CODEWITHSUNDEEP
memory-leakscore-graphics
Adam Evans
Adam Evans

Reputation: 2082

Xcode UIImage imageWithCGImage leaks memory

I have ARC enabled in my app and noticed that if I create a ton of images, my app would crash. As part of my investigation, I created a small project that reproduces the issue which can be found here. The meat of the sample project code is the following:

int width = 10;
int height = 10;
uint8_t data[100];

while (true)
{
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceGray();
    CGContextRef context = CGBitmapContextCreate(data, width, height, 8, width, colorspace, kCGBitmapByteOrderDefault | kCGImageAlphaNone);
    CGImageRef cgimage = CGBitmapContextCreateImage(context);

    // Remove this line and memory is stable, keep it and you lose 15-20 MB per second. Why?
    UIImage* uiimage = [UIImage imageWithCGImage:cgimage];

    CGImageRelease(cgimage);
    CGContextRelease(context);
    CGColorSpaceRelease(colorspace);
}

While running this code, the sidebar in Xcode will show the total memory of the app increasing at around 15-20 MB per second. If you comment out the line that creates the UIImage, the leak disappears.

There are a number of questions on Stack Overflow about whether or not you should release a CGImage after creating a UIImage via imageWithCGImage, and it doesn't look like there is a real consensus. However, if I don't call CGImageRelease(cgimage), then the memory usage increases by over 100 MB per second, so I'm certain that manually releasing the image is the correct thing to do.

Since I have ARC enabled, I tried setting uiimage to nil after releasing everything, which didn't work. Not storing the return value of the call to imageWithCGImage: also doesn't prevent the leak.

Is there something fundamental I'm missing about how to use Core Graphics?

Upvotes: 0

Views: 192

Answers (1)

matt
matt

Reputation: 535178

Is there something fundamental I'm missing about how to use Core Graphics?

It seems more likely that you are missing something fundamental about memory management.

Many Foundation / Cocoa framework commands, especially those that create ready-made objects, make objects that are autoreleased. That means you don't have to release the object, because it will be released later, automatically, when you are done with it. But how is that possible? Such objects go into the autorelease pool and, once their retain count drops to zero, they are drained later on, when there is an opportunity. But you are looping continuously, so there is no such opportunity. So you need to wrap your troublesome line in an @autoreleasepool{} block so as to construct and drain your own pool.

Also note that there can be intermediate autoreleased objects of which you are unaware. The autorelease pool can help with those too.

See this section of my book for more information about autoreleased objects.

Upvotes: 1

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