# 3. Configure Python

## 3.1. Build Requirements

Features required to build CPython:

- A C11 compiler. Optional C11 features are not required.

- Support for IEEE 754 floating point numbers and floating point Not-a-Number (NaN).

- Support for threads.

- OpenSSL 1.1.1 or newer for the `ssl` and `hashlib` modules.

- On Windows, Microsoft Visual Studio 2017 or later is required.

Changed in version 3.11: C11 compiler, IEEE 754 and NaN support are now required. On Windows, Visual Studio 2017 or later is required.

Changed in version 3.10: OpenSSL 1.1.1 is now required.

Changed in version 3.7: Thread support and OpenSSL 1.0.2 are now required.

Changed in version 3.6: Selected C99 features are now required, like `<stdint.h>` and `static inline` functions.

Changed in version 3.5: On Windows, Visual Studio 2015 or later is required.

See also PEP 7 “Style Guide for C Code” and PEP 11 “CPython platform support”.

## 3.2. Configure Options

List all `./configure` script options using:

    ./configure --help

See also the `Misc/SpecialBuilds.txt` in the Python source distribution.

### 3.2.1. General Options

--enable-loadable-sqlite-extensions

  Support loadable extensions in the `_sqlite` extension module (default is no).

  See the `sqlite3.Connection.enable_load_extension()` method of the `sqlite3` module.

  New in version 3.6.

--disable-ipv6

  Disable IPv6 support (enabled by default if supported), see the `socket` module.

--enable-big-digits=[15|30]

  Define the size in bits of Python `int` digits: 15 or 30 bits.

  By default, the digit size is 30.

  Define the `PYLONG_BITS_IN_DIGIT` to `15` or `30`.

  See `sys.int_info.bits_per_digit`.

--with-suffix=SUFFIX

  Set the Python executable suffix to SUFFIX.

  The default suffix is `.exe` on Windows and macOS (`python.exe` executable), `.js` on Emscripten node, `.html` on Emscripten browser, `.wasm` on WASI, and an empty string on other platforms (`python` executable).

  Changed in version 3.11: The default suffix on WASM platform is one of `.js`, `.html` or `.wasm`.

--with-tzpath=<list of absolute paths separated by pathsep>

  Select the default time zone search path for `zoneinfo.TZPATH`. See the Compile-time configuration of the `zoneinfo` module.

  Default: `/usr/share/zoneinfo:/usr/lib/zoneinfo:/usr/share/lib/zoneinfo:/etc/zoneinfo`.

  See `os.pathsep` path separator.

  New in version 3.9.

--without-decimal-contextvar

  Build the `_decimal` extension module using a thread-local context rather than a coroutine-local context (default), see the `decimal` module.

  See `decimal.HAVE_CONTEXTVAR` and the `contextvars` module.

  New in version 3.9.

--with-dbmliborder=<list of backend names>

  Override order to check db backends for the `dbm` module

  A valid value is a colon (`:`) separated string with the backend names:

  - `ndbm`;

  - `gdbm`;

  - `bdb`.

--without-c-locale-coercion

  Disable C locale coercion to a UTF-8 based locale (enabled by default).

  Don’t define the `PY_COERCE_C_LOCALE` macro.

  See `PYTHONCOERCECLOCALE` and the PEP 538.

--without-freelists

  Disable all freelists except the empty tuple singleton.

  New in version 3.11.

--with-platlibdir=DIRNAME

  Python library directory name (default is `lib`).

  Fedora and SuSE use `lib64` on 64-bit platforms.

  See `sys.platlibdir`.

  New in version 3.9.

--with-wheel-pkg-dir=PATH

  Directory of wheel packages used by the `ensurepip` module (none by default).

  Some Linux distribution packaging policies recommend against bundling dependencies. For example, Fedora installs wheel packages in the `/usr/share/python-wheels/` directory and don’t install the `ensurepip._bundled` package.

  New in version 3.10.

--with-pkg-config=[check|yes|no]

  Whether configure should use pkg-config to detect build dependencies.

  - `check` (default): pkg-config is optional

  - `yes`: pkg-config is mandatory

  - `no`: configure does not use pkg-config even when present

  New in version 3.11.

--enable-pystats

  Turn on internal statistics gathering.

