# GCC Frequently Asked Questions

The latest version of this document is always available at http://gcc.gnu.org/faq.html.

This FAQ tries to answer specific questions concerning GCC. For general information regarding C, C++, resp. Fortran please check the comp.lang.c FAQ, comp.lang.c++ FAQ, comp.std.c++ FAQ, and the Fortran Information page.

Other GCC-related FAQs: libstdc++-v3, and GCJ.

# Questions

1. General information
   1. What is the relationship between GCC and EGCS?
   2. What is the relationship between GCC and Cygnus / Red Hat?
   3. What is an open development model?
   4. How do I report a bug?
   5. How do I get a bug fixed or a feature added?
   6. How can I get a development environment for GCC without using CVS?
2. Installation
   1. Problems building the Fortran compiler
   2. How to install multiple versions of GCC
   3. Dynamic linker is unable to find GCC libraries
   4. libstdc++/libio tests fail badly with --enable-shared
   5. GCC can not find GNU as/GNU ld
   6. cpp: Usage:... Error
   7. Optimizing the compiler itself
3. Testsuite problems
   1. Unable to run the testsuite
   2. How do I pass flags like `-fnew-abi` to the testsuite?
   3. How can I run the test suite with multiple options?
4. Bugs and Non-Bugs
   1. Why can't I initialize a static variable with `stdin`?
   2. Why can't I use #if here?
   3. Problems with floating point computations
5. Older versions of GCC or EGCS

1. Why is there no testsuite in GCC 2.95?
2. Is there a stringstream / sstream for gcc 2.95.2?

6. Miscellaneous
   1. Virtual memory exhausted
   2. Friend Templates
   3. Why do I need autoconf, bison, xgettext, automake, etc?
   4. Problems debugging GCC code
   5. Using GCC with GNAT/Ada
   6. Why can't I build a shared library?
   7. How to work around too long C++ symbol names? (`-fsquangle`)
   8. When building C++, the linker says my constructors, destructors or virtual tables are undefined, but I defined them
   9. What is libstdc++-v3 and how can I use it with g++?
   10. Will GCC someday include an incremental linker?

# General information

## What is the relationship between GCC and EGCS?

In 1990/1991 gcc version 1 had reached a point of stability. For the targets it could support, it worked well. It had limitations inherent in its design that would be difficult to resolve, so a major effort was made to resolve those limitiations and gcc version 2 was the result.

When we had gcc2 in a useful state, development efforts on gcc1 stopped and we all concentrated on making gcc2 better than gcc1 could ever be. This is the kind of step forward we wanted to make with the EGCS project when it was formed in 1997.

In April 1999 the Free Software Foundation officially halted development on the gcc2 compiler and appointed the EGCS project as the official GCC maintainers. The net result was a single project which carries forward GCC development under the ultimate control of the GCC Steering Committee.

## What is the relationship between GCC and Cygnus / Red Hat?

It is a common mis-conception that Red Hat controls GCC either directly or indirectly.

While Red Hat does donate hardware, network connections, code and developer time to GCC development, Red Hat does not control GCC.

Overall control of GCC is in the hands of the GCC Steering Committee which includes people from a variety of different organizations and backgrounds. The purpose of the steering committee is to make decisions in the best interest of GCC and to help ensure that no individual or company has control over the project.

To summarize, Red Hat contributes to the GCC project, but does not exert a controlling influence over GCC.

## What is an open development model?

We are using a bazaar style [1] approach to GCC development: we make snapshots publicly available to anyone who wants to try them; we welcome anyone to join the development mailing list. All of the discussions on the development mailing list are available via the web. We're going to be making releases with a much higher frequency than they have been made in the past.

In addition to weekly snapshots of the GCC development sources, we have the sources readable from a CVS server by anyone. Furthermore we are using remote CVS to allow remote maintainers write access to the sources.

There have been many potential GCC developers who were not able to participate in GCC development in the past. We want these people to help in any way they can; we ultimately want GCC to be the best compiler in the world.

A compiler is a complicated piece of software, there will still be strong central maintainers who will reject patches, who will demand documentation of implementations, and who will keep the level of quality as high as it is today. Code that could use wider testing may be integrated--code that is simply ill-conceived won't be.

