EGCS
355 Is there a stringstream / sstream for GCC 2.95.2?
358 [39]http://gcc.gnu.org/ml/libstdc++/2000-q2/msg00700/sstream.
359 _________________________________________________________________
365 In order to make a specialization of a template function a friend of a
366 (possibly template) class, you must explicitly state that the friend
367 function is a template, by appending angle brackets to its name, and
368 this template function must have been declared already. Here's an
370 template <typename T> class foo {
371 friend void bar(foo<T>);
374 The above declaration declares a non-template function named bar, so
375 it must be explicitly defined for each specialization of foo. A
376 template definition of bar won't do, because it is unrelated with the
377 non-template declaration above. So you'd have to end up writing:
378 void bar(foo<int>) { /* ... */ }
379 void bar(foo<void>) { /* ... */ }
381 If you meant bar to be a template function, you should have
382 forward-declared it as follows. Note that, since the template function
383 declaration refers to the template class, the template class must be
384 forward-declared too:
385 template <typename T>
388 template <typename T>
391 template <typename T>
393 friend void bar<>(foo<T>);
396 template <typename T>
397 void bar(foo<T>) { /* ... */ }
399 In this case, the template argument list could be left empty, because
400 it can be implicitly deduced from the function arguments, but the
401 angle brackets must be present, otherwise the declaration will be
402 taken as a non-template function. Furthermore, in some cases, you may
403 have to explicitly specify the template arguments, to remove
406 An error in the last public comment draft of the ANSI/ISO C++ Standard
407 and the fact that previous releases of GCC would accept such friend
408 declarations as template declarations has led people to believe that
409 the forward declaration was not necessary, but, according to the final
410 version of the Standard, it is.
411 _________________________________________________________________
413 dynamic_cast, throw, typeid don't work with shared libraries
415 The new C++ ABI in the GCC 3.0 series uses address comparisons, rather
416 than string compares, to determine type equality. This leads to better
417 performance. Like other objects that have to be present in the final
418 executable, these std::type_info objects have what is called vague
419 linkage because they are not tightly bound to any one particular
420 translation unit (object file). The compiler has to emit them in any
421 translation unit that requires their presence, and then rely on the
422 linking and loading process to make sure that only one of them is
423 active in the final executable. With static linking all of these
424 symbols are resolved at link time, but with dynamic linking, further
425 resolution occurs at load time. You have to ensure that objects within
426 a shared library are resolved against objects in the executable and
427 other shared libraries.
428 * For a program which is linked against a shared library, no
429 additional precautions are needed.
430 * You cannot create a shared library with the "-Bsymbolic" option,
431 as that prevents the resolution described above.
432 * If you use dlopen to explicitly load code from a shared library,
433 you must do several things. First, export global symbols from the
434 executable by linking it with the "-E" flag (you will have to
435 specify this as "-Wl,-E" if you are invoking the linker in the
436 usual manner from the compiler driver, g++). You must also make
437 the external symbols in the loaded library available for
438 subsequent libraries by providing the RTLD_GLOBAL flag to dlopen.
439 The symbol resolution can be immediate or lazy.
441 Template instantiations are another, user visible, case of objects
442 with vague linkage, which needs similar resolution. If you do not take
443 the above precautions, you may discover that a template instantiation
444 with the same argument list, but instantiated in multiple translation
445 units, has several addresses, depending in which translation unit the
446 address is taken. (This is not an exhaustive list of the kind of
447 objects which have vague linkage and are expected to be resolved
448 during linking & loading.)
450 If you are worried about different objects with the same name
451 colliding during the linking or loading process, then you should use
452 namespaces to disambiguate them. Giving distinct objects with global
453 linkage the same name is a violation of the One Definition Rule (ODR)
456 For more details about the way that GCC implements these and other C++
457 features, please read the [40]ABI specification. Note the
458 std::type_info objects which must be resolved all begin with "_ZTS".
459 Refer to ld's documentation for a description of the "-E" &
461 _________________________________________________________________
463 Why do I need autoconf, bison, xgettext, automake, etc?
465 If you're using diffs up dated from one snapshot to the next, or if
466 you're using the CVS repository, you may need several additional
467 programs to build GCC.
469 These include, but are not necessarily limited to autoconf, automake,
472 This is necessary because neither diff nor cvs keep timestamps
473 correct. This causes problems for generated files as "make" may think
474 those generated files are out of date and try to regenerate them.
476 An easy way to work around this problem is to use the gcc_update
477 script in the contrib subdirectory of GCC, which handles this
478 transparently without requiring installation of any additional tools.
