Add support for ssize_t

[nikratio]

BPO	6729
Nosy	@theller, @gpshead
Files	issue6729.patch: Adds C type ssize_t to ctypes.

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/gpshead'
closed_at = <Date 2010-03-01.04:56:34.292>
created_at = <Date 2009-08-18.20:16:39.353>
labels = ['ctypes', 'type-feature']
title = 'Add support for ssize_t'
updated_at = <Date 2010-03-01.04:56:34.290>
user = 'https://bugs.python.org/nikratio'

bugs.python.org fields:

activity = <Date 2010-03-01.04:56:34.290>
actor = 'gregory.p.smith'
assignee = 'gregory.p.smith'
closed = True
closed_date = <Date 2010-03-01.04:56:34.292>
closer = 'gregory.p.smith'
components = ['ctypes']
creation = <Date 2009-08-18.20:16:39.353>
creator = 'nikratio'
dependencies = []
files = ['16405']
hgrepos = []
issue_num = 6729
keywords = ['patch']
message_count = 7.0
messages = ['91713', '92763', '92775', '92843', '93246', '100238', '100246']
nosy_count = 5.0
nosy_names = ['theller', 'gregory.p.smith', 'rcoyner', 'nikratio', 'robotify']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue6729'
versions = ['Python 2.7', 'Python 3.2']

[nikratio]

ctypes currently has a datatype c_size_t which corresponds to size_t in
C, but there is no datatype for the C ssize_t.

[theller]

Would you like to work on a patch?

[nikratio]

I can give it a shot if you give me a rough idea where I have to make
the appropriate changes.

[theller]

Find where c_size_t is defined: in Lib/ctypes/init.py

[nikratio]

Ok, apparently the lines that define c_size_t are these:

if sizeof(c_uint) == sizeof(c_void_p):
    c_size_t = c_uint
elif sizeof(c_ulong) == sizeof(c_void_p):
    c_size_t = c_ulong
elif sizeof(c_ulonglong) == sizeof(c_void_p):
    c_size_t = c_ulonglong

(side remark: wouldn't a simple c_size_t = c_void_p do exactly the same
as the above and be much clearer?)

Unfortunately I have no real idea how to come up with something similar
for ssize_t, since there is no predefined object that is of type ssize_t
(as c_void_p is for type size_t). Any ideas what to do?

[rcoyner]

You don't want to do c_size_t = c_void_p because that will prevent type checking. We want c_size_t to be integers; setting it to c_void_p will accept other values. The lines that define c_size_t are doing a sizeof check to determine how many bits the CPU supports, and c_size_t should represent unsigned integers [1].

On a 16-bit machine: c_size_t = c_uint
On a 32-bit machine: c_size_t = c_ulong
On a 64-bit machine: c_size_t = c_ulonglong

Now, ssize_t is like size_t, except that it is signed [2]. So if I am not mistaken, all we have to do is:

if sizeof(c_uint) == sizeof(c_void_p):
    c_size_t = c_uint
    c_ssize_t = c_int
elif sizeof(c_ulong) == sizeof(c_void_p):
    c_size_t = c_ulong
    c_ssize_t = c_long
elif sizeof(c_ulonglong) == sizeof(c_void_p):
    c_size_t = c_ulonglong
    c_ssize_t = c_longlong

Patch attached with documentation and unit test.

[1] - http://www.gnu.org/software/libc/manual/html_node/Important-Data-Types.html
[2] - http://www.gnu.org/software/libc/manual/html_node/I_002fO-Primitives.html

[gpshead]

committed in r78544.