GCC is not the first piece of software to use this open development process; FreeBSD, the Emacs lisp repository, and the Linux kernel are a few examples of the bazaar style of development.

With GCC, we are adding new features and optimizations at a rate that has not been done since the creation of gcc2; these additions inevitably have a temporarily destabilizing effect. With the help of developers working together with this bazaar style development, the resulting stability and quality levels will be better than we've had before.

> [1] We've been discussing different development models a lot over the past few months. The paper which started all of this introduced two terms: A cathedral development model versus a bazaar development model. The paper is written by Eric S. Raymond, it is called ``The Cathedral and the Bazaar''. The paper is a useful starting point for discussions.

## How do I report a bug?

There are complete instructions here.

## How do I get a bug fixed or a feature added?

There are lots of ways to get something fixed. The list below may be incomplete, but it covers many of the common cases. These are listed roughly in order of increasing difficulty for the average GCC user, meaning someone who is not skilled in the internals of GCC, and where difficulty is measured in terms of the time required to fix the bug. No alternative is better than any other; each has its benefits and disadvantages.

- Hire someone to fix it for you. There are various companies and individuals providing support for GCC. This alternative costs money, but is relatively likely to get results.
- Report the problem to gcc-bugs and hope that someone will be kind enough to fix it for you. While this is certainly possible, and often happens, there is no guarantee that it will. You should not expect the same response from gcc-bugs that you would see from a commercial support organization since the people who read gcc-bugs, if they choose to help you, will be volunteering their time. This alternative will work best if you follow the directions on submitting bugreports.
- Fix it yourself. This alternative will probably bring results, if you work hard enough, but will probably take a lot of time, and, depending on the quality of your work and the perceived benefits of your changes, your code may or may not ever make it into an official release of GCC.

## How can I get a development environment for GCC without using CVS?

GCC snapshots are available from the FTP server and its mirrors; see the GCC mirror list.

# Installation

## Problems building the Fortran compiler

The Fortran front end can not be built with most vendor compilers; it must be built with GCC. As a result, you may get an error if you do not follow the install instructions carefully.

In particular, instead of using "make" to build GCC, you should use "make bootstrap" if you are building a native compiler or "make cross" if you are building a cross compiler.

It has also been reported that the Fortran compiler can not be built on Red Hat 4.X GNU/Linux for the Alpha. Fixing this may require upgrading binutils or to Red Hat 5.0; we'll provide more information as it becomes available.

## How to install multiple versions of GCC

It may be desirable to install multiple versions of the compiler on the same system. This can be done by using different prefix paths at configure time and a few symlinks.

Basically, configure the two compilers with different --prefix options, then build and install each compiler. Assume you want "gcc" to be the latest compiler and available in /usr/local/bin; also assume that you want "gcc2" to be the older gcc2 compiler and also available in /usr/local/bin.

The easiest way to do this is to configure the new GCC with --prefix=/usr/local/gcc and the older gcc2 with --prefix=/usr/local/gcc2. Build and install both compilers. Then make a symlink from /usr/local/bin/gcc to /usr/local/gcc/bin/gcc and from /usr/local/bin/gcc2 to /usr/local/gcc2/bin/gcc. Create similar links for the "g++", "c++" and "g77" compiler drivers.

An alternative to using symlinks is to configure with a --program-transform-name option. This option specifies a sed command to process installed program names with. Using it you can, for instance, have all the new GCC programs installed as "new-gcc" and the like. You will still have to specify different --prefix options for new GCC and old GCC, because it is only the executable program names that are transformed. The difference is that you (as administrator) do not have to set up symlinks, but must specify additional directories in your (as a user) PATH. A complication with --program-transform-name is that the sed command invariably contains characters significant to the shell, and these have to be escaped correctly, also it is not possible to use "^" or "$" in the command. Here is the option to prefix "new-" to the new GCC installed programs "--program-transform-name='s,\\\\(.*\\\\),new-\\\\1,'". With the above --prefix option, that will install the new GCC programs into /usr/local/gcc/bin with names prefixed by "new-". You can use --program-transform-name if you have multiple versions of GCC, and wish to be sure about which version you are invoking.

