# Using

## Command Options

The set of features available in the GNU C++ library is shaped by several GCC Command Options. Options that impact libstdc++ are enumerated and detailed in the table below.

By default, `g++` is equivalent to `g++ -std=gnu++98`. The standard library also defaults to this dialect.

C++ Command Options

Option Flags | Description
--- | ---
`-std=c++98` | Use the 1998 ISO C++ standard plus amendments.
`-std=gnu++98` | As directly above, with GNU extensions.
`-std=c++0x` | Use the working draft of the upcoming ISO C++0x standard.
`-std=gnu++0x` | As directly above, with GNU extensions.
`-fexceptions` | See exception-free dialect
`-frtti` | As above, but RTTI-free dialect.
`-pthread` or `-pthreads` | For ISO C++0x <thread>, <future>, <mutex>, or <condition_variable>.
`-fopenmp` | For parallel mode.

## Headers

### Header Files

The C++ standard specifies the entire set of header files that must be available to all hosted implementations. Actually, the word "files" is a misnomer, since the contents of the headers don't necessarily have to be in any kind of external file. The only rule is that when one `#include`'s a header, the contents of that header become available, no matter how.

That said, in practice files are used.

There are two main types of include files: header files related to a specific version of the ISO C++ standard (called Standard Headers), and all others (TR1, C++ ABI, and Extensions).

Two dialects of standard headers are supported, corresponding to the 1998 standard as updated for 2003, and the draft of the upcoming 200x standard.

C++98/03 include files. These are available in the default compilation mode, i.e. `-std=c++98` or `-std=gnu++98`.

C++ 1998 Library Headers

`algorithm` | `bitset` | `complex` | `deque` | `exception`
`fstream` | `functional` | `iomanip` | `ios` | `iosfwd`
`iostream` | `istream` | `iterator` | `limits` | `list`
`locale` | `map` | `memory` | `new` | `numeric`
`ostream` | `queue` | `set` | `sstream` | `stack`
`stdexcept` | `streambuf` | `string` | `utility` | `typeinfo`
`valarray` | `vector`

C++ 1998 Library Headers for C Library Facilities

`cassert` | `cerrno` | `cctype` | `cfloat` | `ciso646`
`climits` | `clocale` | `cmath` | `csetjmp` | `csignal`
`cstdarg` | `cstddef` | `cstdio` | `cstdlib` | `cstring`
`ctime` | `cwchar` | `cwctype`

C++0x include files. These are only available in C++0x compilation mode, i.e. `-std=c++0x` or `-std=gnu++0x`.

C++ 200x Library Headers

`algorithm` | `array` | `bitset` | `chrono` | `complex`
`condition_variable` | `deque` | `exception` | `forward_list` | `fstream`
`functional` | `future` | `initalizer_list` | `iomanip` | `ios`
`iosfwd` | `iostream` | `istream` | `iterator` | `limits`
`list` | `locale` | `map` | `memory` | `mutex`
`new` | `numeric` | `ostream` | `queue` | `random`
`ratio` | `regex` | `set` | `sstream` | `stack`
`stdexcept` | `streambuf` | `string` | `system_error` | `thread`
`tuple` | `type_traits` | `typeinfo` | `unordered_map` | `unordered_set`
`utility` | `valarray` | `vector`

C++ 200x Library Headers for C Library Facilities

`cassert` | `ccomplex` | `cctype` | `cerrno` | `cfenv`
`cfloat` | `cinttypes` | `ciso646` | `climits` | `clocale`
`cmath` | `csetjmp` | `csignal` | `cstdarg` | `cstdbool`
`cstddef` | `cstdint` | `cstdlib` | `cstdio` | `cstring`
`ctgmath` | `ctime` | `cuchar` | `cwchar` | `cwctype`

In addition, TR1 includes as:

C++ TR 1 Library Headers

`tr1/array` | `tr1/complex` | `tr1/memory` | `tr1/functional` | `tr1/random`
`tr1/regex` | `tr1/tuple` | `tr1/type_traits` | `tr1/unordered_map` | `tr1/unordered_set`
`tr1/utility`

C++ TR 1 Library Headers for C Library Facilities

`tr1/ccomplex` | `tr1/cfenv` | `tr1/cfloat` | `tr1/cmath` | `tr1/cinttypes`
`tr1/climits` | `tr1/cstdarg` | `tr1/cstdbool` | `tr1/cstdint` | `tr1/cstdio`
`tr1/cstdlib` | `tr1/ctgmath` | `tr1/ctime` | `tr1/cwchar` | `tr1/cwctype`

Decimal floating-point arithmetic is available if the C++ compiler supports scalar decimal floating-point types defined via `__attribute__((mode(SD|DD|LD)))`.

