PEP: 353 Title: Using ssize_t as the index type Version: $Revision$
Last-Modified: $Date$ Author: Martin von Löwis <martin@v.loewis.de>
Status: Final Type: Standards Track Content-Type: text/x-rst Created:
18-Dec-2005 Python-Version: 2.5 Post-History:

Abstract

In Python 2.4, indices of sequences are restricted to the C type int. On
64-bit machines, sequences therefore cannot use the full address space,
and are restricted to 2**31 elements. This PEP proposes to change this,
introducing a platform-specific index type Py_ssize_t. An implementation
of the proposed change is in
http://svn.python.org/projects/python/branches/ssize_t.

Rationale

64-bit machines are becoming more popular, and the size of main memory
increases beyond 4GiB. On such machines, Python currently is limited, in
that sequences (strings, unicode objects, tuples, lists, array.arrays,
...) cannot contain more than 2GiElements.

Today, very few machines have memory to represent larger lists: as each
pointer is 8B (in a 64-bit machine), one needs 16GiB to just hold the
pointers of such a list; with data in the list, the memory consumption
grows even more. However, there are three container types for which
users request improvements today:

-   strings (currently restricted to 2GiB)
-   mmap objects (likewise; plus the system typically won't keep the
    whole object in memory concurrently)
-   Numarray objects (from Numerical Python)

As the proposed change will cause incompatibilities on 64-bit machines,
it should be carried out while such machines are not in wide use (IOW,
as early as possible).

Specification

A new type Py_ssize_t is introduced, which has the same size as the
compiler's size_t type, but is signed. It will be a typedef for ssize_t
where available.

The internal representation of the length fields of all container types
is changed from int to ssize_t, for all types included in the standard
distribution. In particular, PyObject_VAR_HEAD is changed to use
Py_ssize_t, affecting all extension modules that use that macro.

All occurrences of index and length parameters and results are changed
to use Py_ssize_t, including the sequence slots in type objects, and the
buffer interface.

New conversion functions PyInt_FromSsize_t and PyInt_AsSsize_t, are
introduced. PyInt_FromSsize_t will transparently return a long int
object if the value exceeds the LONG_MAX; PyInt_AsSsize_t will
transparently process long int objects.

New function pointer typedefs ssizeargfunc, ssizessizeargfunc,
ssizeobjargproc, ssizessizeobjargproc, and lenfunc are introduced. The
buffer interface function types are now called readbufferproc,
writebufferproc, segcountproc, and charbufferproc.

A new conversion code 'n' is introduced for PyArg_ParseTuple
Py_BuildValue, PyObject_CallFunction and PyObject_CallMethod. This code
operates on Py_ssize_t.

The conversion codes 's#' and 't#' will output Py_ssize_t if the macro
PY_SSIZE_T_CLEAN is defined before Python.h is included, and continue to
output int if that macro isn't defined.

At places where a conversion from size_t/Py_ssize_t to int is necessary,
the strategy for conversion is chosen on a case-by-case basis (see next
section).

To prevent loading extension modules that assume a 32-bit size type into
an interpreter that has a 64-bit size type, Py_InitModule4 is renamed to
Py_InitModule4_64.

Conversion guidelines

Module authors have the choice whether they support this PEP in their
code or not; if they support it, they have the choice of different
levels of compatibility.

If a module is not converted to support this PEP, it will continue to
work unmodified on a 32-bit system. On a 64-bit system, compile-time
errors and warnings might be issued, and the module might crash the
interpreter if the warnings are ignored.

Conversion of a module can either attempt to continue using int indices,
or use Py_ssize_t indices throughout.

If the module should continue to use int indices, care must be taken
when calling functions that return Py_ssize_t or size_t, in particular,
for functions that return the length of an object (this includes the
strlen function and the sizeof operator). A good compiler will warn when
a Py_ssize_t/size_t value is truncated into an int. In these cases,
three strategies are available:

-   statically determine that the size can never exceed an int (e.g.
    when taking the sizeof a struct, or the strlen of a file pathname).
    In this case, write:

        some_int = Py_SAFE_DOWNCAST(some_value, Py_ssize_t, int);

    This will add an assertion in debug mode that the value really fits
    into an int, and just add a cast otherwise.

