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         xml:id="manual.intro.setup.configure" xreflabel="Configuring">

      ISO C++

      configure

      options

  When configuring libstdc++, you'll have to configure the entire

  gccsrcdir directory. Consider using the

  toplevel gcc configuration option

  --enable-languages=c++, which saves time by only

  building the C++ toolchain.

  Here are all of the configure options specific to libstdc++.  Keep

  in mind that

   they

   all have opposite forms as well (enable/disable and

   with/without).  The defaults are for the current

   development sources, which may be different than those

   for released versions.

The canonical way to find out the configure options that are

   available for a given set of libstdc++ sources is to go to the

   source directory and then type:./configure --help.

 --enable-multilib[default]

 This is part of the generic multilib support for building cross

        compilers.  As such, targets like "powerpc-elf" will have

        libstdc++ built many different ways:  "-msoft-float"

        and not, etc.  A different libstdc++ will be built for each of

        the different multilib versions.  This option is on by default.

 --enable-sjlj-exceptions

 Forces old, set-jump/long-jump exception handling model.  If

        at all possible, the new, frame unwinding exception handling routines

        should be used instead, as they significantly reduce both

        runtime memory usage and executable size. This option can

        change the library ABI.

 --enable-version-specific-runtime-libs

 Specify that run-time libraries should be installed in the

        compiler-specific subdirectory (i.e.,

        ${libdir}/gcc-lib/${target_alias}/${gcc_version})

        instead of ${libdir}.  This option is useful if you

        intend to use several versions of gcc in parallel.  In addition,

        libstdc++'s include files will be installed in

        ${libdir}/gcc-lib/${target_alias}/${gcc_version}/include/g++,

        unless you also specify

       --with-gxx-include-dir=dirname during configuration.

 --with-gxx-include-dir=<include-files dir>

 Adds support for named libstdc++ include directory.  For instance,

        the following puts all the libstdc++ headers into a directory

        called "4.4-20090404" instead of the usual

        "c++/(version)".

   --with-gxx-include-dir=/foo/H-x86-gcc-3-c-gxx-inc/include/4.4-20090404 

 --enable-cstdio

 This is an abbreviated form of '--enable-cstdio=stdio'

        (described next).

 --enable-cstdio=OPTION

 Select a target-specific I/O package. At the moment, the only

        choice is to use 'stdio', a generic "C" abstraction.

        The default is 'stdio'. This option can change the library ABI.

 --enable-clocale

 This is an abbreviated form of '--enable-clocale=generic'

        (described next).

 --enable-clocale=OPTION

 Select a target-specific underlying locale package.  The

        choices are 'ieee_1003.1-2001' to specify an X/Open, Standard Unix

        (IEEE Std. 1003.1-2001) model based on langinfo/iconv/catgets,

        'gnu' to specify a model based on functionality from the GNU C

        library (langinfo/iconv/gettext) (from glibc, the GNU C

        library), or 'generic' to use a generic "C"

        abstraction which consists of "C" locale info.

     If not explicitly specified, the configure proccess tries

      to guess the most suitable package from the choices above. The

      default is 'generic'. On glibc-based systems of sufficient

      vintage (2.3 and newer), 'gnu' is automatically selected. This option

      can change the library ABI.

 --enable-libstdcxx-allocator

 This is an abbreviated form of

        '--enable-libstdcxx-allocator=auto' (described

        next).

 --enable-libstdcxx-allocator=OPTION  

 Select a target-specific underlying std::allocator.  The

        choices are 'new' to specify a wrapper for new, 'malloc' to

        specify a wrapper for malloc, 'mt' for a fixed power of two allocator,

        'pool' for the SGI pooled allocator or 'bitmap' for a bitmap allocator.

        See this page for more information on allocator

        extensions. This option

        can change the library ABI.

 --enable-cheaders=OPTION

 This allows the user to define the approach taken for C header

        compatibility with C++. Options are c, c_std, and c_global.

        These correspond to the source directory's include/c,

        include/c_std, and include/c_global, and may also include

        include/c_compatibility.  The default is 'c_global'.

 --enable-threads

 This is an abbreviated form of '--enable-threads=yes'

        (described next).

 --enable-threads=OPTION

 Select a threading library.  A full description is

        given in the

        general compiler

        configuration instructions. This option can change the

        library ABI.

 --enable-libstdcxx-threads

 Enable C++11 threads support.  If not explicitly specified,

        the  configure process enables it if possible.  It defaults to 'off'

        on Solaris 8 and 9, where it would break symbol versioning.   This

        option can change the library ABI.

 --enable-libstdcxx-time

 This is an abbreviated form of

        '--enable-libstdcxx-time=yes'(described next).

 --enable-libstdcxx-time=OPTION

 Enables link-type checks for the availability of the

        clock_gettime clocks, used in the implementation of [time.clock],

        and of the nanosleep and sched_yield functions, used in the

        implementation of [thread.thread.this] of the 2011 ISO C++ standard.

        The choice OPTION=yes checks for the availability of the facilities

        in libc and libposix4.  In case of need the latter is also linked

        to libstdc++ as part of the build process.  OPTION=rt also searches

        (and, in case, links) librt.   Note that the latter is not always

        desirable because, in glibc, for example, in turn it triggers the

        linking of libpthread too, which activates locking, a large overhead

        for single-thread programs.  OPTION=no skips the tests completely.

        The default is OPTION=no.

 --enable-libstdcxx-debug

 Build separate debug libraries in addition to what is normally built.

