CGImageRef gets corrupted after returned from a method - objective-c

Question

My code creates an TIFFRepresentation of an image and I want to recode it to something different. This is not problematic.
My ImgUtils function is:
+ (CGImageRef) processImageData:(NSData*)rep {
NSBitmapImageRep *bitmapRep = [NSBitmapImageRep imageRepWithData:rep];
int width = bitmapRep.size.width;
int height = bitmapRep.size.height;
size_t pixels_size = width * height;
Byte raw_bytes[pixels_size * 3];
//
// processing, creates and stores raw byte stream
//
int bitsPerComponent = 8;
int bytesPerPixel = 3;
int bitsPerPixel = bytesPerPixel * bitsPerComponent;
int bytesPerRow = bytesPerPixel * width;
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL,
raw_bytes,
pixels_size * bytesPerPixel,
NULL);
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGImageRef imageRef = CGImageCreate(width,
height,
bitsPerComponent,
bitsPerPixel,
bytesPerRow,
colorSpaceRef,
bitmapInfo,
provider,
NULL,
NO,
renderingIntent);
[ImgUtils saveToPng:imageRef withSuffix:#"-ok"];
CGColorSpaceRelease(colorSpaceRef);
CGDataProviderRelease(provider);
return imageRef;
}
There is another method, that saves a CGImageRef to filesystem.
+ (BOOL) saveToPng:(CGImageRef)imageRef withSuffix:(NSString*)suffix {
CFURLRef url = (__bridge CFURLRef)[NSURL fileURLWithPath:[NSString stringWithFormat:#"~/Downloads/pic%#.png", suffix]];
CGImageDestinationRef destination = CGImageDestinationCreateWithURL(url, kUTTypePNG, 1, NULL);
CGImageDestinationAddImage(destination, imageRef, nil);
CGImageDestinationFinalize(destination);
CFRelease(destination);
return YES;
}
As you can see, immediately after processing the image, I save it on a disk, as pic-ok.png.
Here is the code, that calls the processing function:
CGImageRef cgImage = [ImgUtils processImageData:imageRep];
[ImgUtils saveToPng:cgImage withSuffix:#"-bad"];
The problem is, that the two images differ. Second one, with the -bad suffix is corrupted.
See examples below. Seems like the memory area the CGImageRef pointer is pointing to is released and overwritten immediately after returning from the method.
I tried also return CGImageCreateCopy(imageRef); but it changed nothing.
What am I missing?

Answer 1

CGDataProviderCreateWithData() does not copy the buffer you provide. Its purpose is to allow creation of a data provider that accesses that buffer directly.
Your buffer is created on the stack. It goes invalid after +processImageData: returns. However, the CGImage still refers to the provider and the provider still refers to the now-invalid buffer.
One solution would be to create the buffer on the heap and provide a callback via the releaseData parameter that frees it. Another would be to create a CFData from the buffer (which copies it) and then create the data provider using CGDataProviderCreateWithCFData(). Probably the best would be to create a CFMutableData of the desired capacity, set its length to match, and use its storage (CFDataGetMutableBytePtr()) as your buffer from the beginning. That's heap-allocated, memory-managed, and doesn't require any copying.