  The statistics will be dumped to a arbitrary (probably unique) file in `/tmp/py_stats/`, or `C:\temp\py_stats\` on Windows. If that directory does not exist, results will be printed on stdout.

  Use `Tools/scripts/summarize_stats.py` to read the stats.

  New in version 3.11.

### 3.2.2. WebAssembly Options

--with-emscripten-target=[browser|node]

  Set build flavor for `wasm32-emscripten`.

  - `browser` (default): preload minimal stdlib, default MEMFS.

  - `node`: NODERAWFS and pthread support.

  New in version 3.11.

--enable-wasm-dynamic-linking

  Turn on dynamic linking support for WASM.

  Dynamic linking enables `dlopen`. File size of the executable increases due to limited dead code elimination and additional features.

  New in version 3.11.

--enable-wasm-pthreads

  Turn on pthreads support for WASM.

  New in version 3.11.

### 3.2.3. Install Options

--prefix=PREFIX

  Install architecture-independent files in PREFIX. On Unix, it defaults to `/usr/local`.

  This value can be retrived at runtime using `sys.prefix`.

  As an example, one can use `--prefix="$HOME/.local/"` to install a Python in its home directory.

--exec-prefix=EPREFIX

  Install architecture-dependent files in EPREFIX, defaults to `--prefix`.

  This value can be retrived at runtime using `sys.exec_prefix`.

--disable-test-modules

  Don’t build nor install test modules, like the `test` package or the `_testcapi` extension module (built and installed by default).

  New in version 3.10.

--with-ensurepip=[upgrade|install|no]

  Select the `ensurepip` command run on Python installation:

  - `upgrade` (default): run `python -m ensurepip --altinstall --upgrade` command.

  - `install`: run `python -m ensurepip --altinstall` command;

  - `no`: don’t run ensurepip;

  New in version 3.6.

### 3.2.4. Performance options

Configuring Python using `--enable-optimizations --with-lto` (PGO + LTO) is recommended for best performance. The experimental `--enable-bolt` flag can also be used to improve performance.

--enable-optimizations

  Enable Profile Guided Optimization (PGO) using `PROFILE_TASK` (disabled by default).

  The C compiler Clang requires `llvm-profdata` program for PGO. On macOS, GCC also requires it: GCC is just an alias to Clang on macOS.

  Disable also semantic interposition in libpython if `--enable-shared` and GCC is used: add `-fno-semantic-interposition` to the compiler and linker flags.

  New in version 3.6.

  Changed in version 3.10: Use `-fno-semantic-interposition` on GCC.

PROFILE_TASK

  Environment variable used in the Makefile: Python command line arguments for the PGO generation task.

  Default: `-m test --pgo --timeout=$(TESTTIMEOUT)`.

  New in version 3.8.

--with-lto=[full|thin|no|yes]

  Enable Link Time Optimization (LTO) in any build (disabled by default).

  The C compiler Clang requires `llvm-ar` for LTO (`ar` on macOS), as well as an LTO-aware linker (`ld.gold` or `lld`).

  New in version 3.6.

  New in version 3.11: To use ThinLTO feature, use `--with-lto=thin` on Clang.

  Changed in version 3.12: Use ThinLTO as the default optimization policy on Clang if the compiler accepts the flag.

--enable-bolt

  Enable usage of the BOLT post-link binary optimizer (disabled by default).

  BOLT is part of the LLVM project but is not always included in their binary distributions. This flag requires that `llvm-bolt` and `merge-fdata` are available.

  BOLT is still a fairly new project so this flag should be considered experimental for now. Because this tool operates on machine code its success is dependent on a combination of the build environment + the other optimization configure args + the CPU architecture, and not all combinations are supported.

  New in version 3.12.

--with-computed-gotos

  Enable computed gotos in evaluation loop (enabled by default on supported compilers).

--without-pymalloc

  Disable the specialized Python memory allocator pymalloc (enabled by default).

  See also `PYTHONMALLOC` environment variable.

--without-doc-strings

  Disable static documentation strings to reduce the memory footprint (enabled by default). Documentation strings defined in Python are not affected.

  Don’t define the `WITH_DOC_STRINGS` macro.

  See the `PyDoc_STRVAR()` macro.

--enable-profiling

  Enable C-level code profiling with `gprof` (disabled by default).

