C# struct variables remain null when passed into unmanaged C DLL functions

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Problem:

I am writing a C# wrapper for an unmanaged C DLL. The DLL contains a struct zint_symbol, which contains a variable char[] *bitmap. In my C# struct I have public byte[] bitmap;.

This struct is passed in to an external function ZBarcode_Encode_and_Buffer which is supposed to render a PNG image and write the bitmap data to bitmap in zint_symbol.

When I invoke ZBarcode_Encode_and_Buffer and check bitmap, it remains null.

In fact, every variable in my C# struct that is supposed to be written by the DLL remains null.

What I've tried:

1. 
   

   Marshalling bitmap as a fixed-size array with a very large constant, i.e.,

   
   

   
   [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U1, SizeConst = 25454)]
   public byte[] bitmap; 
   
   

   
   

   ...which yields a SystemAccessViolation:

   
   

   
   An unhandled exception of type 'System.AccessViolationException' occurred in mscorlib.dll
   
   Additional information: Attempted to read or write protected memory. This is often an indication     that other memory is corrupt.
   
   



2. 
   

   Changing the type of ibtmap to char[] and string. This changed nothing.

   
   


3. 
   

   Adding the [In,Out] decorator to the struct parameter I am passing into my C# function.

   
   


4. 
   

   Changing the type of bitmap from byte[] to IntPtr.

   
   

What I cannot do:

I cannot seem to compile the library, despite having the source (the project is open source but abandoned years ago). This means I cannot change the C code.

Code/Working example

I left only certain things in for brevity. A small but complete working example can be downloaded here .

C# class containing my struct:

class ZintLib
{
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
    public struct zint_symbol
    {
        public byte[] bitmap;

        public int bitmap_width;
        public int bitmap_height;
        public IntPtr rendered;
    }

    [DllImport("zint.dll", EntryPoint = "ZBarcode_Create", CallingConvention = CallingConvention.Cdecl)]
    public extern static IntPtr Create();

    [DllImport("zint.dll", EntryPoint = "ZBarcode_Encode_and_Buffer", CallingConvention = CallingConvention.Cdecl)]
    public extern static int EncodeAndBuffer(
      [In, Out] ref zint_symbol symbol,
     String input,
     int length,
     int rotate_angle);
}

The function calling EncodeAndBuffer:

        // call DLL function to generate pointer to initialized struct
        ZintLib.zint_symbol s = (ZintLib.zint_symbol)

        // generate managed counterpart of struct
        Marshal.PtrToStructure(ZintLib.Create(), typeof(ZintLib.zint_symbol));

        // change some settings
        s.symbology = 71;
        s.outfile = "datamatrix.png";

        // DLL function call to generate output file using changed settings -- WORKS --
        //System.Console.WriteLine(ZintLib.EncodeAndPrint(ref s, "12345", 5, 0));

        // DLL function to generate data in s.bitmap, s.bitmapheight, s.bitmapwidth -- DOES NOT WORK managed struct is unaltered --
        System.Console.WriteLine(ZintLib.EncodeAndBuffer(ref s, (String)"12345", 5, 0));

        if (s.bitmap == null)
            Console.WriteLine("bitmap is null.");
        else
            Console.WriteLine("bitmap is not null.");

The struct and export methods in C:

struct zint_symbol {
    int symbology;
    int height;
    int whitespace_width;
    int border_width;
    int output_options;
    char fgcolour[10];
    char bgcolour[10];
    char outfile[256];
    float scale;
    int option_1;
    int option_2;
    int option_3;
    int show_hrt;
    int input_mode;
    unsigned char text[128];
    int rows;
    int width;
    char primary[128];
    unsigned char encoded_data[178][143];
    int row_height[178]; /* Largest symbol is 177x177 QR Code */
    char errtxt[100];
    char *bitmap;
    int bitmap_width;
    int bitmap_height;
    struct zint_render *rendered;
};

#if defined(__WIN32__) || defined(_WIN32) || defined(WIN32) || defined(_MSC_VER)
#  if defined (DLL_EXPORT) || defined(PIC) || defined(_USRDLL)
#    define ZINT_EXTERN __declspec(dllexport)
#  elif defined(ZINT_DLL)
#    define ZINT_EXTERN __declspec(dllimport)
#  else
#    define ZINT_EXTERN extern
#  endif
#else
#  define ZINT_EXTERN extern    
#endif

ZINT_EXTERN struct zint_symbol *ZBarcode_Create(void);
ZINT_EXTERN void ZBarcode_Clear(struct zint_symbol *symbol);
ZINT_EXTERN void ZBarcode_Delete(struct zint_symbol *symbol);

ZINT_EXTERN int ZBarcode_Encode(struct zint_symbol *symbol, unsigned char *input, int length);
ZINT_EXTERN int ZBarcode_Encode_File(struct zint_symbol *symbol, char *filename);
ZINT_EXTERN int ZBarcode_Print(struct zint_symbol *symbol, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_and_Print(struct zint_symbol *symbol, unsigned char *input, int length, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_File_and_Print(struct zint_symbol *symbol, char *filename, int rotate_angle);

ZINT_EXTERN int ZBarcode_Render(struct zint_symbol *symbol, float width, float height);

ZINT_EXTERN int ZBarcode_Buffer(struct zint_symbol *symbol, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_and_Buffer(struct zint_symbol *symbol, unsigned char *input, int length, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_File_and_Buffer(struct zint_symbol *symbol, char *filename, int rotate_angle);

ZINT_EXTERN int ZBarcode_ValidID(int symbol_id);

#ifdef __cplusplus
}
#endif /* __cplusplus */

#endif /* ZINT_H */

Note: A similar question was asked but never resolved in this thread. I've been scratching my head over this for a week. Any ideas?

asked 32 secs ago

josh

1,293

C# struct variables remain null when passed into unmanaged C DLL functions