How to pack/unpack nested structures?

I am accessing the Win32 API from Ruby, and am having trouble dealing
with
structures that nest structures. How should I pack/unpack these objects?
Ruby/DL looks like it would give me a hand but I’m not sure how to use
it.

Here is an example of what I’m doing. I’m calling a function,
InitializeSecurityContext, which as one of its arguments takes a pointer
to
a SecBufferDesc structure. This structure contains a pointer to a
SecBuffer
structure. The structures look mostly like this:

struct SecBufferDesc {
long version;
long numBuffers;
SecurityBuffer * buffers;
}

struct SecurityBuffer {
long bufferSize;
long bufferType;
void * buffer;
}

I am unclear on how to pack a string to contain such a structure. This
sort
of works, but I am unable to access the inner structure:

version = 1
numBuffers = 1

bufferSize = 100
bufferType = 2
buffer = “\0” * bufferSize

struct = [version, numBuffers, [bufferSize, bufferType,
buffer].pack(“LLP”)].pack(“LLP”)

After calling the method with ‘struct’, I can unpack the outer structure

struct.unpack(“LLP12”) # gives SecBufferDesc structure

But when I try to access the inner SecBuffer structure, I get an errror:

secBuffPtr = struct.unpack(“LLP12”)[2]
secBuffPtr.unpack(“LLP12”) # gives ‘no associated pointer’ error

What’s interesting is the packing process seems to work, because the
function called correctly writes to the structures in memory. However, I
cannot then unpack the string to get access to the structures!

How can I pack the structure correctly to get to the inner structure?
Thanks
for any help in advance!

Justin

p.s. Here is a short file that demonstrates the problem, without any
need
for API calls:

version = 1
count = 1

size = 100
type = 2
buffer = “\0” * size

struct = [version, count, [size, type, buffer].pack(“LLP”)].pack(“LLP”)

outerStruct = struct.unpack(“LLP12”)
puts “structure is: #{outerStruct.inspect}”

begin
innerStruct = outerStruct[2].unpack(“LLP12”)
puts “innerStruct is #{innerStruct.inspect}”
rescue ArgumentError
puts “unable to unpack inner structure”
end

Hi,

At Thu, 13 Jul 2006 00:31:37 +0900,
Justin B. wrote in [ruby-talk:201532]:

version = 1
numBuffers = 1

bufferSize = 100
bufferType = 2
buffer = “\0” * bufferSize

I guess this buffer is the answer, unless the function returns
another pointer.

struct = [version, numBuffers, [bufferSize, bufferType,
buffer].pack(“LLP”)].pack(“LLP”)

or:

nested_struct = [bufferSize, bufferType, buffer].pack(“LLP”)
struct = [version, numBuffers, inner_struct].pack(“LLP”)

nested_struct.unpack(“LLP12”)

If the function alters buffer of struct SecurityBuffer, says it
allocates new buffer and returns it, unpack can’t deal with it.
You need DL instead.

On 7/14/06, [email protected] [email protected] wrote:

nested_struct = [bufferSize, bufferType, buffer].pack(“LLP”)
struct = [version, numBuffers, inner_struct].pack(“LLP”)

nested_struct.unpack(“LLP12”)

If the function alters buffer of struct SecurityBuffer, says it
allocates new buffer and returns it, unpack can’t deal with it.
You need DL instead.

Would you mind demonstrating how I can use DL to do it? I’d love to use
that
library but I wasn’t able to figure it out … Thanks for your time and
attention!

Justin

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