--with-strict-overflow

  Add `-fstrict-overflow` to the C compiler flags (by default we add `-fno-strict-overflow` instead).

### 3.2.5. Python Debug Build

A debug build is Python built with the `--with-pydebug` configure option.

Effects of a debug build:

- Display all warnings by default: the list of default warning filters is empty in the `warnings` module.

- Add `d` to `sys.abiflags`.

- Add `sys.gettotalrefcount()` function.

- Add `-X showrefcount` command line option.

- Add `-d` command line option and `PYTHONDEBUG` environment variable to debug the parser.

- Add support for the `__lltrace__` variable: enable low-level tracing in the bytecode evaluation loop if the variable is defined.

- Install debug hooks on memory allocators to detect buffer overflow and other memory errors.

- Define `Py_DEBUG` and `Py_REF_DEBUG` macros.

- Add runtime checks: code surrounded by `#ifdef Py_DEBUG` and `#endif`. Enable `assert(...)` and `_PyObject_ASSERT(...)` assertions: don’t set the `NDEBUG` macro (see also the `--with-assertions` configure option). Main runtime checks:

  - Add sanity checks on the function arguments.

  - Unicode and int objects are created with their memory filled with a pattern to detect usage of uninitialized objects.

  - Ensure that functions which can clear or replace the current exception are not called with an exception raised.

  - Check that deallocator functions don’t change the current exception.

  - The garbage collector (`gc.collect()` function) runs some basic checks on objects consistency.

  - The `Py_SAFE_DOWNCAST()` macro checks for integer underflow and overflow when downcasting from wide types to narrow types.

See also the Python Development Mode and the `--with-trace-refs` configure option.

Changed in version 3.8: Release builds and debug builds are now ABI compatible: defining the `Py_DEBUG` macro no longer implies the `Py_TRACE_REFS` macro (see the `--with-trace-refs` option), which introduces the only ABI incompatibility.

### 3.2.6. Debug options

--with-pydebug

  Build Python in debug mode: define the `Py_DEBUG` macro (disabled by default).

--with-trace-refs

  Enable tracing references for debugging purpose (disabled by default).

  Effects:

  - Define the `Py_TRACE_REFS` macro.

  - Add `sys.getobjects()` function.

  - Add `PYTHONDUMPREFS` environment variable.

  This build is not ABI compatible with release build (default build) or debug build (`Py_DEBUG` and `Py_REF_DEBUG` macros).

  New in version 3.8.

--with-assertions

  Build with C assertions enabled (default is no): `assert(...);` and `_PyObject_ASSERT(...);`.

  If set, the `NDEBUG` macro is not defined in the `OPT` compiler variable.

  See also the `--with-pydebug` option (debug build) which also enables assertions.

  New in version 3.6.

--with-valgrind

  Enable Valgrind support (default is no).

--with-dtrace

  Enable DTrace support (default is no).

  See Instrumenting CPython with DTrace and SystemTap.

  New in version 3.6.

--with-address-sanitizer

  Enable AddressSanitizer memory error detector, `asan` (default is no).

  New in version 3.6.

--with-memory-sanitizer

  Enable MemorySanitizer allocation error detector, `msan` (default is no).

  New in version 3.6.

--with-undefined-behavior-sanitizer

  Enable UndefinedBehaviorSanitizer undefined behaviour detector, `ubsan` (default is no).

  New in version 3.6.

### 3.2.7. Linker options

--enable-shared

  Enable building a shared Python library: `libpython` (default is no).

--without-static-libpython

  Do not build `libpythonMAJOR.MINOR.a` and do not install `python.o` (built and enabled by default).

  New in version 3.10.

### 3.2.8. Libraries options

--with-libs='lib1 ...'

  Link against additional libraries (default is no).

--with-system-expat

  Build the `pyexpat` module using an installed `expat` library (default is no).

--with-system-libmpdec

  Build the `_decimal` extension module using an installed `mpdec` library, see the `decimal` module (default is no).

  New in version 3.3.

--with-readline=editline

  Use `editline` library for backend of the `readline` module.

  Define the `WITH_EDITLINE` macro.

  New in version 3.10.

--without-readline

  Don’t build the `readline` module (built by default).

  Don’t define the `HAVE_LIBREADLINE` macro.

  New in version 3.10.