        By default, the debug libraries are compiled with

         CXXFLAGS='-g3 -O0 -fno-inline'

        , are installed in ${libdir}/debug, and have the

        same names and versioning information as the non-debug

        libraries. This option is off by default.

     Note this make command, executed in

        the build directory, will do much the same thing, without the

        configuration difference and without building everything twice:

        make CXXFLAGS='-g3 -O0 -fno-inline' all

 --enable-libstdcxx-debug-flags=FLAGS

 This option is only valid when  --enable-debug 

        is also specified, and applies to the debug builds only. With

        this option, you can pass a specific string of flags to the

        compiler to use when building the debug versions of libstdc++.

        FLAGS is a quoted string of options, like

  --enable-libstdcxx-debug-flags='-g3 -O1 -fno-inline'

 --enable-cxx-flags=FLAGS

 With this option, you can pass a string of -f (functionality)

        flags to the compiler to use when building libstdc++. This

        option can change the library ABI. FLAGS is a quoted string of

        options, like

  --enable-cxx-flags='-fvtable-gc -fomit-frame-pointer -ansi'

        Note that the flags don't necessarily have to all be -f flags,

        as shown, but usually those are the ones that will make sense

        for experimentation and configure-time overriding.

     The advantage of --enable-cxx-flags over setting CXXFLAGS in

        the 'make' environment is that, if files are automatically

        rebuilt, the same flags will be used when compiling those files

        as well, so that everything matches.

     Fun flags to try might include combinations of

  -fstrict-aliasing

  -fno-exceptions

  -ffunction-sections

  -fvtable-gc

     and opposite forms (-fno-) of the same.  Tell us (the libstdc++

        mailing list) if you discover more!

 --enable-c99

 The "long long" type was introduced in C99, along

        with many other functions for wide characters, and math

        classification macros, etc.  If enabled, all C99 functions not

        specified by the C++ standard will be put into namespace

        __gnu_cxx, and then all these names will

        be injected into namespace std, so that C99 functions can be

        used "as if" they were in the C++ standard (as they

        will eventually be in some future revision of the standard,

        without a doubt).  By default, C99 support is on, assuming the

        configure probes find all the necessary functions and bits

        necessary. This option can change the library ABI.

 --enable-wchar_t[default]

 Template specializations for the "wchar_t" type are

        required for wide character conversion support.  Disabling

        wide character specializations may be expedient for initial

        porting efforts, but builds only a subset of what is required by

        ISO, and is not recommended.  By default, this option is on.

        This option can change the library ABI.

 --enable-long-long  

 The "long long" type was introduced in C99.  It is

        provided as a GNU extension to C++98 in g++.  This flag builds

        support for "long long" into the library (specialized

        templates and the like for iostreams).  This option is on by default:

        if enabled, users will have to either use the new-style "C"

        headers by default (i.e., <cmath> not <math.h>)

        or add appropriate compile-time flags to all compile lines to

        allow "C" visibility of this feature (on GNU/Linux,

        the flag is -D_ISOC99_SOURCE, which is added automatically via

        CPLUSPLUS_CPP_SPEC's addition of _GNU_SOURCE).

        This option can change the library ABI.

 --enable-fully-dynamic-string

 This option enables a special version of basic_string avoiding

        the optimization that allocates empty objects in static memory.

        Mostly useful together with shared memory allocators, see PR

        libstdc++/16612 for details.

 --enable-concept-checks

 This turns on additional compile-time checks for instantiated

        library templates, in the form of specialized templates,

        described here.  They

        can help users discover when they break the rules of the STL, before

        their programs run.

 --enable-symvers[=style]

 In 3.1 and later, tries to turn on symbol versioning in the

        shared library (if a shared library has been

        requested). Values for 'style' that are currently supported

        are 'gnu', 'gnu-versioned-namespace', 'darwin',

        'darwin-export', and 'sun'. Both gnu- options require that a recent

        version of the GNU linker be in use. Both darwin options are

        equivalent. With no style given, the configure script will try

        to guess correct defaults for the host system, probe to see if

        additional requirements are necessary and present for

        activation, and if so, will turn symbol versioning on. This

        option can change the library ABI.

 --enable-libstdcxx-visibility

  In 4.2 and later, enables or disables visibility

        attributes. If enabled (as by default), and the compiler seems

        capable of passing the simple sanity checks thrown at it, adjusts

        items in namespace std, namespace std::tr1, namespace std::tr2,

        and namespace __gnu_cxx to have visibility ("default")

        so that -fvisibility options can be used without affecting the

        normal external-visibility of namespace std entities.

        Prior to 4.7 this option was spelled --enable-visibility.

 --enable-libstdcxx-pch

 In 3.4 and later, tries to turn on the generation of

        stdc++.h.gch, a pre-compiled file including all the standard

        C++ includes. If enabled (as by default), and the compiler

        seems capable of passing the simple sanity checks thrown at

        it, try to build stdc++.h.gch as part of the make process.

        In addition, this generated file is used later on (by appending 

        --include bits/stdc++.h  to CXXFLAGS) when running the

        testsuite.

 --enable-extern-template[default]

 Use extern template to pre-instantiate all required

        specializations for certain types defined in the standard libraries.

        These types include string and dependents like

        char_traits, the templateized io classes,

        allocator, and others.

        Disabling means that implicit

        template generation will be used when compiling these types.  By

        default, this option is on. This option can change the library ABI.

 --disable-hosted-libstdcxx

     By default, a complete hosted C++ library is

     built.  The C++ Standard also describes a

     freestanding environment, in which only a

     minimal set of headers are provided.  This option builds such an

     environment.