479 (Note: Up to and including GCC 2.95 this script was called egcs_update
482 When building from diffs or CVS or if you modified some sources, you
483 may also need to obtain development versions of some GNU tools, as the
484 production versions do not necessarily handle all features needed to
487 In general, the current versions of these tools from
488 [41]ftp://ftp.gnu.org/gnu/ will work. At present, Autoconf 2.50 is not
489 supported, and you will need to use Autoconf 2.13; work is in progress
490 to fix this problem. Also look at
491 [42]ftp://gcc.gnu.org/pub/gcc/infrastructure/ for any special versions
493 _________________________________________________________________
495 Why can't I build a shared library?
497 When building a shared library you may get an error message from the
498 linker like `assert pure-text failed:' or `DP relative code in file'.
500 This kind of error occurs when you've failed to provide proper flags
501 to gcc when linking the shared library.
503 You can get this error even if all the .o files for the shared library
504 were compiled with the proper PIC option. When building a shared
505 library, gcc will compile additional code to be included in the
506 library. That additional code must also be compiled with the proper
509 Adding the proper PIC option (-fpic or -fPIC) to the link line which
510 creates the shared library will fix this problem on targets that
511 support PIC in this manner. For example:
512 gcc -c -fPIC myfile.c
513 gcc -shared -o libmyfile.so -fPIC myfile.o
514 _________________________________________________________________
516 When building C++, the linker says my constructors, destructors or virtual
517 tables are undefined, but I defined them
519 The ISO C++ Standard specifies that all virtual methods of a class
520 that are not pure-virtual must be defined, but does not require any
521 diagnostic for violations of this rule [class.virtual]/8. Based on
522 this assumption, GCC will only emit the implicitly defined
523 constructors, the assignment operator, the destructor and the virtual
524 table of a class in the translation unit that defines its first such
527 Therefore, if you fail to define this particular method, the linker
528 may complain about the lack of definitions for apparently unrelated
529 symbols. Unfortunately, in order to improve this error message, it
530 might be necessary to change the linker, and this can't always be
533 The solution is to ensure that all virtual methods that are not pure
534 are defined. Note that a destructor must be defined even if it is
535 declared pure-virtual [class.dtor]/7.
536 _________________________________________________________________
538 Will GCC someday include an incremental linker?
540 Incremental linking is part of the linker, not the compiler. As such,
541 GCC doesn't have anything to do with incremental linking. Depending on
542 what platform you use, it may be possible to tell GCC to use the
543 platform's native linker (e.g., Solaris' ild(1)).
547 1. http://gcc.gnu.org/faq.html
548 2. http://www.eskimo.com/~scs/C-faq/top.html
549 3. http://www.jamesd.demon.co.uk/csc/faq.html
550 4. http://www.fortran.com/fortran/info.html
551 5. http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html
552 6. http://gcc.gnu.org/java/faq.html
553 7. http://gcc.gnu.org/faq.html#general
554 8. http://gcc.gnu.org/faq.html#gcc
555 9. http://gcc.gnu.org/faq.html#open-development
556 10. http://gcc.gnu.org/faq.html#support
557 11. http://gcc.gnu.org/faq.html#platforms
558 12. http://gcc.gnu.org/faq.html#installation
559 13. http://gcc.gnu.org/faq.html#multiple
560 14. http://gcc.gnu.org/faq.html#rpath
561 15. http://gcc.gnu.org/faq.html#rpath
562 16. http://gcc.gnu.org/faq.html#gas
563 17. http://gcc.gnu.org/faq.html#environ
564 18. http://gcc.gnu.org/faq.html#optimizing
565 19. http://gcc.gnu.org/faq.html#iconv
566 20. http://gcc.gnu.org/faq.html#testsuite
567 21. http://gcc.gnu.org/faq.html#testoptions
568 22. http://gcc.gnu.org/faq.html#multipletests
569 23. http://gcc.gnu.org/faq.html#old
570 24. http://gcc.gnu.org/faq.html#2.95sstream
571 25. http://gcc.gnu.org/faq.html#misc
572 26. http://gcc.gnu.org/faq.html#friend
573 27. http://gcc.gnu.org/faq.html#dso
574 28. http://gcc.gnu.org/faq.html#generated_files
575 29. http://gcc.gnu.org/faq.html#picflag-needed
576 30. http://gcc.gnu.org/faq.html#vtables
577 31. http://gcc.gnu.org/faq.html#incremental
578 32. http://gcc.gnu.org/steering.html
579 33. http://gcc.gnu.org/faq.html#cathedral-vs-bazaar
580 34. http://gcc.gnu.org/bugs.html
581 35. http://gcc.gnu.org/install/specific.html
582 36. http://gcc.gnu.org/buildstat.html
583 37. http://gcc.gnu.org/faq.html#gas
584 38. http://gcc.gnu.org/install/specific.html
585 39. http://gcc.gnu.org/ml/libstdc++/2000-q2/msg00700/sstream
586 40. http://www.codesourcery.com/cxx-abi/
587 41. ftp://ftp.gnu.org/gnu/
588 42. ftp://gcc.gnu.org/pub/gcc/infrastructure/