If you use --prefix, GCC may have difficulty locating a GNU assembler or linker on your system, GCC can not find GNU as/GNU ld explains how to deal with this.

Another option that may be easier is to use the --program-prefix= or --program-suffix= options to configure. So if you're installing GCC 2.95.2 and don't want to disturb the current version of GCC in /usr/local/bin/, you could do 'configure --program-suffix=-2.95.2 '. This should result in GCC being installed as /usr/local/bin/gcc-2.95.2 instead of /usr/local/bin/gcc.

## Dynamic linker is unable to find GCC libraries

This problem manifests itself by programs not finding shared libraries they depend on when the programs are started. Note this problem often manifests itself with failures in the libio/libstdc++ tests after configuring with --enable-shared and building GCC.

GCC does not specify a runpath so that the dynamic linker can find dynamic libraries at runtime.

The short explanation is that if you always pass a -R option to the linker, then your programs become dependent on directories which may be NFS mounted, and programs may hang unnecessarily when an NFS server goes down.

The problem is not programs that do require the directories; those programs are going to hang no matter what you do. The problem is programs that do not require the directories.

SunOS effectively always passed a -R option for every -L option; this was a bad idea, and so it was removed for Solaris. We should not recreate it.

However, if you feel you really need such an option to be passed automatically to the linker, you may add it to the GCC specs file. This file can be found in the same directory that contains cc1 (run `gcc -print-prog-name=cc1` to find it). You may add linker flags such as `-R` or `-rpath`, depending on platform and linker, to the `*link` or `*lib` specs.

Another alternative is to install a wrapper script around gcc, g++ or ld that adds the appropriate directory to the environment variable `LD_RUN_PATH` or equivalent (again, it's platform-dependent).

Yet another option, that works on a few platforms, is to hard-code the full pathname of the library into its soname. This can only be accomplished by modifying the appropriate `.ml` file within `libstdc++/config` (and also `libg++/config`, if you are building libg++), so that `$(libdir)/` appears just before the library name in `-soname` or `-h` options.

## GCC can not find GNU as/GNU ld

GCC searches the PATH for an assembler and a loader, but it only does so after searching a directory list hard-coded in the GCC executables. Since, on most platforms, the hard-coded list includes directories in which the system asembler and loader can be found, you may have to take one of the following actions to arrange that GCC uses the GNU versions of those programs.

To ensure that GCC finds the GNU assembler (the GNU loader), which are required by some configurations, you should configure these with the same --prefix option as you used for GCC. Then build & install GNU as (GNU ld) and proceed with building GCC.

Another alternative is to create links to GNU as and ld in any of the directories printed by the command ``gcc -print-search-dirs | grep '^programs:'`'. The link to ``ld`' should be named ``real-ld`' if ``ld`' already exists. If such links do not exist while you're compiling GCC, you may have to create them in the build directories too, within the `gcc` directory and in all the `gcc/stage*` subdirectories.

GCC 2.95 allows you to specify the full pathname of the assembler and the linker to use. The configure flags are ``--with-as=/path/to/as`' and ``--with-ld=/path/to/ld`'. GCC will try to use these pathnames before looking for ``as`' or ``(real-)ld`' in the standard search dirs. If, at configure-time, the specified programs are found to be GNU utilities, ``--with-gnu-as`' and ``--with-gnu-ld`' need not be used; these flags will be auto-detected. One drawback of this option is that it won't allow you to override the search path for assembler and linker with command-line options `-B/path/` if the specified filenames exist.

## cpp: Usage:... Error

If you get an error like this when building GCC (particularly when building __mulsi3), then you likely have a problem with your environment variables.

      cpp: Usage: /usr/lib/gcc-lib/i586-unknown-linux-gnulibc1/2.7.2.3/cpp
      [switches] input output

First look for an explicit '.' in either LIBRARY_PATH or GCC_EXEC_PREFIX from your environment. If you do not find an explicit '.', look for an empty pathname in those variables. Note that ':' at either the start or end of these variables is an implicit '.' and will cause problems.

Also note '::' in these paths will also cause similar problems.