-   statically determine that the value shouldn't overflow an int unless
    there is a bug in the C code somewhere. Test whether the value is
    smaller than INT_MAX, and raise an InternalError if it isn't.

-   otherwise, check whether the value fits an int, and raise a
    ValueError if it doesn't.

The same care must be taken for tp_as_sequence slots, in addition, the
signatures of these slots change, and the slots must be explicitly
recast (e.g. from intargfunc to ssizeargfunc). Compatibility with
previous Python versions can be achieved with the test:

    #if PY_VERSION_HEX < 0x02050000 && !defined(PY_SSIZE_T_MIN)
    typedef int Py_ssize_t;
    #define PY_SSIZE_T_MAX INT_MAX
    #define PY_SSIZE_T_MIN INT_MIN
    #endif

and then using Py_ssize_t in the rest of the code. For the
tp_as_sequence slots, additional typedefs might be necessary;
alternatively, by replacing:

    PyObject* foo_item(struct MyType* obj, int index)
    {
      ...
    }

with:

    PyObject* foo_item(PyObject* _obj, Py_ssize_t index)
    {
       struct MyType* obj = (struct MyType*)_obj;
       ...
    }

it becomes possible to drop the cast entirely; the type of foo_item
should then match the sq_item slot in all Python versions.

If the module should be extended to use Py_ssize_t indices, all usages
of the type int should be reviewed, to see whether it should be changed
to Py_ssize_t. The compiler will help in finding the spots, but a manual
review is still necessary.

Particular care must be taken for PyArg_ParseTuple calls: they need all
be checked for s# and t# converters, and PY_SSIZE_T_CLEAN must be
defined before including Python.h if the calls have been updated
accordingly.

Fredrik Lundh has written a scanner which checks the code of a C module
for usage of APIs whose signature has changed.

Discussion

Why not size_t

An initial attempt to implement this feature tried to use size_t. It
quickly turned out that this cannot work: Python uses negative indices
in many places (to indicate counting from the end). Even in places where
size_t would be usable, too many reformulations of code where necessary,
e.g. in loops like:

    for(index = length-1; index >= 0; index--)

This loop will never terminate if index is changed from int to size_t.

Why not Py_intptr_t

Conceptually, Py_intptr_t and Py_ssize_t are different things:
Py_intptr_t needs to be the same size as void*, and Py_ssize_t the same
size as size_t. These could differ, e.g. on machines where pointers have
segment and offset. On current flat-address space machines, there is no
difference, so for all practical purposes, Py_intptr_t would have worked
as well.

Doesn't this break much code?

With the changes proposed, code breakage is fairly minimal. On a 32-bit
system, no code will break, as Py_ssize_t is just a typedef for int.

On a 64-bit system, the compiler will warn in many places. If these
warnings are ignored, the code will continue to work as long as the
container sizes don't exceed 2**31, i.e. it will work nearly as good as
it does currently. There are two exceptions to this statement: if the
extension module implements the sequence protocol, it must be updated,
or the calling conventions will be wrong. The other exception is the
places where Py_ssize_t is output through a pointer (rather than a
return value); this applies most notably to codecs and slice objects.

If the conversion of the code is made, the same code can continue to
work on earlier Python releases.

Doesn't this consume too much memory?

One might think that using Py_ssize_t in all tuples, strings, lists,
etc. is a waste of space. This is not true, though: on a 32-bit machine,
there is no change. On a 64-bit machine, the size of many containers
doesn't change, e.g.

-   in lists and tuples, a pointer immediately follows the ob_size
    member. This means that the compiler currently inserts a 4 padding
    bytes; with the change, these padding bytes become part of the size.
-   in strings, the ob_shash field follows ob_size. This field is of
    type long, which is a 64-bit type on most 64-bit systems (except
    Win64), so the compiler inserts padding before it as well.

Open Issues

-   Marc-Andre Lemburg commented that complete backwards compatibility
    with existing source code should be preserved. In particular,
    functions that have Py_ssize_t* output arguments should continue to
    run correctly even if the callers pass int*.

    It is not clear what strategy could be used to implement that
    requirement.

Copyright

This document has been placed in the public domain.



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