Problem with calling function from dll

[deepblu]

I have problem with using function from dll.

import ctypes

b = ctypes.windll.LoadLibrary("kernel32")

a = ""

b.GetComputerNameA(a,20)


But I got exception:

Traceback (most recent call last):
File "<pyshell#323>", line 1, in <module>
b.GetComputerNameA(a,20)
WindowsError: exception: access violation reading 0x00000014

Even when I write:
a = ctypes.c_char_p('.' * 20)
I got the same result.

Here I found this function description:
http://sd2cx1.webring.org/l/rd?ring...ty.net/~azakrze3/html/win32_api_functios.html


Please help.

 

[Ulrich Eckhardt]

Am 13.12.2012 08:40, schrieb (e-mail address removed):

I have problem with using function from dll.

import ctypes

b = ctypes.windll.LoadLibrary("kernel32")

a = ""

b.GetComputerNameA(a,20)

GetComputerNameA takes a pointer to a writable char string. You give it
a pointer to an immutable string. You will have to create a buffer first
and pass that to the function. Also, I would use GetComputerNameW
instead, although it makes little difference for this name (provided
that's the hostname, which may only contain an ASCII subset).

But I got exception:

Traceback (most recent call last):
File "<pyshell#323>", line 1, in <module>
b.GetComputerNameA(a,20)
WindowsError: exception: access violation reading 0x00000014

Here is something else that is wrong: The address 0x00000014 that the
function tries to access is neither the address of a mutable nor an
immutable string but simply the value 20 that you passed as second
parameter. In other words, the way that the parameters are passed to the
function is wrong.

Even when I write:
a = ctypes.c_char_p('.' * 20)
I got the same result.

This looks much better than passing the string above, but it still seems
the (hopefully correct) parameters are passed wrongly.

Here I found this function description:
http://sd2cx1.webring.org/l/rd?ring...ty.net/~azakrze3/html/win32_api_functios.html

Use the primary source for such info:

http://msdn.microsoft.com/en-us/library/windows/desktop/ms724295(v=vs.85).aspx

One possibly important point there is the WINAPI part, which describes
how parameters are passed to and from the function, which might be the
only cause that this doesn't work. However, instead of try-and-error,
rather go to the documentation of the ctypes API and search for WINAPI.
There, you will find an example that uses a function from the win32 API,
too. A bit further down, you will also find a create_string_buffer()
function that could be useful for you.

http://docs.python.org/2/library/ctypes.html


Greetings!

Uli

 

[deepblu]

sb = ctypes.create_string_buffer(20)
i = ctypes.byref(ctypes.c_int(20))
w = b.GetComputerNameA(sb,i)

w -> 1
sb.value -> nazwa komputera