win32utils devel - Dec 2010 - How to initialize and pass a VARIANT to IAccessible::get_accName?

Hi all,

First off, thanks for Win32Utils!  The libraries have been really beneficial
for me.

I have a question (hope this is an appropriate place for it):

I''m trying to write some demo scripts that use the IAccessible
interface:

http://msdn.microsoft.com/en-us/library/dd318466%28v=vs.85%29.aspx

I''ve had some success so far (for example, I can successfully call the
"IAccessible::get_accChildCount" method).

But I''m stuck on IAccessible::get_accName.

In fact, any method like get_accName that takes a VARIANT (note: not a
VARIANT*) seems to be failing for me.

Does anyone have experience using win32-api to define and call a function
that takes a VARIANT?

In the case of get_accName, I can''t get it to return successfully...

Either ruby.exe crashes (when I pass a 16-byte packed string as the VARIANT)
or I get the HRESULT ''The parameter is incorrect.'' (when I use
a >=28-byte
packed string as the VARIANT).


I''m wondering if the problem is one of:

- the Win32::API function prototype I''m using for the VARIANT param is
wrong, or:

- the way I''m initializing the VARIANT before passing it is incorrect

I''d be really grateful if anyone could weigh in on how to achieve this.
:)

Right now I''m treating the VARIANT as a pointer in the get_accName
prototype
(note the full example program this came from is at the end of the email):

  Get_accName = Win32::API::Function.new(table[10], ''PPP'',
''L'')

The C prototype for the function (from OleAcc.h) is:

  /* [id][propget][hidden] */ HRESULT ( STDMETHODCALLTYPE *get_accName )(
      __RPC__in IAccessible * This,
      /* [optional][in] */ VARIANT varChild,
      /* [retval][out] */ __RPC__deref_out_opt BSTR *pszName);


I combed through the windows-pr code looking for examples of functions that
take a VARIANT param, but so far I''ve only found functions that take a
pointer-to-VARIANT.

Also, I suspect this code, which initializes my VARIANT, might be causing my
problem (it was cobbled together from snippets on the win32utils-devel list
and the pr-win32ole source):

  # Initialize VARIANT that holds CHILDID_SELF
  self_var = 0.chr * 16
  VariantInit(self_var)
  self_var[0, 2] = [VT_I4].pack(''S'')
  self_var[2, 2] = [0].pack(''S'')
  self_var[4, 2] = [0].pack(''S'')
  self_var[6, 2] = [0].pack(''S'')
  self_var[8, 4] = [CHILDID_SELF].pack(''L'')
  self_var[12, 4] = [0].pack(''L'')


Here''s a complete script that shows the get_accName problem - please
let me
know if there are any questions I could answer that might help resolve the
issue.  If it would help I also have a working C demo program where
get_accName is used.

# print_desktop_name.rb

require ''rubygems''
require ''win32/api''
require ''windows/com''
require ''windows/com/variant''
require ''windows/error''
require ''windows/msvcrt/buffer''
require ''windows/unicode''
require ''windows/window''
require ''windows/window/menu''

include Windows::COM
include Windows::COM::Variant
include Windows::Error
include Windows::MSVCRT::Buffer
include Windows::Unicode
include Windows::Window
include Windows::Window::Menu

IID_IAccessible = [0x618736e0, 0x3c3d, 0x11cf, 0x81, 0x0c, 0x00, 0xaa, 0x00,
0x38, 0x9b, 0x71].pack(''LSSC8'')
CHILDID_SELF = 0

CoInitialize(nil) # Initialize COM

desktop_hwnd = GetDesktopWindow.call() # Get the desktop''s HWND

# Use the HWND to get the desktop''s IAccessible object
AccessibleObjectFromWindow =
Win32::API.new(''AccessibleObjectFromWindow'',
                                            ''LLPP'',
''L'', ''oleacc'')
desktop_iacc = 0.chr * 4
hr = AccessibleObjectFromWindow.call(desktop_hwnd,
                                     OBJID_WINDOW,
                                     IID_IAccessible,
                                     desktop_iacc)
raise "Failed to instantiate IAcc object!" if (hr != S_OK)

# Get the memory address of the IAcc pointer - need to use it later as a
# ''this'' pointer to pass to get_accName
desktop_iacc_ptr = desktop_iacc.unpack(''L'').first

# Get the IAccessibleVtbl C interface. It contains 28 functions (see
OleAcc.h).
lpVtbl = 0.chr * 4
table = 0.chr * (4 * 28)

# Unpack the IAccessibleVtbl:
memcpy(lpVtbl, desktop_iacc_ptr, 4)
memcpy(table, lpVtbl.unpack(''L'').first, 4 * 28)
table = table.unpack(''L*'')