--with-libm=STRING

  Override `libm` math library to STRING (default is system-dependent).

--with-libc=STRING

  Override `libc` C library to STRING (default is system-dependent).

--with-openssl=DIR

  Root of the OpenSSL directory.

  New in version 3.7.

--with-openssl-rpath=[no|auto|DIR]

  Set runtime library directory (rpath) for OpenSSL libraries:

  - `no` (default): don’t set rpath;

  - `auto`: auto-detect rpath from `--with-openssl` and `pkg-config`;

  - DIR: set an explicit rpath.

  New in version 3.10.

### 3.2.9. Security Options

--with-hash-algorithm=[fnv|siphash13|siphash24]

  Select hash algorithm for use in `Python/pyhash.c`:

  - `siphash13` (default);

  - `siphash24`;

  - `fnv`.

  New in version 3.4.

  New in version 3.11: `siphash13` is added and it is the new default.

--with-builtin-hashlib-hashes=md5,sha1,sha256,sha512,sha3,blake2

  Built-in hash modules:

  - `md5`;

  - `sha1`;

  - `sha256`;

  - `sha512`;

  - `sha3` (with shake);

  - `blake2`.

  New in version 3.9.

--with-ssl-default-suites=[python|openssl|STRING]

  Override the OpenSSL default cipher suites string:

  - `python` (default): use Python’s preferred selection;

  - `openssl`: leave OpenSSL’s defaults untouched;

  - STRING: use a custom string

  See the `ssl` module.

  New in version 3.7.

  Changed in version 3.10: The settings `python` and STRING also set TLS 1.2 as minimum protocol version.

### 3.2.10. macOS Options

See `Mac/README.rst`.

--enable-universalsdk

--enable-universalsdk=SDKDIR

  Create a universal binary build. SDKDIR specifies which macOS SDK should be used to perform the build (default is no).

--enable-framework

--enable-framework=INSTALLDIR

  Create a Python.framework rather than a traditional Unix install. Optional INSTALLDIR specifies the installation path (default is no).

--with-universal-archs=ARCH

  Specify the kind of universal binary that should be created. This option is only valid when `--enable-universalsdk` is set.

  Options:

  - `universal2`;

  - `32-bit`;

  - `64-bit`;

  - `3-way`;

  - `intel`;

  - `intel-32`;

  - `intel-64`;

  - `all`.

--with-framework-name=FRAMEWORK

  Specify the name for the python framework on macOS only valid when `--enable-framework` is set (default: `Python`).

### 3.2.11. Cross Compiling Options

Cross compiling, also known as cross building, can be used to build Python for another CPU architecture or platform. Cross compiling requires a Python interpreter for the build platform. The version of the build Python must match the version of the cross compiled host Python.

--build=BUILD

  configure for building on BUILD, usually guessed by config.guess.

--host=HOST

  cross-compile to build programs to run on HOST (target platform)

--with-build-python=path/to/python

  path to build `python` binary for cross compiling

  New in version 3.11.

CONFIG_SITE=file

  An environment variable that points to a file with configure overrides.

  Example config.site file:

      # config.site-aarch64
      ac_cv_buggy_getaddrinfo=no
      ac_cv_file__dev_ptmx=yes
      ac_cv_file__dev_ptc=no

Cross compiling example:

    CONFIG_SITE=config.site-aarch64 ../configure \
        --build=x86_64-pc-linux-gnu \
        --host=aarch64-unknown-linux-gnu \
        --with-build-python=../x86_64/python

## 3.3. Python Build System

### 3.3.1. Main files of the build system

- `configure.ac` => `configure`;

- `Makefile.pre.in` => `Makefile` (created by `configure`);

- `pyconfig.h` (created by `configure`);

- `Modules/Setup`: C extensions built by the Makefile using `Module/makesetup` shell script;

- `setup.py`: C extensions built using the `setuptools` package.

### 3.3.2. Main build steps

- C files (`.c`) are built as object files (`.o`).

- A static `libpython` library (`.a`) is created from objects files.

- `python.o` and the static `libpython` library are linked into the final `python` program.

- C extensions are built by the Makefile (see `Modules/Setup`) and `python setup.py build`.

### 3.3.3. Main Makefile targets

- `make`: Build Python with the standard library.

- `make platform:`: build the `python` program, but don’t build the standard library extension modules.

