3. Configure Python
*******************


3.1. Build Requirements
=======================

Features and minimum versions required to build CPython:

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

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

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

* Support for threads.

* OpenSSL 1.1.1 is the minimum version and OpenSSL 3.0.9 is the
  recommended minimum version for the "ssl" and "hashlib" extension
  modules.

* SQLite 3.15.2 for the "sqlite3" extension module.

* Tcl/Tk 8.5.12 for the "tkinter" module.

* Autoconf 2.71 and aclocal 1.16.5 are required to regenerate the
  "configure" script.

Changed in version 3.1: Tcl/Tk version 8.3.1 is now required.

Changed in version 3.5: On Windows, Visual Studio 2015 or later is now
required. Tcl/Tk version 8.4 is now required.

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

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

Changed in version 3.10: OpenSSL 1.1.1 is now required. Require SQLite
3.7.15.

Changed in version 3.11: C11 compiler, IEEE 754 and NaN support are
now required. On Windows, Visual Studio 2017 or later is required.
Tcl/Tk version 8.5.12 is now required for the "tkinter" module.

Changed in version 3.13: Autoconf 2.71, aclocal 1.16.5 and SQLite
3.15.2 are now required.

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


3.2. Generated files
====================

To reduce build dependencies, Python source code contains multiple
generated files. Commands to regenerate all generated files:

   make regen-all
   make regen-stdlib-module-names
   make regen-limited-abi
   make regen-configure

The "Makefile.pre.in" file documents generated files, their inputs,
and tools used to regenerate them. Search for "regen-*" make targets.


3.2.1. configure script
-----------------------

The "make regen-configure" command regenerates the "aclocal.m4" file
and the "configure" script using the "Tools/build/regen-configure.sh"
shell script which uses an Ubuntu container to get the same tools
versions and have a reproducible output.

The container is optional, the following command can be run locally:

   autoreconf -ivf -Werror

The generated files can change depending on the exact "autoconf-
archive", "aclocal" and "pkg-config" versions.


3.3. Configure Options
======================

List all "configure" script options using:

   ./configure --help

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


3.3.1. General Options
----------------------

--enable-loadable-sqlite-extensions

   Support loadable extensions in the "_sqlite" extension module
   (default is no) of the "sqlite3" module.

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

   Added 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/zon
   einfo:/etc/zoneinfo".

   See "os.pathsep" path separator.

   Added 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.

   Added 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.

   Added 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".

   Added 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.

   Added 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

   Added in version 3.11.

--enable-pystats

   Turn on internal Python performance statistics gathering.

   By default, statistics gathering is off. Use "python3 -X pystats"
   command or set "PYTHONSTATS=1" environment variable to turn on
   statistics gathering at Python startup.

   At Python exit, dump statistics if statistics gathering was on and
   not cleared.

   Effects:

   * Add "-X pystats" command line option.

   * Add "PYTHONSTATS" environment variable.

   * Define the "Py_STATS" macro.

   * Add functions to the "sys" module:

     * "sys._stats_on()": Turns on statistics gathering.

     * "sys._stats_off()": Turns off statistics gathering.

     * "sys._stats_clear()": Clears the statistics.

     * "sys._stats_dump()": Dump statistics to file, and clears the
       statistics.

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

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

   Statistics:

   * Opcode:

     * Specialization: success, failure, hit, deferred, miss, deopt,
       failures;

     * Execution count;

     * Pair count.

   * Call:

     * Inlined Python calls;

     * PyEval calls;

     * Frames pushed;

     * Frame object created;

     * Eval calls: vector, generator, legacy, function VECTORCALL,
       build class, slot, function “ex”, API, method.

   * Object:

     * incref and decref;

     * interpreter incref and decref;

     * allocations: all, 512 bytes, 4 kiB, big;

     * free;

     * to/from free lists;

     * dictionary materialized/dematerialized;

     * type cache;

     * optimization attempts;

     * optimization traces created/executed;

     * uops executed.

   * Garbage collector:

     * Garbage collections;

     * Objects visited;

     * Objects collected.

   Added in version 3.11.

--disable-gil

   Enables **experimental** support for running Python without the
   *global interpreter lock* (GIL): free threading build.

   Defines the "Py_GIL_DISABLED" macro and adds ""t"" to
   "sys.abiflags".

