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


3.1. Configure Options
======================

List all "./configure" script options using:

   ./configure --help

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


3.1.1. 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 number of bits is selected depending on
   "sizeof(void*)": 30 bits if "void*" size is 64-bit or larger, 15
   bits otherwise.

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

   See "sys.int_info.bits_per_digit".

--with-cxx-main

--with-cxx-main=COMPILER

   Compile the Python "main()" function and link Python executable
   with C++ compiler: "$CXX", or *COMPILER* if specified.

--with-suffix=SUFFIX

   Set the Python executable suffix to *SUFFIX*.

   The default suffix is ".exe" on Windows and macOS ("python.exe"
   executable), and an empty string on other platforms ("python"
   executable).

--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.

   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=db1:db2:...

   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**.

--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.


3.1.2. 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.


3.1.3. Performance options
--------------------------

Configuring Python using "--enable-optimizations --with-lto" (PGO +
LTO) is recommended for best 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

   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.

--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).


3.1.4. 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 "PYTHONTHREADDEBUG" environment variable.

* Add support for the "__ltrace__" 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 surroundeded 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.

  * 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.


3.1.5. 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.


3.1.6. 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.


3.1.7. 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-ffi

   Build the "_ctypes" extension module using an installed "ffi"
   library, see the "ctypes" module (default is system-dependent).

--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-tcltk-includes='-I...'

   Override search for Tcl and Tk include files.

--with-tcltk-libs='-L...'

   Override search for Tcl and Tk libraries.

--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.


3.1.8. Security Options
-----------------------

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

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

   * "siphash24" (default).

   * "fnv";

   New in version 3.4.

--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.


3.1.9. 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. Python Build System
========================


3.2.1. 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 "distutils" module.


3.2.2. 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".


3.2.3. 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.


3.2.4. 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 extensins 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.


3.3. Compiler and linker flags
==============================

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


3.3.1. 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.


3.3.2. Compiler flags
---------------------

CC

   C compiler command.

   Example: "gcc -pthread".

MAINCC

   C compiler command used to build the "main()" function of programs
   like "python".

   Variable set by the "--with-cxx-main" option of the configure
   script.

   Default: "$(CC)".

CXX

   C++ compiler command.

   Used if the "--with-cxx-main" option is used.

   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.

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).


3.3.3. Linker flags
-------------------

LINKCC

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

   Default: "$(PURIFY) $(MAINCC)".

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.
