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.

Would you like to work on a patch?

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

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

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?

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

committed in r78544.