# Define get_accName
Get_accName = Win32::API::Function.new(table[10], ''PPP'',
''L'')

# Create buffer for the BSTR that will receive the IAcc object name
name_bstr = 0.chr * 4

# Initialize CHILDID_SELF variant
self_var = 0.chr * 16
VariantInit(self_var)
self_var[0, 2] = [VT_I4].pack(''S'')
self_var[2, 2] = [0].pack(''S'')
self_var[4, 2] = [0].pack(''S'')
self_var[6, 2] = [0].pack(''S'')
self_var[8, 4] = [CHILDID_SELF].pack(''L'')
self_var[12, 4] = [0].pack(''L'')

# This segfaults on XP and Win7:
# (using ruby 1.8.7 i386-mingw32, 2010-08-16 patchlevel 302)
hr = Get_accName.call(desktop_iacc_ptr, self_var, name_bstr)
puts "HRESULT from get_accName is: " + hr.to_s
puts "HRESULT message is: ''#{get_last_error(hr)}''"
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Hi,

2010/12/15 Bill Agee <billagee at gmail.com>:
> Hi all,
>
> First off, thanks for Win32Utils!? The libraries have been really
beneficial
> for me.
>
> I have a question (hope this is an appropriate place for it):
>
> I''m trying to write some demo scripts that use the IAccessible
interface:
>
> http://msdn.microsoft.com/en-us/library/dd318466%28v=vs.85%29.aspx
>
> I''ve had some success so far (for example, I can successfully call
the
> "IAccessible::get_accChildCount" method).
>
> But I''m stuck on IAccessible::get_accName.
>
> In fact, any method like get_accName that takes a VARIANT (note: not a
> VARIANT*) seems to be failing for me.
>
> Does anyone have experience using win32-api to define and call a function
> that takes a VARIANT?
>
> In the case of get_accName, I can''t get it to return
successfully...
>
> Either ruby.exe crashes (when I pass a 16-byte packed string as the
VARIANT)
> or I get the HRESULT ''The parameter is incorrect.'' (when
I use a >=28-byte
> packed string as the VARIANT).
>
>
> I''m wondering if the problem is one of:
>
> - the Win32::API function prototype I''m using for the VARIANT
param is
> wrong, or:
>
> - the way I''m initializing the VARIANT before passing it is
incorrect
>
> I''d be really grateful if anyone could weigh in on how to achieve
this. :)
>
> Right now I''m treating the VARIANT as a pointer in the get_accName
prototype
> (note the full example program this came from is at the end of the email):
>
> ? Get_accName = Win32::API::Function.new(table[10],
''PPP'', ''L'')
>
> The C prototype for the function (from OleAcc.h) is:
>
> ? /* [id][propget][hidden] */ HRESULT ( STDMETHODCALLTYPE *get_accName )(
> ????? __RPC__in IAccessible * This,
> ????? /* [optional][in] */ VARIANT varChild,
> ????? /* [retval][out] */ __RPC__deref_out_opt BSTR *pszName);
>
>
> I combed through the windows-pr code looking for examples of functions that
> take a VARIANT param, but so far I''ve only found functions that
take a
> pointer-to-VARIANT.
>
> Also, I suspect this code, which initializes my VARIANT, might be causing
my
> problem (it was cobbled together from snippets on the win32utils-devel list
> and the pr-win32ole source):
>
> ? # Initialize VARIANT that holds CHILDID_SELF
> ? self_var = 0.chr * 16
> ? VariantInit(self_var)
> ? self_var[0, 2] = [VT_I4].pack(''S'')
> ? self_var[2, 2] = [0].pack(''S'')
> ? self_var[4, 2] = [0].pack(''S'')
> ? self_var[6, 2] = [0].pack(''S'')
> ? self_var[8, 4] = [CHILDID_SELF].pack(''L'')
> ? self_var[12, 4] = [0].pack(''L'')
>
>
> Here''s a complete script that shows the get_accName problem -
please let me
> know if there are any questions I could answer that might help resolve the
> issue.? If it would help I also have a working C demo program where
> get_accName is used.
>
> # print_desktop_name.rb
>
> require ''rubygems''
> require ''win32/api''
> require ''windows/com''
> require ''windows/com/variant''
> require ''windows/error''
> require ''windows/msvcrt/buffer''
> require ''windows/unicode''
> require ''windows/window''
> require ''windows/window/menu''
>
> include Windows::COM
> include Windows::COM::Variant
> include Windows::Error
> include Windows::MSVCRT::Buffer
> include Windows::Unicode
> include Windows::Window
> include Windows::Window::Menu
>
> IID_IAccessible = [0x618736e0, 0x3c3d, 0x11cf, 0x81, 0x0c, 0x00, 0xaa,
0x00,
> 0x38, 0x9b, 0x71].pack(''LSSC8'')
> CHILDID_SELF = 0
>
> CoInitialize(nil) # Initialize COM
>
> desktop_hwnd = GetDesktopWindow.call() # Get the desktop''s HWND
>
> # Use the HWND to get the desktop''s IAccessible object
> AccessibleObjectFromWindow =
Win32::API.new(''AccessibleObjectFromWindow'',
> ??????????????????????????????????????????? ''LLPP'',
''L'', ''oleacc'')
> desktop_iacc = 0.chr * 4
> hr = AccessibleObjectFromWindow.call(desktop_hwnd,
> ???????????????????????????????????? OBJID_WINDOW,
> ???????????????????????????????????? IID_IAccessible,
> ???????????????????????????????????? desktop_iacc)
> raise "Failed to instantiate IAcc object!" if (hr != S_OK)
>
> # Get the memory address of the IAcc pointer - need to use it later as a
> # ''this'' pointer to pass to get_accName
> desktop_iacc_ptr = desktop_iacc.unpack(''L'').first
>
> # Get the IAccessibleVtbl C interface. It contains 28 functions (see
> OleAcc.h).
> lpVtbl = 0.chr * 4
> table = 0.chr * (4 * 28)
>
> # Unpack the IAccessibleVtbl:
> memcpy(lpVtbl, desktop_iacc_ptr, 4)
> memcpy(table, lpVtbl.unpack(''L'').first, 4 * 28)
> table = table.unpack(''L*'')
>
> # Define get_accName
> Get_accName = Win32::API::Function.new(table[10], ''PPP'',
''L'')
>
> # Create buffer for the BSTR that will receive the IAcc object name
> name_bstr = 0.chr * 4
>
> # Initialize CHILDID_SELF variant
> self_var = 0.chr * 16
> VariantInit(self_var)
> self_var[0, 2] = [VT_I4].pack(''S'')
> self_var[2, 2] = [0].pack(''S'')
> self_var[4, 2] = [0].pack(''S'')
> self_var[6, 2] = [0].pack(''S'')
> self_var[8, 4] = [CHILDID_SELF].pack(''L'')
> self_var[12, 4] = [0].pack(''L'')
>
> # This segfaults on XP and Win7:
> # (using ruby 1.8.7 i386-mingw32, 2010-08-16 patchlevel 302)
> hr = Get_accName.call(desktop_iacc_ptr, self_var, name_bstr)
> puts "HRESULT from get_accName is: " + hr.to_s
> puts "HRESULT message is:
''#{get_last_error(hr)}''"
>
After some debugging, I found that passing VARIANT parameter is same
to passing four DWORD parameters  with each VARIANT content element.

