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passing array to C DLL

hey

i am having a few problems calling a C DLL from C#. i am using a simple
function that takes an array of floats and an integer as an input, but
i cannot seem to get it to work. when i try to compile i get the
following error:

Attempted to read or write protected memory

the C function should not be manipulating the input arra, only reading
it. can anyone help? code is below:

namespace DLLTest
{
class HellDLL
{
[DllImport("FFT. DLL")]
public static extern unsafe void FFT(float[] x, int n);
}
class Program
{
static void Main(string[] args)
{
int i, N = 4096;
float[] noise = new float[N];

for (i = 0; i < N; i++)
{
noise[i] = (float)i;
}

unsafe
{
HellDLL.FFT(noi se, N);
}

}
}
}

Apr 4 '06 #1
17 3668
Can you provide the declaration of the function in C? It's impossible
to say what is wrong without knowing that first.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m

<mr*********@gm ail.com> wrote in message
news:11******** **************@ i40g2000cwc.goo glegroups.com.. .
hey

i am having a few problems calling a C DLL from C#. i am using a simple
function that takes an array of floats and an integer as an input, but
i cannot seem to get it to work. when i try to compile i get the
following error:

Attempted to read or write protected memory

the C function should not be manipulating the input arra, only reading
it. can anyone help? code is below:

namespace DLLTest
{
class HellDLL
{
[DllImport("FFT. DLL")]
public static extern unsafe void FFT(float[] x, int n);
}
class Program
{
static void Main(string[] args)
{
int i, N = 4096;
float[] noise = new float[N];

for (i = 0; i < N; i++)
{
noise[i] = (float)i;
}

unsafe
{
HellDLL.FFT(noi se, N);
}

}
}
}

Apr 4 '06 #2
hey

here is the C function declaration:

DLLIMPORT void FFT(float x[], int n);

the function is a high performance FFT algorithm, it mallocs a bunch of
arrays, copies the input array x into one of them, does a load of maths
using SSE intrinsics, then copies the result back into x.

is this a suitable way to go about this? for some strange reason the
program worked the first time i ran it, but since then it gives me a
corrupt memory error.

can anyone help?

Apr 4 '06 #3
>public static extern unsafe void FFT(float[] x, int n);
You might want to try
public static extern unsafe void FFT([In,Out]float[] x, int n);
^^^^^^
In think you might need the Out attribute since the native method is updating the array.

Also, do really need to mark it as unsafe?

Marcus

mr*********@gma il.com wrote:
hey

i am having a few problems calling a C DLL from C#. i am using a simple
function that takes an array of floats and an integer as an input, but
i cannot seem to get it to work. when i try to compile i get the
following error:

Attempted to read or write protected memory

the C function should not be manipulating the input arra, only reading
it. can anyone help? code is below:

namespace DLLTest
{
class HellDLL
{
[DllImport("FFT. DLL")]
public static extern unsafe void FFT(float[] x, int n);
}
class Program
{
static void Main(string[] args)
{
int i, N = 4096;
float[] noise = new float[N];

for (i = 0; i < N; i++)
{
noise[i] = (float)i;
}

unsafe
{
HellDLL.FFT(noi se, N);
}

}
}
}

Apr 4 '06 #4
ok, have removed unsafe, currently calling it as:

[DllImport("FFT. dll")]
public static extern void FFT([Out]float[] x, int n);

but with the same problem. i have worked out that the error only occurs
when the C DLL tries to use the SSE commands such as:

_mm_load_ps
_mm_store_ps

etc. can you think why this error is occuring? i can call the DLL just
fine from a C program, so it is definitely functioning.

Apr 4 '06 #5
Note that you can't pass the C# float array to SSE instructions directly.
There is no guarantee that the managed array is 16 byte aligned, so you'll
have to marshal the array to a 16 byte aligned array before passing to SSE.
I'm not entirely clear why you want to use this from managed code anyway,
you won't realize any performance gain by marshaling from managed to
unmanaged, you better use unmanaged or managed code only for this.

Willy.

<mr*********@gm ail.com> wrote in message
news:11******** **************@ z34g2000cwc.goo glegroups.com.. .
| ok, have removed unsafe, currently calling it as:
|
| [DllImport("FFT. dll")]
| public static extern void FFT([Out]float[] x, int n);
|
| but with the same problem. i have worked out that the error only occurs
| when the C DLL tries to use the SSE commands such as:
|
| _mm_load_ps
| _mm_store_ps
|
| etc. can you think why this error is occuring? i can call the DLL just
| fine from a C program, so it is definitely functioning.
|
Apr 4 '06 #6
Willy,
I'm not entirely clear why you want to use this from managed code anyway,
you won't realize any performance gain by marshaling from managed to
unmanaged, you better use unmanaged or managed code only for this. For my own understanding, you are only referring to marshallings the 16 byte aligned arrays here? For the non-aligned
array, my understanding (from [1]) is that that marshallings overhead is minimal when marshallings a float/double array.
The runtime just pins the pointer to the array and passes it on to the unmanaged function.

I'm curious if using _mm_loadu_ps and _mm_storeu_ps instead of _mm_load_ps and _mm_store_ps would solve the problem.

Thanks,
Marcus
[1] http://msdn2.microsoft.com/en-us/library/75dwhxf7.aspx
Willy Denoyette [MVP] wrote: Note that you can't pass the C# float array to SSE instructions directly.
There is no guarantee that the managed array is 16 byte aligned, so you'll
have to marshal the array to a 16 byte aligned array before passing to SSE.
I'm not entirely clear why you want to use this from managed code anyway,
you won't realize any performance gain by marshaling from managed to
unmanaged, you better use unmanaged or managed code only for this.

Willy.

