PInvoke - Issue while calling DJVU function from C# code. Attempted to read or write protected memory - pinvoke

Question

UPDATE 3-4-15:11IS
As recommended by David modified PInvoke as below, this time I am getting different error "Unhandled exception of type System.ExecutionEngineException occurred in mscorlib.dll"
[DllImport("C:\\Program Files\\DJVULIBRE\\LIBDJVULIBRE.dll", CharSet=CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
private unsafe static extern int ddjvu_page_render(IntPtr page, ddjvu_render_mode_t mode, ref ddjvu_rect_t pagerect,
ref ddjvu_rect_t renderrect,
IntPtr pixelformat,
uint rowsize,
[Out][MarshalAs(UnmanagedType.LPArray)]byte[] imagebuffer);
Thanks David for your valuable time, I think is close to a fix.
HISTORY
I know there are many questions in this subject but none of them help to resolve the issue I am currently facing.
Below is the API function in C Language
DDJVUAPI int
ddjvu_page_render(ddjvu_page_t *page,
const ddjvu_render_mode_t mode,
const ddjvu_rect_t *pagerect,
const ddjvu_rect_t *renderrect,
const ddjvu_format_t *pixelformat,
unsigned long rowsize,
char *imagebuffer );
Below is the PInvoke signature of C Function added in .NET code
[DllImport("C:\\Program Files\\DJVULIBRE\\LIBDJVULIBRE.dll", CharSet=CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
private unsafe static extern int ddjvu_page_render(IntPtr page, ddjvu_render_mode_t mode, IntPtr pagerect,
IntPtr renderrect,
IntPtr pixelformat,
ulong rowsize,
[Out][MarshalAs(UnmanagedType.LPArray)]byte[] imagebuffer);
Below is how I am calling this function in the c# code
byte* buffer = (byte *)Memory.Alloc(nSize);
try
{
IntPtr ptr1 = (IntPtr)Memory.Alloc(Marshal.SizeOf(prect));
Marshal.StructureToPtr(prect, ptr1, false);
IntPtr ptr2 = (IntPtr)Memory.Alloc(Marshal.SizeOf(rrect));
Marshal.StructureToPtr(rrect, ptr2, false);
byte[] array = new byte[nSize];
fixed (byte* p = array) Memory.Copy(buffer, p, nSize);
ddjvu_page_render(page, ddjvu_render_mode_t.DDJVU_RENDER_MASKONLY, ptr1, ptr2, fmt, (ulong)stride, array);
}
finally
{
Memory.Free(buffer);
}
call to ddjvu_page_render in above code is throwing "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."
Prior to this post I must have tried all the option could find in various blogs.
Appreciate any help, is almost a day I am clueless, your timely help could save my job
UPDATE 3-4-15:7:44IS
This code is using DJVULibre
UPDATE 3-4-15:8:35IS
Here is the code I have in Form Load
ctx = ddjvu_context_create(System.AppDomain.CurrentDomain.FriendlyName);
if (ctx != null)
{
string djFile = "C:\\Users\\rammohan.chavakula\\Documents\\eiasample.djvu";
doc = ddjvu_document_create_by_filename(ctx, djFile, 100);
if (doc != null)
{
while (ddjvu_job_status(ddjvu_document_job(doc)) >= ddjvu_status_t.DDJVU_JOB_OK)
SpinDdjvuMessageLoop(ctx, true);
int pageCount = ddjvu_document_get_pagenum(doc);
mediaboxes = new Rectangle[pageCount];
for (int i = 0; i < pageCount; i++)
{
ddjvu_status_t status;
ddjvu_pageinfo_t info = new ddjvu_pageinfo_t();
while ((status = ddjvu_document_get_pageinfo_imp(doc, i, ref info, (uint)System.Runtime.InteropServices.Marshal.SizeOf(info))) < ddjvu_status_t.DDJVU_JOB_OK)
SpinDdjvuMessageLoop(ctx, true);
if (status != ddjvu_status_t.DDJVU_JOB_OK)
continue;
mediaboxes[i] = new Rectangle(0, 0, info.width / info.dpi,
info.height / info.dpi);
}
}
ddjvu_context_release(ctx);
}
In OnPaint function I have this below code
