PEP 353 – Using ssize_t as the index type

PEP: 353
Title: Using ssize_t as the index type
Author: Martin von Löwis <martin at v.loewis.de>
Status: Final
Type: Standards Track
Created: 18-Dec-2005
Post-History

Abstract
Rationale
Specification
Conversion guidelines
Discussion
Open Issues
References
Copyright

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 1 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.