<mr*********@gm ail.com> wrote in message
news:11******** **************@ z34g2000cwc.goo glegroups.com.. .
| ok, have removed unsafe, currently calling it as:
|
| [DllImport("FFT. dll")]
| public static extern void FFT([Out]float[] x, int n);
|
| but with the same problem. i have worked out that the error only occurs
| when the C DLL tries to use the SSE commands such as:
|
| _mm_load_ps
| _mm_store_ps
|
| etc. can you think why this error is occuring? i can call the DLL just
| fine from a C program, so it is definitely functioning.
|

Apr 5 '06 #7
i am not using the float array directly, in the C DLL i am assigning
aligned space using _aligned_malloc for 3 arrays and the float[] input
is being copied to one of these arrays. all the SSE work is then
performed on the 3 aligned arrays, and this is what causes the error
for some reason.
I'm not entirely clear why you want to use this from managed code anyway,
you won't realize any performance gain by marshaling from managed to
unmanaged, you better use unmanaged or managed code only for this.


i was also under the impression that the performance overhead would be
minimal for marshalling the float[]. if i could get the thing to run i
would give you some numbers :)

i am doing this using managed code because i am trying to create a
simple app for recording hydrophone data, and the only demanding work
it does is a bunch of FFTs. i have written an FFT algorithm in managed
code but the performance hit is quite large, a 16k FFT in C# takes
something on the order of 5-8ms whereas the C DLL using SIMD commands
rarely takes more than 1ms. i thought it would be easy to call it from
C# thus speeding up the app considerably (each iteration performs 12
FFTs roughly) and saving me the hassle of writing the rest of the app
in C++.

Apr 5 '06 #8
Marcus,

Yes, it should, but you will loose another couple of cycles as these
intrinsics are a lot slower than the aligned ones.

Willy.

"Marcus Cuda" <ma****@newsgro up.nospam> wrote in message
news:%2******** *******@TK2MSFT NGP04.phx.gbl.. .
| Willy,
|
| > I'm not entirely clear why you want to use this from managed code
anyway,
| > you won't realize any performance gain by marshaling from managed to
| > unmanaged, you better use unmanaged or managed code only for this.
| For my own understanding, you are only referring to marshallings the 16
byte aligned arrays here? For the non-aligned
| array, my understanding (from [1]) is that that marshallings overhead is
minimal when marshallings a float/double array.
| The runtime just pins the pointer to the array and passes it on to the
unmanaged function.
|
| I'm curious if using _mm_loadu_ps and _mm_storeu_ps instead of _mm_load_ps
and _mm_store_ps would solve the problem.
|
| Thanks,
| Marcus
| [1] http://msdn2.microsoft.com/en-us/library/75dwhxf7.aspx
|
|
| Willy Denoyette [MVP] wrote:
| > Note that you can't pass the C# float array to SSE instructions
directly.
| > There is no guarantee that the managed array is 16 byte aligned, so
you'll
| > have to marshal the array to a 16 byte aligned array before passing to
SSE.
| > I'm not entirely clear why you want to use this from managed code
anyway,
| > you won't realize any performance gain by marshaling from managed to
| > unmanaged, you better use unmanaged or managed code only for this.
| >
| > Willy.
| >
| > <mr*********@gm ail.com> wrote in message
| > news:11******** **************@ z34g2000cwc.goo glegroups.com.. .
| > | ok, have removed unsafe, currently calling it as:
| > |
| > | [DllImport("FFT. dll")]
| > | public static extern void FFT([Out]float[] x, int n);
| > |
| > | but with the same problem. i have worked out that the error only
occurs
| > | when the C DLL tries to use the SSE commands such as:
| > |
| > | _mm_load_ps
| > | _mm_store_ps
| > |
| > | etc. can you think why this error is occuring? i can call the DLL just
| > | fine from a C program, so it is definitely functioning.
| > |
| >
| >
Apr 5 '06 #9
Sorry, but you got a AV exception, that means you are reading/writing
from/to a read/write protected piece of memory. You also said that the C DLL
is working correctly when called from C, that would mean that: or, the AV is
due to the managed/unmanaged interop, or due to the piece of code that
passes the float[] elements from unmanaged code to managed code. I would
suggest you run this in the (unmanaged) debugger.
I'm also not clear on what you are doing exactly in your FFT alorithm and
what version of the framework you are using, but, as CLR sits in top of the
exact same C runtime when doing float maths (using SIMD when available), the
performance figures should be comparable.
Willy.
<mr*********@gm ail.com> wrote in message
news:11******** **************@ t31g2000cwb.goo glegroups.com.. .
|i am not using the float array directly, in the C DLL i am assigning
| aligned space using _aligned_malloc for 3 arrays and the float[] input
| is being copied to one of these arrays. all the SSE work is then
| performed on the 3 aligned arrays, and this is what causes the error
| for some reason.
|
| > I'm not entirely clear why you want to use this from managed code
anyway,
| > you won't realize any performance gain by marshaling from managed to
| > unmanaged, you better use unmanaged or managed code only for this.
|
| i was also under the impression that the performance overhead would be
| minimal for marshalling the float[]. if i could get the thing to run i
| would give you some numbers :)
|
| i am doing this using managed code because i am trying to create a
| simple app for recording hydrophone data, and the only demanding work
| it does is a bunch of FFTs. i have written an FFT algorithm in managed
| code but the performance hit is quite large, a 16k FFT in C# takes
| something on the order of 5-8ms whereas the C DLL using SIMD commands
| rarely takes more than 1ms. i thought it would be easy to call it from
| C# thus speeding up the app considerably (each iteration performs 12
| FFTs roughly) and saving me the hassle of writing the rest of the app
| in C++.
|
Apr 5 '06 #10

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