c# - 绑定(bind)到 Monotouch 中的第 3 方库 - 映射 uint8_t * 和 uint8_t **

Question

我正在尝试使用 monotouch 绑定(bind)到第 3 方条形码扫描库 - 目前一切正常，但库头文件中定义的以下方法除外:

/**
 * Main scan function. Invokes all activated decoders by priority.
 * For successful scan, allocates pp_data buffer and pass it to user.
 * User should deallocate *pp_data pointer when no more needed.
 *
 * @param[in]   pp_image                Byte array representing grayscale value of image pixels.
 *                                      Array shold be stored in row after row fashion, starting with top row.
 * @param[in]   lenX                    X axis size (width) of image.
 * @param[in]   lenY                    Y axis size (length) of image.
 * @param[out]  pp_data                 On successful decode, library allocates new byte array where it stores decoded
 *                                      string result. Pointer to string is passed here. User application is responsible
 *                                      for deallocating this buffer after use.
 *
 * @retval      >0                      Result string length for successful decode
 * @retval      MWB_RT_BAD_PARAM        Null pointer or out of range parameters
 * @retval      MWB_RT_NOT_SUPPORTED    Unsupported decoder found in execution list - library error
 */
extern int MWB_scanGrayscaleImage(uint8_t *  pp_image,  int lenX,  int lenY, uint8_t **pp_data);

我已经有一段时间没有直接处理 C 数据结构了，我不清楚如何映射 uint8_t * pp_image 和 uint8_t **pp_data。

第一个参数处理来自像素缓冲区的灰度图像。我正在从 CMSampleBuffer 获取图像缓冲区。它需要一个亮度转换的字节数组，字节数组的内存地址，还是将 pixelBuffer.GetBaseAddress(0) 作为 IntPtr 传递就足够了？

最后一个参数在一个指针中传递，该指针在 Objective-C 演示中初始化为 unsigned char *pResult=NULL;，然后在找到有效扫描时填充数据。同样，我不确定如何初始化和传入它，因为您不能在 C# 中传入未初始化/空字节数组。

目前我的绑定(bind)库代码如下(虽然我也尝试过使用IntPtr，通过ref传递，在不安全模式下传递直接地址):

[DllImport ("__Internal")]
extern static int MWB_scanGrayscaleImage (byte[] pp_image, int lenX, int lenY, out byte[] pp_data);
public static int ScanGrayscaleImage (byte[] pp_image, int lenX, int lenY, out byte[] pp_data)
{
    int result = MWB_scanGrayscaleImage(pp_image, lenX, lenY, out pp_data);
    return result;
}

到目前为止，对于我尝试过的所有操作，不断返回的结果值是 -1，映射到“扫描失败”。任何解决这个问题的帮助将不胜感激。

Answer 1

我建议将 uint8_t** 绑定(bind)为 IntPtr，因为该库将无法分配托管字节 []

也许是这样的:

[DllImport ("__Internal")]
extern static int MWB_scanGrayscaleImage (byte[] pp_image, int lenX, int lenY, out IntPtr pp_data);
public static int ScanGrayscaleImage (byte[] image, int lenX, int lenY, out byte[] data)
{
    IntPtr pp_data;

    int result = MWB_scanGrayscaleImage (image, lenX, lenY, out pp_data);
    if (result > 0) {
        data = new byte[result];
        Marshal.Copy (pp_data, data, 0, result);
        Marshal.FreeHGlobal (pp_data); // I think this is what you want...
    } else {
        data = null;
    }

    return result;
}