## Optimizing the compiler itself

If you want to test a particular optimization option, it's useful to try bootstrapping the compiler with that option turned on. For example, to test the `-fssa` option, you could bootstrap like this:

    make BOOT_CFLAGS="-O2 -fssa" bootstrap

# Testsuite problems

## Unable to run the testsuite

If you get a message about unable to find "standard.exp" when trying to run the GCC testsuites, then your dejagnu is too old to run the GCC tests. You will need to get a newer version of dejagnu from http://www.gnu.org/software/dejagnu/dejagnu.html.

## How do I pass flags like `-fnew-abi` to the testsuite?

If you invoke `runtest` directly, you can use the `--tool_opts` option, e.g:

      runtest --tool_opts "-fnew-abi -fno-honor-std" <other options>

Or, if you use `make check` you can use the `make` variable `RUNTESTFLAGS`, e.g:

      make RUNTESTFLAGS="--tool_opts '-fnew-abi -fno-honor-std'" check-g++

## How can I run the test suite with multiple options?

If you invoke `runtest` directly, you can use the `--target_board` option, e.g:

      runtest --target_board "unix{-fPIC,-fpic,}" <other options>

Or, if you use `make check` you can use the `make` variable `RUNTESTFLAGS`, e.g:

      make RUNTESTFLAGS="--target_board 'unix{-fPIC,-fpic,}'" check-gcc

Either of these examples will run the tests three times. Once with `-fPIC`, once with `-fpic`, and once with no additional flags.

This technique is particularly useful on multilibbed targets.

# Bugs and Non-Bugs

Unfortunately, improvements in tools that are widely used are sooner or later bound to break something. Sometimes, the code that breaks was wrong, and then that code should be fixed, even if it works for earlier versions of GCC or other compilers. The following problems with some releases of widely used packages have been identified:

There is a separate list of well-known bugs describing known deficiencies. Naturally we'd like that list to be of zero length.

To report a bug, see How to report bugs.

## Why can't I initialize a static variable with `stdin`?

This has nothing to do with GCC, but people ask us about it a lot. Code like this:

      #include <stdio.h>

      FILE *yyin = stdin;

will not compile with GNU libc (Linux libc6), because `stdin` is not a constant. This was done deliberately, in order for there to be no limit on the number of open FILE objects. It is surprising for people used to traditional Unix C libraries, but it is permitted by the C standard.

This construct commonly occurs in code generated by old versions of lex or yacc. We suggest you try regenerating the parser with a current version of flex or bison, respectively. In your own code, the appropriate fix is to move the initialization to the beginning of main.

There is a common misconception that the GCC developers are responsible for GNU libc. These are in fact two entirely separate projects. The appropriate place to ask questions relating to GNU libc is libc-alpha@sources.redhat.com.

## Why can't I use #if here?

Let me guess... you wrote code that looks something like this:

      memcpy(dest, src,
    #ifdef PLATFORM1
    	 12
    #else
    	 24
    #endif
    	);

and you got a whole pile of error messages:

    test.c:11: warning: preprocessing directive not recognized within macro arg
    test.c:11: warning: preprocessing directive not recognized within macro arg
    test.c:11: warning: preprocessing directive not recognized within macro arg
    test.c: In function `foo':
    test.c:6: undefined or invalid # directive
    test.c:8: undefined or invalid # directive
    test.c:9: parse error before `24'
    test.c:10: undefined or invalid # directive
    test.c:11: parse error before `#'

The problem, simply put, is that GCC's preprocessor does not allow you to put #ifdef (or any other directive) inside the arguments of a macro. Your C library's `string.h` happens to define `memcpy` as a macro - this is perfectly legitimate. The code therefore will not compile.

We have two good reasons for not allowing directives inside macro arguments. First, it is not portable. It is "undefined behavior" according to the C standard; that means different compilers will do different things with it. Some will give you errors. Some will dump core. Some will silently mangle your code - you could get the equivalent of

    	memcpy(dest, src, 1224);

from the above example. A very few might do what you expected it to. We therefore feel it is most useful for GCC to reject this construct immediately so that it is found and fixed.