C++ TR 24733 Decimal Floating-Point Header

`decimal/decimal`

Also included are files for the C++ ABI interface:

C++ ABI Headers

`cxxabi.h` | `cxxabi_forced.h`

And a large variety of extensions.

Extension Headers

`ext/algorithm` | `ext/atomicity.h` | `ext/array_allocator.h` | `ext/bitmap_allocator.h` | `ext/cast.h`
`ext/codecvt_specializations.h` | `ext/concurrence.h` | `ext/debug_allocator.h` | `ext/enc_filebuf.h` | `ext/extptr_allocator.h`
`ext/functional` | `ext/iterator` | `ext/malloc_allocator.h` | `ext/memory` | `ext/mt_allocator.h`
`ext/new_allocator.h` | `ext/numeric` | `ext/numeric_traits.h` | `ext/pb_ds/assoc_container.h` | `ext/pb_ds/priority_queue.h`
`ext/pod_char_traits.h` | `ext/pool_allocator.h` | `ext/rb_tree` | `ext/rope` | `ext/slist`
`ext/stdio_filebuf.h` | `ext/stdio_sync_filebuf.h` | `ext/throw_allocator.h` | `ext/typelist.h` | `ext/type_traits.h`
`ext/vstring.h`

Extension Debug Headers

`debug/bitset` | `debug/deque` | `debug/list` | `debug/map` | `debug/set`
`debug/string` | `debug/unordered_map` | `debug/unordered_set` | `debug/vector`

Extension Profile Headers

`profile/bitset` | `profile/deque` | `profile/list` | `profile/map`
`profile/set` | `profile/unordered_map` | `profile/unordered_set` | `profile/vector`

Extension Parallel Headers

`parallel/algorithm` | `parallel/numeric`

### Mixing Headers

A few simple rules.

First, mixing different dialects of the standard headers is not possible. It's an all-or-nothing affair. Thus, code like

    #include <array>
    #include <functional>

Implies C++0x mode. To use the entities in <array>, the C++0x compilation mode must be used, which implies the C++0x functionality (and deprecations) in <functional> will be present.

Second, the other headers can be included with either dialect of the standard headers, although features and types specific to C++0x are still only enabled when in C++0x compilation mode. So, to use rvalue references with `__gnu_cxx::vstring`, or to use the debug-mode versions of `std::unordered_map`, one must use the `std=gnu++0x` compiler flag. (Or `std=c++0x`, of course.)

A special case of the second rule is the mixing of TR1 and C++0x facilities. It is possible (although not especially prudent) to include both the TR1 version and the C++0x version of header in the same translation unit:

    #include <tr1/type_traits>
    #include <type_traits>

Several parts of C++0x diverge quite substantially from TR1 predecessors.

### The C Headers and `namespace std`

The standard specifies that if one includes the C-style header (<math.h> in this case), the symbols will be available in the global namespace and perhaps in namespace `std::` (but this is no longer a firm requirement.) On the other hand, including the C++-style header (<cmath>) guarantees that the entities will be found in namespace std and perhaps in the global namespace.

Usage of C++-style headers is recommended, as then C-linkage names can be disambiguated by explicit qualification, such as by `std::abort`. In addition, the C++-style headers can use function overloading to provide a simpler interface to certain families of C-functions. For instance in <cmath>, the function `std::sin` has overloads for all the builtin floating-point types. This means that `std::sin` can be used uniformly, instead of a combination of `std::sinf`, `std::sin`, and `std::sinl`.

### Precompiled Headers

There are three base header files that are provided. They can be used to precompile the standard headers and extensions into binary files that may the be used to speed compiles that use these headers.

- stdc++.h

  Includes all standard headers. Actual content varies depending on language dialect.

- stdtr1c++.h

  Includes all of <stdc++.h>, and adds all the TR1 headers.

