Re: LP_c_ubyte buffer and length to python string?

Dan Stromberg wrote, On 07/23/2011 11:27 PM:
>
> I've googled on this for hours, and found nothing that seems close to 
> what I want to do.
>
> I have a C function that accepts a preallocated buffer and a pointer 
> to an integer.  It then stashes data in the buffer, and passes back 
> the buffer's "in use" length in the integer.
>
> I'm creating the buffer with:
>
>       ctypes_compressed_buffer_char_p = 
> ctypes.create_string_buffer(maximum_size)
>       ctypes_compressed_buffer = 
> ctypes.cast(ctypes_compressed_buffer_char_p, 
> ctypes.POINTER(ctypes.c_ubyte))
>
> Then I create my length with:
>
>       ctypes_compressed_size = ctypes.c_size_t(0)
>       ctypes_compressed_size_pointer = 
> ctypes.cast(ctypes.addressof(ctypes_compressed_size), ctypes.c_void_p)
>
> The length is actually a size_t, but passing a pointer to size_t in 
> ctypes was giving me type errors, hence the void *.
>
> Anyway, the function executes without reporting any errors, so I seem 
> to have my result - in a ctypes format.  Printing its type reports 
> that its an LP_c_ubyte.
>
> How can I convert the resulting LP_c_ubyte (array) and int to a Python 
> 2.x str and a 3.x bytes?

I assume you want to get the content of the chunk of memory that was 
allocated by create_string_buffer. The simplest way to get that is
   ctypes_compressed_buffer_char_p.raw[:length]
but you can also do
   ''.join(chr(x[i]) for i in range(length))

Besides that: It seems strange that you have to cast your buffer and 
your size_t. ctypes gives you all the power and danger of both Python 
and C at once, so just because it executes without reporting errors 
doesn't mean that you got it right. You might want to post a minimal 
self-contained code snippet that shows what function you are calling and 
how you are doing it.

/Mads


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