- `make profile-opt`: build Python using Profile Guided Optimization (PGO). You can use the configure `--enable-optimizations` option to make this the default target of the `make` command (`make all` or just `make`).

- `make buildbottest`: Build Python and run the Python test suite, the same way than buildbots test Python. Set `TESTTIMEOUT` variable (in seconds) to change the test timeout (1200 by default: 20 minutes).

- `make install`: Build and install Python.

- `make regen-all`: Regenerate (almost) all generated files; `make regen-stdlib-module-names` and `autoconf` must be run separately for the remaining generated files.

- `make clean`: Remove built files.

- `make distclean`: Same than `make clean`, but remove also files created by the configure script.

### 3.3.4. C extensions

Some C extensions are built as built-in modules, like the `sys` module. They are built with the `Py_BUILD_CORE_BUILTIN` macro defined. Built-in modules have no `__file__` attribute:

    >>> import sys
    >>> sys
    <module 'sys' (built-in)>
    >>> sys.__file__
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: module 'sys' has no attribute '__file__'

Other C extensions are built as dynamic libraries, like the `_asyncio` module. They are built with the `Py_BUILD_CORE_MODULE` macro defined. Example on Linux x86-64:

    >>> import _asyncio
    >>> _asyncio
    <module '_asyncio' from '/usr/lib64/python3.9/lib-dynload/_asyncio.cpython-39-x86_64-linux-gnu.so'>
    >>> _asyncio.__file__
    '/usr/lib64/python3.9/lib-dynload/_asyncio.cpython-39-x86_64-linux-gnu.so'

`Modules/Setup` is used to generate Makefile targets to build C extensions. At the beginning of the files, C extensions are built as built-in modules. Extensions defined after the `*shared*` marker are built as dynamic libraries.

The `setup.py` script only builds C extensions as shared libraries using the `distutils` module.

The `PyAPI_FUNC()`, `PyAPI_API()` and `PyMODINIT_FUNC()` macros of `Include/pyport.h` are defined differently depending if the `Py_BUILD_CORE_MODULE` macro is defined:

- Use `Py_EXPORTED_SYMBOL` if the `Py_BUILD_CORE_MODULE` is defined

- Use `Py_IMPORTED_SYMBOL` otherwise.

If the `Py_BUILD_CORE_BUILTIN` macro is used by mistake on a C extension built as a shared library, its `PyInit_xxx()` function is not exported, causing an `ImportError` on import.

## 3.4. Compiler and linker flags

Options set by the `./configure` script and environment variables and used by `Makefile`.

### 3.4.1. Preprocessor flags

CONFIGURE_CPPFLAGS

  Value of `CPPFLAGS` variable passed to the `./configure` script.

  New in version 3.6.

CPPFLAGS

  (Objective) C/C++ preprocessor flags, e.g. `-I<include dir>` if you have headers in a nonstandard directory `<include dir>`.

  Both `CPPFLAGS` and `LDFLAGS` need to contain the shell’s value for setup.py to be able to build extension modules using the directories specified in the environment variables.

BASECPPFLAGS

  New in version 3.4.

PY_CPPFLAGS

  Extra preprocessor flags added for building the interpreter object files.

  Default: `$(BASECPPFLAGS) -I. -I$(srcdir)/Include $(CONFIGURE_CPPFLAGS) $(CPPFLAGS)`.

  New in version 3.2.

### 3.4.2. Compiler flags

CC

  C compiler command.

  Example: `gcc -pthread`.

CXX

  C++ compiler command.

  Example: `g++ -pthread`.

CFLAGS

  C compiler flags.

CFLAGS_NODIST

  `CFLAGS_NODIST` is used for building the interpreter and stdlib C extensions. Use it when a compiler flag should not be part of the distutils `CFLAGS` once Python is installed (bpo-21121).

  In particular, `CFLAGS` should not contain:

  - the compiler flag `-I` (for setting the search path for include files). The `-I` flags are processed from left to right, and any flags in `CFLAGS` would take precedence over user- and package-supplied `-I` flags.

  - hardening flags such as `-Werror` because distributions cannot control whether packages installed by users conform to such heightened standards.

  New in version 3.5.

COMPILEALL_OPTS

  Options passed to the `compileall` command line when building PYC files in `make install`. Default: `-j0`.