- extc++.h

  Includes all of <stdtr1c++.h>, and adds all the Extension headers.

How to construct a .gch file from one of these base header files.

First, find the include directory for the compiler. One way to do this is:

    g++ -v hello.cc

    #include <...> search starts here:
     /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0
    ...
    End of search list.

Then, create a precompiled header file with the same flags that will be used to compile other projects.

    g++ -Winvalid-pch -x c++-header -g -O2 -o ./stdc++.h.gch /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0/x86_64-unknown-linux-gnu/bits/stdc++.h

The resulting file will be quite large: the current size is around thirty megabytes.

How to use the resulting file.

    g++ -I. -include stdc++.h  -H -g -O2 hello.cc

Verification that the PCH file is being used is easy:

    g++ -Winvalid-pch -I. -include stdc++.h -H -g -O2 hello.cc -o test.exe
    ! ./stdc++.h.gch
    . /mnt/share/bld/H-x86-gcc.20071201/include/c++/4.3.0/iostream
    . /mnt/share/bld/H-x86-gcc.20071201include/c++/4.3.0/string

The exclamation point to the left of the `stdc++.h.gch` listing means that the generated PCH file was used, and thus the

Detailed information about creating precompiled header files can be found in the GCC documentation.

## Macros

All library macros begin with `_GLIBCXX_`.

Furthermore, all pre-processor macros, switches, and configuration options are gathered in the file `c++config.h`, which is generated during the libstdc++ configuration and build process. This file is then included when needed by files part of the public libstdc++ API, like <ios>. Most of these macros should not be used by consumers of libstdc++, and are reserved for internal implementation use. These macros cannot be redefined.

A select handful of macros control libstdc++ extensions and extra features, or provide versioning information for the API. Only those macros listed below are offered for consideration by the general public.

Below is the macro which users may check for library version information.

`__GLIBCXX__`

  The current version of libstdc++ in compressed ISO date format, form of an unsigned long. For details on the value of this particular macro for a particular release, please consult this document.

Below are the macros which users may change with #define/#undef or with -D/-U compiler flags. The default state of the symbol is listed.

Configurable (or Not configurable) means that the symbol is initially chosen (or not) based on --enable/--disable options at library build and configure time (documented here), with the various --enable/--disable choices being translated to #define/#undef).

ABI means that changing from the default value may mean changing the ABI of compiled code. In other words, these choices control code which has already been compiled (i.e., in a binary such as libstdc++.a/.so). If you explicitly #define or #undef these macros, the headers may see different code paths, but the libraries which you link against will not. Experimenting with different values with the expectation of consistent linkage requires changing the config headers before building/installing the library.

`_GLIBCXX_USE_DEPRECATED`

  Defined by default. Not configurable. ABI-changing. Turning this off removes older ARM-style iostreams code, and other anachronisms from the API. This macro is dependent on the version of the standard being tracked, and as a result may give different results for `-std=c++98` and `-std=c++0x`. This may be useful in updating old C++ code which no longer meet the requirements of the language, or for checking current code against new language standards.

`_GLIBCXX_FORCE_NEW`

  Undefined by default. When defined, memory allocation and allocators controlled by libstdc++ call operator new/delete without caching and pooling. Configurable via `--enable-libstdcxx-allocator`. ABI-changing.

`_GLIBCXX_CONCEPT_CHECKS`

  Undefined by default. Configurable via `--enable-concept-checks`. When defined, performs compile-time checking on certain template instantiations to detect violations of the requirements of the standard. This is described in more detail here.

`_GLIBCXX_DEBUG`

  Undefined by default. When defined, compiles user code using the debug mode.

`_GLIBCXX_DEBUG_PEDANTIC`

  Undefined by default. When defined while compiling with the debug mode, makes the debug mode extremely picky by making the use of libstdc++ extensions and libstdc++-specific behavior into errors.

`_GLIBCXX_PARALLEL`

  Undefined by default. When defined, compiles user code using the parallel mode.

`_GLIBCXX_PROFILE`

  Undefined by default. When defined, compiles user code using the profile mode.

## Namespaces

### Available Namespaces

There are three main namespaces.

- std

  The ISO C++ standards specify that "all library entities are defined within namespace std." This includes namespaces nested within `namespace std`, such as `namespace std::tr1`.