   See Free-threaded CPython for more detail.

   Added in version 3.13.

--enable-experimental-jit=[no|yes|yes-off|interpreter]

   Indicate how to integrate the JIT compiler.

   * "no" - build the interpreter without the JIT.

   * "yes" - build the interpreter with the JIT.

   * "yes-off" - build the interpreter with the JIT but disable it by
     default.

   * "interpreter" - build the interpreter without the JIT, but with
     the tier 2 enabled interpreter.

   By convention, "--enable-experimental-jit" is a shorthand for "--
   enable-experimental-jit=yes".

   Note:

     When building CPython with JIT enabled, ensure that your system
     has Python 3.11 or later installed.

   Added in version 3.13.

PKG_CONFIG

   Path to "pkg-config" utility.

PKG_CONFIG_LIBDIR

PKG_CONFIG_PATH

   "pkg-config" options.


3.3.2. C compiler options
-------------------------

CC

   C compiler command.

CFLAGS

   C compiler flags.

CPP

   C preprocessor command.

CPPFLAGS

   C preprocessor flags, e.g. "-I*include_dir*".


3.3.3. Linker options
---------------------

LDFLAGS

   Linker flags, e.g. "-L*library_directory*".

LIBS

   Libraries to pass to the linker, e.g. "-l*library*".

MACHDEP

   Name for machine-dependent library files.


3.3.4. Options for third-party dependencies
-------------------------------------------

Added in version 3.11.

BZIP2_CFLAGS

BZIP2_LIBS

   C compiler and linker flags to link Python to "libbz2", used by
   "bz2" module, overriding "pkg-config".

CURSES_CFLAGS

CURSES_LIBS

   C compiler and linker flags for "libncurses" or "libncursesw", used
   by "curses" module, overriding "pkg-config".

GDBM_CFLAGS

GDBM_LIBS

   C compiler and linker flags for "gdbm".

LIBB2_CFLAGS

LIBB2_LIBS

   C compiler and linker flags for "libb2" (BLAKE2), used by "hashlib"
   module, overriding "pkg-config".

LIBEDIT_CFLAGS

LIBEDIT_LIBS

   C compiler and linker flags for "libedit", used by "readline"
   module, overriding "pkg-config".

LIBFFI_CFLAGS

LIBFFI_LIBS

   C compiler and linker flags for "libffi", used by "ctypes" module,
   overriding "pkg-config".

LIBMPDEC_CFLAGS

LIBMPDEC_LIBS

   C compiler and linker flags for "libmpdec", used by "decimal"
   module, overriding "pkg-config".

   Note:

     These environment variables have no effect unless "--with-system-
     libmpdec" is specified.

LIBLZMA_CFLAGS

LIBLZMA_LIBS

   C compiler and linker flags for "liblzma", used by "lzma" module,
   overriding "pkg-config".

LIBREADLINE_CFLAGS

LIBREADLINE_LIBS

   C compiler and linker flags for "libreadline", used by "readline"
   module, overriding "pkg-config".

LIBSQLITE3_CFLAGS

LIBSQLITE3_LIBS

   C compiler and linker flags for "libsqlite3", used by "sqlite3"
   module, overriding "pkg-config".

LIBUUID_CFLAGS

LIBUUID_LIBS

   C compiler and linker flags for "libuuid", used by "uuid" module,
   overriding "pkg-config".

PANEL_CFLAGS

PANEL_LIBS

   C compiler and linker flags for PANEL, overriding "pkg-config".

   C compiler and linker flags for "libpanel" or "libpanelw", used by
   "curses.panel" module, overriding "pkg-config".

TCLTK_CFLAGS

TCLTK_LIBS

   C compiler and linker flags for TCLTK, overriding "pkg-config".

ZLIB_CFLAGS

ZLIB_LIBS

   C compiler and linker flags for "libzlib", used by "gzip" module,
   overriding "pkg-config".


3.3.5. 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.

   Added 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.

   Added in version 3.11.

--enable-wasm-pthreads

   Turn on pthreads support for WASM.

   Added in version 3.11.


3.3.6. Install Options
----------------------

--prefix=PREFIX

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

   This value can be retrieved 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 retrieved 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).

   Added 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;

   Added in version 3.6.


3.3.7. 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.

