ios - unsigned char 分配在偏移值 objective-c 中为零

Question

我正在从触摸点获取像素颜色值。我成功地做到了这一点，但有时应用程序给出错误(EXC_BAD_ACCESS(CODE = 1，address = 0x41f6864)。它的内存分配问题这里是供您引用的源代码。

- (UIColor *) getPixelColorAtLocation:(CGPoint)point {
UIColor* color = nil;

@try{

    {

    CGImageRef inImage =  drawImage.image.CGImage;

// Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
if (cgctx == NULL)
{
    return nil; /* error */

}  
size_t w = CGImageGetWidth(inImage);
size_t h = CGImageGetHeight(inImage);
CGRect rect = {{0,0},{w,h}};

// Draw the image to the bitmap context. Once we draw, the memory
// allocated for the context for rendering will then contain the
// raw image data in the specified color space.
   CGContextDrawImage (cgctx, rect, inImage);


// Now we can get a pointer to the image data associated with the bitmap
// context.
        unsigned char *data = {0};
       data=(unsigned char*) calloc(CGImageGetHeight(inImage) * CGImageGetWidth(inImage) , CGBitmapContextGetHeight(cgctx)*CGBitmapContextGetWidth(cgctx));

data= CGBitmapContextGetData (cgctx);






  if( data !=NULL  ) {



    //offset locates the pixel in the data from x,y.
    //4 for 4 bytes of data per pixel, w is width of one row of data.


      int offset = 4*((w*round(point.y))+round(point.x));

           // NSLog(@"%s111111",data);

     int alpha =  data[offset]; /////// EXC_BAD_ACCESS(CODE=1,address=0x41f6864)           

    int red = data[offset+1];
    int green = data[offset+2];
    int blue = data[offset+3];



    //NSLog(@"offset: %i colors: RGB A %i %i %i  %i",offset,red,green,blue,alpha);
     color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];
    }
// When finished, release the context
//CGImageRelease(*data);
CGContextRelease(cgctx);

// Free image data memory for the context
if (data)
{

 free(data);  
}
}
@catch (NSException *exception) {
}
return color;
}

Answer 1

您代码中的内存管理似乎有误:

声明 data 并毫无意义地为其赋值:

unsigned char *data = {0};

分配一个内存块并将对它的引用存储在 data - 覆盖无意义的初始化:

data = (unsigned char *)calloc(CGImageGetHeight(inImage) * CGImageGetWidth(inImage), CGBitmapContextGetHeight(cgctx) * CGBitmapContextGetWidth(cgctx));

现在获取对不同 内存块的引用并将其存储在data 中，丢弃对calloc block 的引用:

data = CGBitmapContextGetData (cgctx);

做一些其他的事情然后释放你没有calloc的 block :

free(data);

如果您正在分配自己的内存缓冲区，您应该将其传递给 CGBitmapContextCreate，但是如果您使用的是 iOS 4+，则无需分配您自己的缓冲区。

至于内存访问错误，您没有对 point 的值进行检查，并且您的计算似乎产生了一个不正确的 offset 值。添加对 point 和 offset 值的检查，并在它们超出范围时采取适当的操作(您必须决定应该是什么)。

HTH