- abi

  Specified by the C++ ABI. This ABI specifies a number of type and function APIs supplemental to those required by the ISO C++ Standard, but necessary for interoperability.

- __gnu_

  Indicating one of several GNU extensions. Choices include `__gnu_cxx`, `__gnu_debug`, `__gnu_parallel`, and `__gnu_pbds`.

A complete list of implementation namespaces (including namespace contents) is available in the generated source documentation.

### namespace std

One standard requirement is that the library components are defined in `namespace std::`. Thus, in order to use these types or functions, one must do one of two things:

- put a kind of using-declaration in your source (either `using namespace std;` or i.e. `using std::string;`) This approach works well for individual source files, but should not be used in a global context, like header files.

- use a fully qualified name for each library symbol (i.e. `std::string`, `std::cout`) Always can be used, and usually enhanced, by strategic use of typedefs. (In the cases where the qualified verbiage becomes unwieldy.)

### Using Namespace Composition

Best practice in programming suggests sequestering new data or functionality in a sanely-named, unique namespace whenever possible. This is considered an advantage over dumping everything in the global namespace, as then name look-up can be explicitly enabled or disabled as above, symbols are consistently mangled without repetitive naming prefixes or macros, etc.

For instance, consider a project that defines most of its classes in `namespace gtk`. It is possible to adapt `namespace gtk` to `namespace std` by using a C++-feature called namespace composition. This is what happens if a using-declaration is put into a namespace-definition: the imported symbol(s) gets imported into the currently active namespace(s). For example:

    namespace gtk
    {
      using std::string;
      using std::tr1::array;

      class Window { ... };
    }

In this example, `std::string` gets imported into `namespace gtk`. The result is that use of `std::string` inside namespace gtk can just use `string`, without the explicit qualification. As an added bonus, `std::string` does not get imported into the global namespace. Additionally, a more elaborate arrangement can be made for backwards compatibility and portability, whereby the `using`-declarations can wrapped in macros that are set based on autoconf-tests to either "" or i.e. `using std::string;` (depending on whether the system has libstdc++ in `std::` or not). (ideas from Llewelly and Karl Nelson)

## Linking

### Almost Nothing

Or as close as it gets: freestanding. This is a minimal configuration, with only partial support for the standard library. Assume only the following header files can be used:

- `cstdarg`

- `cstddef`

- `cstdlib`

- `exception`

- `limits`

- `new`

- `exception`

- `typeinfo`

In addition, throw in

- `cxxabi.h`.

In the C++0x dialect add

- `initializer_list`

- `type_traits`

There exists a library that offers runtime support for just these headers, and it is called `libsupc++.a`. To use it, compile with `gcc` instead of `g++`, like so:

`gcc foo.cc -lsupc++`

No attempt is made to verify that only the minimal subset identified above is actually used at compile time. Violations are diagnosed as undefined symbols at link time.

### Finding Dynamic or Shared Libraries

If the only library built is the static library (`libstdc++.a`), or if specifying static linking, this section is can be skipped. But if building or using a shared library (`libstdc++.so`), then additional location information will need to be provided.

But how?

A quick read of the relevant part of the GCC manual, Compiling C++ Programs, specifies linking against a C++ library. More details from the GCC FAQ, which states GCC does not, by default, specify a location so that the dynamic linker can find dynamic libraries at runtime.

Users will have to provide this information.

Methods vary for different platforms and different styles, and are printed to the screen during installation. To summarize:

- At runtime set `LD_LIBRARY_PATH` in your environment correctly, so that the shared library for libstdc++ can be found and loaded. Be certain that you understand all of the other implications and behavior of `LD_LIBRARY_PATH` first.

- Compile the path to find the library at runtime into the program. This can be done by passing certain options to `g++`, which will in turn pass them on to the linker. The exact format of the options is dependent on which linker you use:

  - GNU ld (default on Linux): `-Wl,-rpath,` `destdir/lib`

  - IRIX ld: `-Wl,-rpath,` `destdir/lib`

  - Solaris ld: `-Wl,-R` `destdir/lib`

- Some linkers allow you to specify the path to the library by setting `LD_RUN_PATH` in your environment when linking.