   Note:

     During the build, you may encounter compiler warnings about
     profile data not being available for some source files. These
     warnings are harmless, as only a subset of the code is exercised
     during profile data acquisition. To disable these warnings on
     Clang, manually suppress them by adding "-Wno-profile-instr-
     unprofiled" to "CFLAGS".

   Added 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)".

   Added in version 3.8.

   Changed in version 3.13: Task failure is no longer ignored
   silently.

--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").

   Added in version 3.6.

   Added 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. BOLT versions
   before LLVM 16 are known to crash BOLT under some scenarios. Use of
   LLVM 16 or newer for BOLT optimization is strongly encouraged.

   The "BOLT_INSTRUMENT_FLAGS" and "BOLT_APPLY_FLAGS" **configure**
   variables can be defined to override the default set of arguments
   for **llvm-bolt** to instrument and apply BOLT data to binaries,
   respectively.

   Added in version 3.12.

BOLT_APPLY_FLAGS

   Arguments to "llvm-bolt" when creating a BOLT optimized binary.

   Added in version 3.12.

BOLT_INSTRUMENT_FLAGS

   Arguments to "llvm-bolt" when instrumenting binaries.

   Added in version 3.12.

--with-computed-gotos

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

--without-mimalloc

   Disable the fast mimalloc allocator (enabled by default).

   See also "PYTHONMALLOC" environment variable.

--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).


3.3.8. 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).


3.3.9. 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.

   The "PYTHONDUMPREFS" environment variable can be used to dump
   objects and reference counts still alive at Python exit.

   Statically allocated objects are not traced.

   Added in version 3.8.

   Changed in version 3.13: This build is now ABI compatible with
   release build and debug build.

--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.

   Added 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.

   Added in version 3.6.

--with-address-sanitizer

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

   Added in version 3.6.

--with-memory-sanitizer

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

   Added in version 3.6.

--with-undefined-behavior-sanitizer

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

   Added in version 3.6.

--with-thread-sanitizer

   Enable ThreadSanitizer data race detector, "tsan" (default is no).

   Added in version 3.13.


3.3.10. 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).

   Added in version 3.10.


3.3.11. 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
   "mpdecimal" library, see the "decimal" module (default is yes).

   Added in version 3.3.

   Changed in version 3.13: Default to using the installed "mpdecimal"
   library.

   Deprecated since version 3.13, will be removed in version 3.15: A
   copy of the "mpdecimal" library sources will no longer be
   distributed with Python 3.15.

   See also: "LIBMPDEC_CFLAGS" and "LIBMPDEC_LIBS".

--with-readline=readline|editline

   Designate a backend library for the "readline" module.

   * readline: Use readline as the backend.

   * editline: Use editline as the backend.

   Added in version 3.10.

--without-readline

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

   Don’t define the "HAVE_LIBREADLINE" macro.

   Added 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.

   Added 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.

   Added in version 3.10.


3.3.12. Security Options
------------------------

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

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

   * "siphash13" (default);

   * "siphash24";

   * "fnv".

   Added in version 3.4.

   Added 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".

   Added 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.

   Added in version 3.7.

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


3.3.13. 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").

--with-app-store-compliance

--with-app-store-compliance=PATCH-FILE

   The Python standard library contains strings that are known to
   trigger automated inspection tool errors when submitted for
   distribution by the macOS and iOS App Stores. If enabled, this
   option will apply the list of patches that are known to correct app
   store compliance. A custom patch file can also be specified. This
   option is disabled by default.

   Added in version 3.13.


3.3.14. iOS Options
-------------------

See iOS/README.rst.

--enable-framework=INSTALLDIR

   Create a Python.framework. Unlike macOS, the *INSTALLDIR* argument
   specifying the installation path is mandatory.

--with-framework-name=FRAMEWORK

   Specify the name for the framework (default: "Python").


3.3.15. 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

   Added 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

HOSTRUNNER

   Program to run CPython for the host platform for cross-compilation.

   Added in version 3.11.

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


3.4. Python Build System
========================


3.4.1. 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;


3.4.2. 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").