if (doc == null)
{
base.OnPaint(e);
return;
}
Rectangle pageRc = PageMediabox(1);
Rectangle screen = Transform(pageRc, 1, zoom, rotation, false);
Rectangle full = Transform(PageMediabox(1), 1, zoom, rotation, false);
full.Intersect(screen);
IntPtr page = ddjvu_page_create_by_pageno(doc, 1);
if (page == null )
{
base.OnPaint(e);
return;
}
int rotation4 = (((-rotation / 90) % 4) + 4) % 4;
ddjvu_page_set_rotation(page, (ddjvu_page_rotation_t)rotation4);
while (ddjvu_job_status(ddjvu_page_job(page)) >= ddjvu_status_t.DDJVU_JOB_OK)
SpinDdjvuMessageLoop(ctx, true);
if (ddjvu_job_status(ddjvu_page_job(page)) >= ddjvu_status_t.DDJVU_JOB_FAILED)
{
base.OnPaint(e);
return;
}
IntPtr fmt = ddjvu_format_create(ddjvu_format_style_t.DDJVU_FORMAT_BGR24, 0, (UIntPtr)null);
ddjvu_format_set_row_order(fmt, /* top_to_bottom */1);
ddjvu_rect_t prect = new ddjvu_rect_t(full.X, full.Y, (uint)full.Width, (uint)full.Height);
ddjvu_rect_t rrect = new ddjvu_rect_t(screen.X, 2 * full.Y + screen.Y + full.Height - screen.Height, (uint)screen.Width, (uint)screen.Height);
int stride = ((screen.Width * 3 + 3) / 4) * 4;
//byte tmp;
////ScopedMem<char> bmpData(SAZA(char, stride * (screen.dy + 5)));
//for (int y = 0; y < rrect.h; y++) {
// int step_y = y * SCREEN_WIDTH;
// for (int x=0; x < rrect.w; x++) {
// tmp = (byte)((imagebuf[x + step_y] >> 5) << 5);
// }
//}
int rowsize = mediaboxes[0].Width * 3;
int nSize = rowsize * (mediaboxes[0].Height) * 10;
unsafe
{
byte* buffer = (byte *)Memory.Alloc(nSize);
try
{
IntPtr ptr1 = (IntPtr)Memory.Alloc(Marshal.SizeOf(prect));
Marshal.StructureToPtr(prect, ptr1, false);
IntPtr ptr2 = (IntPtr)Memory.Alloc(Marshal.SizeOf(rrect));
Marshal.StructureToPtr(rrect, ptr2, false);
byte[] array = new byte[nSize];
fixed (byte* p = array) Memory.Copy(buffer, p, nSize);
ddjvu_page_render(page, ddjvu_render_mode_t.DDJVU_RENDER_MASKONLY, ptr1, ptr2, fmt, (ulong)stride, array);
}
finally
{
Memory.Free(buffer);
}
ddjvu_page_render should return arbitrary data of page which is to be rendered in a given rectangle area. Once after that I should be able to create image from arbitrary data & display in the screen

Answer 1

Here's how I would write the p/invoke:
[DllImport(dllname, CharSet=CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
private static extern int ddjvu_page_render(
[In] IntPtr page,
[In] ddjvu_render_mode_t mode,
[In] ref ddjvu_rect_t pagerect,
[In] ref ddjvu_rect_t renderrect,
[In] ref ddjvu_format_t pixelformat,
[In] uint rowsize,
[Out] byte[] imagebuffer
);
Note that I've stopped using unsafe, and am letting the marshaler handle the structs automatically.
There are quite a few assumptions here that I cannot verify:
The calling convention. It might not be cdecl. The C++ header will would have the definitive answer.
The first parameter is a void* handle type, I presume.
ddjvu_render_mode_t is an enum that you have translated correctly.
The three ref parameters are structs, passed by reference. I cannot check that you have translated them correctly.
Calling this function would be something like this:
byte[] imagebuffer = new byte[nSize];
int retval = ddjvu_page_render(
page,
ddjvu_render_mode_t.DDJVU_RENDER_MASKONLY,
ref prect,
ref rrect,
ref fmt,
(uint)stride,
imagebuffer
);
if (retval != ??)
// handle error
This is quite a broad brush answer. I hope it points you in the right direction. I don't think that all your problems will be solved by this.