Keywords: ISO C++, configure, options

Configure
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## Configure

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-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), 'generic' to use a generic "C" abstraction which consists of "C" locale info, 'newlib' to specify the Newlib C library model which only differs from the 'generic' model in the handling of ctype, or 'darwin' which omits the wchar_t specializations needed by the 'generic' model.

  If not explicitly specified, the configure process 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. On newlib-based systems (`'--with_newlib=yes'`) and OpenBSD, 'newlib' is automatically selected. On Mac OS X 'darwin' 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. 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 it's needed the latter is also linked to libstdc++ as part of the build process. OPTION=rt also checks in librt (and, if it's needed, links to it). Note that linking to librt is not always desirable because for glibc it requires linking to libpthread too, which causes all reference counting to use atomic operations, resulting in a potentially large overhead for single-threaded programs. OPTION=no skips the tests completely. The default is OPTION=auto, which skips the checks and enables the features only for targets known to support them. For Linux targets, if `clock_gettime` is not used then the [time.clock] implementation will use a system call to access the realtime and monotonic clocks, which is significantly slower than the C library's `clock_gettime` function.

`--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 in the Concept Checking section. They can help users discover when they break the rules of the STL, before their programs run. These checks are based on C++03 rules and some of them are not compatible with correct C++11 code.

`--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 templatized 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.

`--disable-libstdcxx-verbose`

  By default, the library is configured to write descriptive messages to standard error for certain events such as calling a pure virtual function or the invocation of the standard terminate handler. Those messages cause the library to depend on the demangler and standard I/O facilities, which might be undesirable in a low-memory environment or when standard error is not available. This option disables those messages. This option does not change the library ABI.

`--disable-libstdcxx-dual-abi`

  Disable support for the new, C++11-conforming implementations of `std::string`, `std::list` etc. so that the library only provides definitions of types using the old ABI (see Dual ABI). This option changes the library ABI.

`--with-default-libstdcxx-abi=``OPTION`

  Set the default value for the _GLIBCXX_USE_CXX11_ABI macro (see Macros). The default is `OPTION=new` which sets the macro to `1`, use `OPTION=gcc4-compatible` to set it to `0`. This option does not change the library ABI.

`--with-libstdcxx-lock-policy=OPTION`

  Sets the lock policy that controls how `shared_ptr` reference counting is synchronized. The choice OPTION=atomic enables use of atomics for updates to `shared_ptr` reference counts. The choice OPTION=mutex enables use of a mutex to synchronize updates to `shared_ptr` reference counts. If the compiler's thread model is "single" then this option has no effect, as no synchronization is used for the reference counts. The default is OPTION=auto, which checks for the availability of compiler built-ins for 2-byte and 4-byte atomic compare-and-swap, and uses OPTION=atomic if they're available, OPTION=mutex otherwise. This option can change the library ABI. If the library is configured to use atomics and user programs are compiled using a target that doesn't natively support the atomic operations (e.g. the library is configured for armv7 and then code is compiled with `-march=armv5t`) then the program might rely on support in libgcc to provide the atomics.

`--enable-vtable-verify`[default]

  Use `-fvtable-verify=std` to compile the C++ runtime with instrumentation for vtable verification. All virtual functions in the standard library will be verified at runtime. Types impacted include `locale` and `iostream`, and others. Disabling means that the C++ runtime is compiled without support for vtable verification. By default, this option is off.

`--enable-libstdcxx-filesystem-ts`[default]

  Build `libstdc++fs.a` as well as the usual libstdc++ and libsupc++ libraries. This is enabled by default on select POSIX targets where it is known to work and disabled otherwise.

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