[Tutor] ctypes question

[Albert-Jan Roskam]

----- Original Message -----
> From: eryksun <eryksun at gmail.com>
> To: Albert-Jan Roskam <fomcl at yahoo.com>
> Cc: Python Mailing List <tutor at python.org>
> Sent: Wednesday, October 10, 2012 1:40 PM
> Subject: Re: [Tutor] ctypes question
> 
> On Wed, Oct 10, 2012 at 6:02 AM, Albert-Jan Roskam <fomcl at yahoo.com> 
> wrote:
>> 
>> elif pf.startswith("lin"):
>>     libc = ctypes.CDLL("libc.so.6")
>>     fopen = libc.fdopen
> 
>> fh = fopen(ctypes.c_char_p(fn), "rb")
>> fhPtr = ctypes.byref(ctypes.c_int(fh))
>> buff = ctypes.create_string_buffer(lines)
>> ret = libc.fread(buff, ctypes.c_int(1), ctypes.c_int(len(lines)), fhPtr)
>> print buff.value
> 
> fdopen takes a file descriptor, not a filename. You're using it with a
> filename like fopen (and you even called it fopen). Also, the byref
> usage is incorrect. fopen() returns a pointer to a FILE. fread() needs
> the pointer, not a reference to the pointer. You're giving it the
> address of an int, but it expects to find a FILE. You can use
> c_void_p(fh), or set the argtypes.

Aha, thanks. Yes, my total lack of knowledge of C speaks volumes here.
 
>     >>> from ctypes import *
>     >>> import ctypes.util
> 
>     >>> libc = cdll.LoadLibrary(ctypes.util.find_library('c'))

That's a nice little trick!
 
>     >>> fopen = libc.fopen
>     >>> fread = libc.fread
>     >>> fread.argtypes = [c_void_p, c_size_t, c_size_t, c_void_p]
> 
>     >>> fh = fopen("spam.txt", "rb")
>     >>> buf = create_string_buffer(128)
>     >>> fread(buf, 1, 128, fh)
>     5
>     >>> buf.value
>     'spam\n'
> 
> To use fdopen, you can get a fileno() from an open Python file object:
> 
>     >>> f = open("spam.txt", "rb")
> 
>     >>> fdopen = libc.fdopen
>     >>> fh = fdopen(f.fileno(), "rb")
>     >>> fread(buf, 1, 128, fh)
>     5

Very insightful. I'll modify my code an see if it might solve my original problem. At least I've learnt a lot now!
 
Thanks all for replying!!