Second, it is extraordinarily difficult to implement the preprocessor such that it does what you would expect for every possible directive found inside a macro argument. The best example is perhaps

    #define foo(arg) ... arg ...
    foo(blah
    #undef foo
    blah)

which is impossible to implement in portable C without leaking memory. Allowing only a subset of directives would be confusing.

It is always possible to rewrite code which uses conditionals inside macros so that it doesn't. You could write the above example

    #ifdef PLATFORM1
       memcpy(dest, src, 12);
    #else
       memcpy(dest, src, 24);
    #endif

This is a bit more typing, but I personally think it's better style in addition to being more portable.

In recent versions of glibc, `printf` is among the functions which are implemented as macros.

## Problems with floating point computations

In a number of cases, GCC appears to perform floating point computations incorrectly. For example, the program

    #include <iostream>

    int main() {

     double min = 0.0;
     double max = 0.5;
     double width = 0.01;
     std::cout <<  (int)(((max - min) / width) - 1) << std::endl;

    }

might print 50 on some systems and optimization levels, and 51 on others.

The is the result of rounding: The computer cannot represent all real numbers exactly, so it has to use approximations. When computing with approximation, the computer needs to round to the nearest representable number.

This is not a bug in the compiler, but an inherent limitation of the float and double types. Please study this paper for more information.

# Older versions of GCC and EGCS

## Why is there no testsuite in GCC 2.95?

The GCC testsuite is not included in the GCC 2.95 release due to the uncertain copyright status of some tests.

The GCC team has reviewed the entire testsuite to find and remove any tests with uncertain copyright status, following guidelines from Prof. Eben Moglen. The testsuite is included in GCC 3.0 and subsequent releases. Only a few tests needed to be removed from the testsuite.

## Is there a stringstream / sstream for gcc 2.95.2?

Yes, it's at: http://gcc.gnu.org/ml/libstdc++/2000-q2/msg00700/sstream.

# Miscellaneous

## Virtual memory exhausted error

This error means your system ran out of memory; this can happen for large files, particularly when optimizing. If you're getting this error you should consider trying to simplify your files or reducing the optimization level.

Note that using -pedantic or -Wreturn-type can cause an explosion in the amount of memory needed for template-heavy C++ code, such as code that uses STL. Also note that -Wall includes -Wreturn-type, so if you use -Wall you will need to specify -Wno-return-type to turn it off.

## Friend Templates

In order to make a specialization of a template function a friend of a (possibly template) class, you must explicitly state that the friend function is a template, by appending angle brackets to its name, and this template function must have been declared already. Here's an example:

    template <typename T> class foo {
      friend void bar(foo<T>);
    }

The above declaration declares a non-template function named `bar`, so it must be explicitly defined for each specialization of `foo`. A template definition of `bar` won't do, because it is unrelated with the non-template declaration above. So you'd have to end up writing:

    void bar(foo<int>) { /* ... */ }
    void bar(foo<void>) { /* ... */ }

If you meant `bar` to be a template function, you should have forward-declared it as follows. Note that, since the template function declaration refers to the template class, the template class must be forward-declared too:

    template <typename T>
    class foo;

    template <typename T>
    void bar(foo<T>);

    template <typename T>
    class foo {
      friend void bar<>(foo<T>);
    };

    template <typename T>
    void bar(foo<T>) { /* ... */ }

In this case, the template argument list could be left empty, because it can be implicitly deduced from the function arguments, but the angle brackets must be present, otherwise the declaration will be taken as a non-template function. Furthermore, in some cases, you may have to explicitly specify the template arguments, to remove ambiguity.

An error in the last public comment draft of the ANSI/ISO C++ Standard and the fact that previous releases of GCC would accept such friend declarations as template declarations has led people to believe that the forward declaration was not necessary, but, according to the final version of the Standard, it is.

## Why do I need autoconf, bison, xgettext, automake, etc?

If you're using diffs up dated from one snapshot to the next, or if you're using the CVS repository, you may need several additional programs to build GCC.

These include, but are not necessarily limited to autoconf, automake, bison, and xgettext.

This is necessary because neither diff nor cvs keep timestamps correct. This causes problems for generated files as "make" may think those generated files are out of date and try to regenerate them.