3.4.3. Main Makefile targets
----------------------------


3.4.3.1. make
~~~~~~~~~~~~~

For the most part, when rebuilding after editing some code or
refreshing your checkout from upstream, all you need to do is execute
"make", which (per Make’s semantics) builds the default target, the
first one defined in the Makefile.  By tradition (including in the
CPython project) this is usually the "all" target. The "configure"
script expands an "autoconf" variable, "@DEF_MAKE_ALL_RULE@" to
describe precisely which targets "make all" will build. The three
choices are:

* "profile-opt" (configured with "--enable-optimizations")

* "build_wasm" (configured with "--with-emscripten-target")

* "build_all" (configured without explicitly using either of the
  others)

Depending on the most recent source file changes, Make will rebuild
any targets (object files and executables) deemed out-of-date,
including running "configure" again if necessary. Source/target
dependencies are many and maintained manually however, so Make
sometimes doesn’t have all the information necessary to correctly
detect all targets which need to be rebuilt.  Depending on which
targets aren’t rebuilt, you might experience a number of problems. If
you have build or test problems which you can’t otherwise explain,
"make clean && make" should work around most dependency problems, at
the expense of longer build times.


3.4.3.2. make platform
~~~~~~~~~~~~~~~~~~~~~~

Build the "python" program, but don’t build the standard library
extension modules. This generates a file named "platform" which
contains a single line describing the details of the build platform,
e.g., "macosx-14.3-arm64-3.12" or "linux-x86_64-3.13".


3.4.3.3. 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").


3.4.3.4. make clean
~~~~~~~~~~~~~~~~~~~

Remove built files.


3.4.3.5. make distclean
~~~~~~~~~~~~~~~~~~~~~~~

In addition to the work done by "make clean", remove files created by
the configure script.  "configure" will have to be run before building
again. [1]


3.4.3.6. make install
~~~~~~~~~~~~~~~~~~~~~

Build the "all" target and install Python.


3.4.3.7. make test
~~~~~~~~~~~~~~~~~~

Build the "all" target and run the Python test suite with the "--fast-
ci" option. Variables:

* "TESTOPTS": additional regrtest command-line options.

* "TESTPYTHONOPTS": additional Python command-line options.

* "TESTTIMEOUT": timeout in seconds (default: 10 minutes).


3.4.3.8. make buildbottest
~~~~~~~~~~~~~~~~~~~~~~~~~~

This is similar to "make test", but uses the "--slow-ci" option and
default timeout of 20 minutes, instead of "--fast-ci" option.


3.4.3.9. make regen-all
~~~~~~~~~~~~~~~~~~~~~~~

Regenerate (almost) all generated files. These include (but are not
limited to) bytecode cases, and parser generator file. "make regen-
stdlib-module-names" and "autoconf" must be run separately for the
remaining generated files.


3.4.4. 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 "PyAPI_FUNC()", "PyAPI_DATA()" and "PyMODINIT_FUNC" macros of
"Include/exports.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.


3.5. Compiler and linker flags
==============================

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


3.5.1. Preprocessor flags
-------------------------

CONFIGURE_CPPFLAGS

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

   Added 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 to
   be able to build extension modules using the directories specified
   in the environment variables.

BASECPPFLAGS

   Added 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)".

   Added in version 3.2.


3.5.2. 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
   "CFLAGS" once Python is installed (gh-65320).

   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.

   Added in version 3.5.

COMPILEALL_OPTS

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

   Added in version 3.12.

EXTRA_CFLAGS

   Extra C compiler flags.

CONFIGURE_CFLAGS

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

   Added in version 3.2.

CONFIGURE_CFLAGS_NODIST

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

   Added 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".

   Added 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".

   Added 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)".

   Added in version 3.7.

PY_CORE_CFLAGS

   Default: "$(PY_STDMODULE_CFLAGS) -DPy_BUILD_CORE".

   Added 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".

   Added in version 3.8.

PURIFY

   Purify command. Purify is a memory debugger program.

   Default: empty string (not used).


3.5.3. 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.

   Added 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 "LDFLAGS" once
   Python is installed (gh-65320).

   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.

   Added 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 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)".

   Added in version 3.8.

PY_CORE_LDFLAGS

   Linker flags used for building the interpreter object files.

   Added in version 3.8.

-[ Footnotes ]-

[1] "git clean -fdx" is an even more extreme way to “clean” your
    checkout. It removes all files not known to Git. When bug hunting
    using "git bisect", this is recommended between probes to
    guarantee a completely clean build. **Use with care**, as it will
    delete all files not checked into Git, including your new,
    uncommitted work.
