Gregory Ewing at 2018/4/13 上午 07:25 wrote:
On Thu, Apr 12, 2018 at 2:16 PM, <jf...@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?
I am working on a DLL's function.
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 had missed this statement:-(
To make a quick try, I set the function's restype to
ctypes.POINTER(ctypes.c_ubyte), instead of ctypes.c_char_p. It's
amazing, the \x00 trap can be avoided in this way. Now I can use "mydata
= bytes(buf[:n])" to extract n bytes of data and write it to file.
The problem was solved, and thanks for all your help.
--Jach
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.
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
--
https://mail.python.org/mailman/listinfo/python-list
