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

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.