  New in version 3.12.

EXTRA_CFLAGS

  Extra C compiler flags.

CONFIGURE_CFLAGS

  Value of `CFLAGS` variable passed to the `./configure` script.

  New in version 3.2.

CONFIGURE_CFLAGS_NODIST

  Value of `CFLAGS_NODIST` variable passed to the `./configure` script.

  New in version 3.5.

BASECFLAGS

  Base compiler flags.

OPT

  Optimization flags.

CFLAGS_ALIASING

  Strict or non-strict aliasing flags used to compile `Python/dtoa.c`.

  New in version 3.7.

CCSHARED

  Compiler flags used to build a shared library.

  For example, `-fPIC` is used on Linux and on BSD.

CFLAGSFORSHARED

  Extra C flags added for building the interpreter object files.

  Default: `$(CCSHARED)` when `--enable-shared` is used, or an empty string otherwise.

PY_CFLAGS

  Default: `$(BASECFLAGS) $(OPT) $(CONFIGURE_CFLAGS) $(CFLAGS) $(EXTRA_CFLAGS)`.

PY_CFLAGS_NODIST

  Default: `$(CONFIGURE_CFLAGS_NODIST) $(CFLAGS_NODIST) -I$(srcdir)/Include/internal`.

  New in version 3.5.

PY_STDMODULE_CFLAGS

  C flags used for building the interpreter object files.

  Default: `$(PY_CFLAGS) $(PY_CFLAGS_NODIST) $(PY_CPPFLAGS) $(CFLAGSFORSHARED)`.

  New in version 3.7.

PY_CORE_CFLAGS

  Default: `$(PY_STDMODULE_CFLAGS) -DPy_BUILD_CORE`.

  New in version 3.2.

PY_BUILTIN_MODULE_CFLAGS

  Compiler flags to build a standard library extension module as a built-in module, like the `posix` module.

  Default: `$(PY_STDMODULE_CFLAGS) -DPy_BUILD_CORE_BUILTIN`.

  New in version 3.8.

PURIFY

  Purify command. Purify is a memory debugger program.

  Default: empty string (not used).

### 3.4.3. Linker flags

LINKCC

  Linker command used to build programs like `python` and `_testembed`.

  Default: `$(PURIFY) $(CC)`.

CONFIGURE_LDFLAGS

  Value of `LDFLAGS` variable passed to the `./configure` script.

  Avoid assigning `CFLAGS`, `LDFLAGS`, etc. so users can use them on the command line to append to these values without stomping the pre-set values.

  New in version 3.2.

LDFLAGS_NODIST

  `LDFLAGS_NODIST` is used in the same manner as `CFLAGS_NODIST`. Use it when a linker flag should not be part of the distutils `LDFLAGS` once Python is installed (bpo-35257).

  In particular, `LDFLAGS` should not contain:

  - the compiler flag `-L` (for setting the search path for libraries). The `-L` flags are processed from left to right, and any flags in `LDFLAGS` would take precedence over user- and package-supplied `-L` flags.

CONFIGURE_LDFLAGS_NODIST

  Value of `LDFLAGS_NODIST` variable passed to the `./configure` script.

  New in version 3.8.

LDFLAGS

  Linker flags, e.g. `-L<lib dir>` if you have libraries in a nonstandard directory `<lib dir>`.

  Both `CPPFLAGS` and `LDFLAGS` need to contain the shell’s value for setup.py to be able to build extension modules using the directories specified in the environment variables.

LIBS

  Linker flags to pass libraries to the linker when linking the Python executable.

  Example: `-lrt`.

LDSHARED

  Command to build a shared library.

  Default: `@LDSHARED@ $(PY_LDFLAGS)`.

BLDSHARED

  Command to build `libpython` shared library.

  Default: `@BLDSHARED@ $(PY_CORE_LDFLAGS)`.

PY_LDFLAGS

  Default: `$(CONFIGURE_LDFLAGS) $(LDFLAGS)`.

PY_LDFLAGS_NODIST

  Default: `$(CONFIGURE_LDFLAGS_NODIST) $(LDFLAGS_NODIST)`.

  New in version 3.8.

PY_CORE_LDFLAGS

  Linker flags used for building the interpreter object files.

  New in version 3.8.