How to write partial of a buffer which was returned from a C function to a file?

Gregory Ewing greg.ewing at canterbury.ac.nz
Thu Apr 12 19:25:48 EDT 2018


>>> On Thu, Apr 12, 2018 at 2:16 PM,  <jfong at ms4.hinet.net> wrote:
>>> 
>>>> This C function returns a buffer which I declared it as a
>>>> ctypes.c_char_p. The buffer has size 0x10000 bytes long and the valid
>>>> data may vary from a few bytes to the whole size.

I think we need to see the code you're using to call this
C function.

The crucial issue is: are *you* allocating this 0x10000 byte
buffer and telling the function to read data into it, or
does the function allocate the memory itself and return a
pointer to it?

If the function is allocating the buffer, then I don't
think there's any way to make this work. The ctypes docs
say this:

> Fundamental data types, when returned as foreign function call results ... are
> transparently converted to native Python types. In other words, if a foreign
> function has a restype of c_char_p, you will always receive a Python bytes
> object, not a c_char_p instance.

The problem is that the only way ctypes can tell how long
a bytes object to create for a c_char_p is by assuming that
it points to a nul-terminated string. If it actually points
to a char array that can legitimately contain zero bytes,
then you're out of luck.

To get around this, you may need to declare the return type
as POINTER(c_char) instead:

> For a general character pointer that may also point to binary data,
 > POINTER(c_char) must be used.

I'm not sure where to go from here, though, because the
ctypes documentation peters out before explaining exactly
what can be done with a POINTER object.

Another approach would be to allocate the buffer yourself
and pass it into the C function, but whether that's possible
depends on the details of the C API you're using.

-- 
Greg


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