- On some platforms the system administrator can configure the dynamic linker to always look for libraries in `destdir/lib`, for example by using the `ldconfig` utility on Linux or the `crle` utility on Solaris. This is a system-wide change which can make the system unusable so if you are unsure then use one of the other methods described above.

Use the `ldd` utility on the linked executable to show which `libstdc++.so` library the system will get at runtime.

A `libstdc++.la` file is also installed, for use with Libtool. If you use Libtool to create your executables, these details are taken care of for you.

## Concurrency

This section discusses issues surrounding the proper compilation of multithreaded applications which use the Standard C++ library. This information is GCC-specific since the C++ standard does not address matters of multithreaded applications.

### Prerequisites

All normal disclaimers aside, multithreaded C++ application are only supported when libstdc++ and all user code was built with compilers which report (via `gcc/g++ -v` ) the same thread model and that model is not single. As long as your final application is actually single-threaded, then it should be safe to mix user code built with a thread model of single with a libstdc++ and other C++ libraries built with another thread model useful on the platform. Other mixes may or may not work but are not considered supported. (Thus, if you distribute a shared C++ library in binary form only, it may be best to compile it with a GCC configured with --enable-threads for maximal interchangeability and usefulness with a user population that may have built GCC with either --enable-threads or --disable-threads.)

When you link a multithreaded application, you will probably need to add a library or flag to g++. This is a very non-standardized area of GCC across ports. Some ports support a special flag (the spelling isn't even standardized yet) to add all required macros to a compilation (if any such flags are required then you must provide the flag for all compilations not just linking) and link-library additions and/or replacements at link time. The documentation is weak. Here is a quick summary to display how ad hoc this is: On Solaris, both -pthreads and -threads (with subtly different meanings) are honored. On OSF, -pthread and -threads (with subtly different meanings) are honored. On Linux/i386, -pthread is honored. On FreeBSD, -pthread is honored. Some other ports use other switches. AFAIK, none of this is properly documented anywhere other than in ``gcc -dumpspecs'' (look at lib and cpp entries).

### Thread Safety

We currently use the SGI STL definition of thread safety.

The library strives to be thread-safe when all of the following conditions are met:

- The system's libc is itself thread-safe,

- The compiler in use reports a thread model other than 'single'. This can be tested via output from `gcc -v`. Multi-thread capable versions of gcc output something like this:

      %gcc -v
      Using built-in specs.
      ...
      Thread model: posix
      gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)

  Look for "Thread model" lines that aren't equal to "single."

- Requisite command-line flags are used for atomic operations and threading. Examples of this include `-pthread` and `-march=native`, although specifics vary depending on the host environment. See Machine Dependent Options.

- An implementation of atomicity.h functions exists for the architecture in question. See the internals documentation for more details.

The user-code must guard against concurrent method calls which may access any particular library object's state. Typically, the application programmer may infer what object locks must be held based on the objects referenced in a method call. Without getting into great detail, here is an example which requires user-level locks:

         library_class_a shared_object_a;

         thread_main () {
           library_class_b *object_b = new library_class_b;
           shared_object_a.add_b (object_b);   // must hold lock for shared_object_a
           shared_object_a.mutate ();          // must hold lock for shared_object_a
         }

         // Multiple copies of thread_main() are started in independent threads.

Under the assumption that object_a and object_b are never exposed to another thread, here is an example that should not require any user-level locks:

         thread_main () {
           library_class_a object_a;
           library_class_b *object_b = new library_class_b;
           object_a.add_b (object_b);
           object_a.mutate ();
         }

All library objects are safe to use in a multithreaded program as long as each thread carefully locks out access by any other thread while it uses any object visible to another thread, i.e., treat library objects like any other shared resource. In general, this requirement includes both read and write access to objects; unless otherwise documented as safe, do not assume that two threads may access a shared standard library object at the same time.

### Atomics

### IO

This gets a bit tricky. Please read carefully, and bear with me.

#### Structure

A wrapper type called `__basic_file` provides our abstraction layer for the `std::filebuf` classes. Nearly all decisions dealing with actual input and output must be made in `__basic_file`.

A generic locking mechanism is somewhat in place at the filebuf layer, but is not used in the current code. Providing locking at any higher level is akin to providing locking within containers, and is not done for the same reasons (see the links above).

#### Defaults

The __basic_file type is simply a collection of small wrappers around the C stdio layer (again, see the link under Structure). We do no locking ourselves, but simply pass through to calls to `fopen`, `fwrite`, and so forth.