Get_accName = Win32::API::Function.new(table[10], ''PPP'',
''L'')
should be
Get_accName = Win32::API::Function.new(table[10], ''PLLLLP'',
''L'')

hr = Get_accName.call(desktop_iacc_ptr, self_var, name_bstr)
should be
hr =
Get_accName.call(desktop_iacc_ptr,self_var[0,4].unpack(''L'').first,self_var[4,4].unpack(''L'').first,self_var[8,4].unpack(''L'').first,self_var[12,4].unpack(''L'').first,name_bstr)

Regards,
Park Heesob

On Tue, Dec 14, 2010 at 6:36 PM, Heesob Park <phasis at gmail.com> wrote:

> After some debugging, I found that passing VARIANT parameter is same
> to passing four DWORD parameters  with each VARIANT content element.
>
> Get_accName = Win32::API::Function.new(table[10], ''PPP'',
''L'')
> should be
> Get_accName = Win32::API::Function.new(table[10],
''PLLLLP'', ''L'')
>
> hr = Get_accName.call(desktop_iacc_ptr, self_var, name_bstr)
> should be
> hr >
Get_accName.call(desktop_iacc_ptr,self_var[0,4].unpack(''L'').first,self_var[4,4].unpack(''L'').first,self_var[8,4].unpack(''L'').first,self_var[12,4].unpack(''L'').first,name_bstr)
>

Outstanding!  The code works great now.  With your changes in place,
get_accName return S_OK, and the BSTR contains the expected string
(''Desktop'') when I print it with wprintf:

  Wprintf = Windows::API.new(''wprintf'',
''PP'', ''I'', ''msvcrt'')

  format_str = multi_to_wide("The object name is:
''%s''\n")
  format_str_ptr =
[format_str].pack(''p'').unpack(''L'').first
  Wprintf.call(format_str_ptr, name_bstr.unpack(''L'').first)

This is huge; being able to pass a VARIANT means that the IAccessible and
IUIAutomation interfaces are now wide open. :)

Thanks so much!

Bill
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