An easy way to work around this problem is to use the `gcc_update` script in the contrib subdirectory of GCC, which handles this transparently without requiring installation of any additional tools. (Note: Up to and including GCC 2.95 this script was called `egcs_update` .)

When building from diffs or CVS or if you modified some sources, you may also need to obtain development versions of some GNU tools, as the production versions do not necessarily handle all features needed to rebuild GCC.

In general, the current versions of these tools from ftp://ftp.gnu.org/gnu/ will work. At present, Autoconf 2.50 is not supported, and you will need to use Autoconf 2.13; work is in progress to fix this problem. Also look at ftp://gcc.gnu.org/pub/gcc/infrastructure/ for any special versions of packages.

## Problems debugging GCC code

On some systems GCC will produce dwarf debug records by default; however the gdb-4.16 release may not be able to read such debug records.

You can either use the argument "-gstabs" instead of "-g" or pick up a copy of gdb-4.17 to work around the problem.

## Using GCC with GNAT/Ada

The GNU Ada front-end is not currently supported by GCC, but work is in progress to integrate GNU Ada into the GCC CVS repository and produce new releases based on current versions of GCC.

## Why can't I build a shared library?

When building a shared library you may get an error message from the linker like `assert pure-text failed:' or `DP relative code in file'.

This kind of error occurs when you've failed to provide proper flags to gcc when linking the shared library.

You can get this error even if all the .o files for the shared library were compiled with the proper PIC option. When building a shared library, gcc will compile additional code to be included in the library. That additional code must also be compiled with the proper PIC option.

Adding the proper PIC option (`-fpic` or `-fPIC`) to the link line which creates the shared library will fix this problem on targets that support PIC in this manner. For example:

    	gcc -c -fPIC myfile.c
    	gcc -shared -o libmyfile.so -fPIC myfile.o

## How to work around too long C++ symbol names (`-fsquangle`)

This question does not apply to GCC 3.0 or later versions, which have a new C++ ABI with much shorter mangled names.

If the standard assembler of your platform can't cope with the large symbol names that the default g++ name mangling mechanism produces, your best bet is to use GNU as, from the GNU binutils package.

Unfortunately, GNU as does not support all platforms supported by GCC, so you may have to use an experimental work-around: the `-fsquangle` option, that enables compression of symbol names.

Note that this option is still under development, and subject to change. Since it modifies the name mangling mechanism, you'll need to build libstdc++ and any other C++ libraries with this option enabled. Furthermore, if this option changes its behavior in the future, you'll have to rebuild them all again. :-(

This option can be enabled by default by initializing `flag_do_squangling' with `1' in `gcc/cp/decl2.c' (it is not initialized by default), then rebuilding GCC and any C++ libraries.

## When building C++, the linker says my constructors, destructors or virtual tables are undefined, but I defined them

The ISO C++ Standard specifies that all virtual methods of a class that are not pure-virtual must be defined, but does not require any diagnostic for violations of this rule [class.virtual]/8. Based on this assumption, GCC will only emit the implicitly defined constructors, the assignment operator, the destructor and the virtual table of a class in the translation unit that defines its first such non-inline method.

Therefore, if you fail to define this particular method, the linker may complain about the lack of definitions for apparently unrelated symbols. Unfortunately, in order to improve this error message, it might be necessary to change the linker, and this can't always be done.

The solution is to ensure that all virtual methods that are not pure are defined. Note that a destructor must be defined even if it is declared pure-virtual [class.dtor]/7.

## What is libstdc++-v3 and how can I use it with g++?

From the libstdc++-v3 FAQ: "The GNU Standard C++ Library v3, or libstdc++-2.9x, is an ongoing project to implement the ISO 14882 Standard C++ library as described in chapters 17 through 27 and annex D."

libstdc++-v3 is enabled by default in 3.x releases of GCC.

## Will GCC someday include an incremental linker?

Incremental linking is part of the linker, not the compiler. As such, GCC doesn't have anything to do with incremental linking. Depending on what platform you use, it may be possible to tell GCC to use the platform's native linker (e.g., Solaris' ild(1)).