So, for 3.0, the question of "is multithreading safe for I/O" must be answered with, "is your platform's C library threadsafe for I/O?" Some are by default, some are not; many offer multiple implementations of the C library with varying tradeoffs of threadsafety and efficiency. You, the programmer, are always required to take care with multiple threads.

(As an example, the POSIX standard requires that C stdio FILE* operations are atomic. POSIX-conforming C libraries (e.g, on Solaris and GNU/Linux) have an internal mutex to serialize operations on FILE*s. However, you still need to not do stupid things like calling `fclose(fs)` in one thread followed by an access of `fs` in another.)

So, if your platform's C library is threadsafe, then your `fstream` I/O operations will be threadsafe at the lowest level. For higher-level operations, such as manipulating the data contained in the stream formatting classes (e.g., setting up callbacks inside an `std::ofstream`), you need to guard such accesses like any other critical shared resource.

#### Future

A second choice may be available for I/O implementations: libio. This is disabled by default, and in fact will not currently work due to other issues. It will be revisited, however.

The libio code is a subset of the guts of the GNU libc (glibc) I/O implementation. When libio is in use, the `__basic_file` type is basically derived from FILE. (The real situation is more complex than that... it's derived from an internal type used to implement FILE. See libio/libioP.h to see scary things done with vtbls.) The result is that there is no "layer" of C stdio to go through; the filebuf makes calls directly into the same functions used to implement `fread`, `fwrite`, and so forth, using internal data structures. (And when I say "makes calls directly," I mean the function is literally replaced by a jump into an internal function. Fast but frightening. *grin*)

Also, the libio internal locks are used. This requires pulling in large chunks of glibc, such as a pthreads implementation, and is one of the issues preventing widespread use of libio as the libstdc++ cstdio implementation.

But we plan to make this work, at least as an option if not a future default. Platforms running a copy of glibc with a recent-enough version will see calls from libstdc++ directly into the glibc already installed. For other platforms, a copy of the libio subsection will be built and included in libstdc++.

#### Alternatives

Don't forget that other cstdio implementations are possible. You could easily write one to perform your own forms of locking, to solve your "interesting" problems.

### Containers

This section discusses issues surrounding the design of multithreaded applications which use Standard C++ containers. All information in this section is current as of the gcc 3.0 release and all later point releases. Although earlier gcc releases had a different approach to threading configuration and proper compilation, the basic code design rules presented here were similar. For information on all other aspects of multithreading as it relates to libstdc++, including details on the proper compilation of threaded code (and compatibility between threaded and non-threaded code), see Chapter 17.

Two excellent pages to read when working with the Standard C++ containers and threads are SGI's http://www.sgi.com/tech/stl/thread_safety.html and SGI's http://www.sgi.com/tech/stl/Allocators.html.

However, please ignore all discussions about the user-level configuration of the lock implementation inside the STL container-memory allocator on those pages. For the sake of this discussion, libstdc++ configures the SGI STL implementation, not you. This is quite different from how gcc pre-3.0 worked. In particular, past advice was for people using g++ to explicitly define _PTHREADS or other macros or port-specific compilation options on the command line to get a thread-safe STL. This is no longer required for any port and should no longer be done unless you really know what you are doing and assume all responsibility.

Since the container implementation of libstdc++ uses the SGI code, we use the same definition of thread safety as SGI when discussing design. A key point that beginners may miss is the fourth major paragraph of the first page mentioned above (For most clients...), which points out that locking must nearly always be done outside the container, by client code (that'd be you, not us). There is a notable exceptions to this rule. Allocators called while a container or element is constructed uses an internal lock obtained and released solely within libstdc++ code (in fact, this is the reason STL requires any knowledge of the thread configuration).

For implementing a container which does its own locking, it is trivial to provide a wrapper class which obtains the lock (as SGI suggests), performs the container operation, and then releases the lock. This could be templatized to a certain extent, on the underlying container and/or a locking mechanism. Trying to provide a catch-all general template solution would probably be more trouble than it's worth.

The library implementation may be configured to use the high-speed caching memory allocator, which complicates thread safety issues. For all details about how to globally override this at application run-time see here. Also useful are details on allocator options and capabilities.