Weird EXC_BAD_ACCESS in a trivial PDFKit program

Question

I have written this trivial action method associated to a textfield.
Every time I enter text into a textfield a search in a PDF is performed and PDFView automatically scroll to selection:

- (IBAction) search:(id)id
{
    NSString *query = [self.searchView stringValue]; // get from textfield
    selection = [document findString: query fromSelection:NULL withOptions:NSCaseInsensitiveSearch]; 
    if (selection != nil)
    {
        [self.pdfView setCurrentSelection:selection];
        [self.pdfView scrollSelectionToVisible:self.searchView];
    } 
}

Problem is that after 3 or 4 searches I get EXC_BAD_ACCESS on row (i).
If I debug I see that query contains an NSCFString and not an NSString.
I think it is a memory management problem..but where?

I replicated the same issue inside a trivial testcase:

  @interface PDFRef_protoTests : SenTestCase {
   @private

      PDFDocument *document;

   }

  ........

 - (void)setUp
  {
     [super setUp];
     document = [[PDFDocument alloc] initWithURL: @"a local url ..."];
  }

- (void)test_exc_bad_access_in_pdfdocument
{
    for (int i =0 ;i<100; i++)
    {
        NSString *temp;
        if (i % 2 == 0) temp = @"home";
        else if (i % 3 ==0) temp = @"cocoa";
        else temp=@"apple";
        PDFSelection *selection = [document findString: temp 
                                   fromSelection:nil 
                                withOptions:NSCaseInsensitiveSearch]; 
        NSLog(@"Find=%@, iteration=%d", selection, i);
    }
}

Update:

1) It seems that it happens also if I use asynchronous search (method beginFindString: withOptions) every time I perform second search.

2) I found a similar problem to mine in MacRuby Issue Tracking: http://www.macruby.org/trac/ticket/1029

3) It seems that if I disable temporarily garbage collection it works but memory goes up. I wrote something like:

[[NSGarbageCollector defaultCollector] disable];
[[NSGarbageCollector defaultCollector] enable];

surrounding search code

Another Update

Very weird thing is that sometimes all works. Than I clean and Rebuild and problem arises again. From a certain point of view is is not 100% reproducible. I suspect a bug in PDFKit or some compiler setting I have to do

Update Again

Dears it seems very crazy. I'd concentrate on testcase which is very trivial and which replicates easily the problem. What's wrong with it? This testcase works only if I disable (by code or by project setting) GC

Another Update

Boys it seems a bug but I downloaded an example called PDFLinker from Apple website (http://developer.apple.com/library/mac/#samplecode/PDFKitLinker2/Introduction/Intro.html#//apple_ref/doc/uid/DTS10003594). This example implements a PDFViewer. Code of my app and this example are quite similars. For the same search action on same PDF, my memory rises at 300/400 MB while PDFLinker rises at 190MB. Clearly there is something wrong in my code. But I am comparing it bit by bit and I don't think I am inserting memory leaks (and Instrument doesn't give me any evidence). Maybe is there some project-wide setting ?

Update Yet Changing from 64 bit to 32 bit memory consumption lowered. Surely there is a problem with 64bit and PDFKit. BTW still EXC_BAD_ACCESS on second search

SOLUTION Crucial point is that PDFKit with Garbage collection is bugged. If I disable GC all works correctly. I had another issue that had complicated my analysis: I disabled GC on Project Setting but GC remained enabled on Target Settings. So Apple's example PDFLinked2 worked while mine not.

Answer 1

This sort of thing is usually evidence that you're hanging onto a pointer to an object that has been destroyed. Turn on zombie objects (with NSZombieEnabled) to see exactly where and when you're accessing a bad object.

Answer 2

I agree you have found a bug in PDFKit.

I got various forms of errors (segmentation fault, selector not understood, etc) running your test case. Wrapping the code in @try/@catch doesn't prevent all errors associated with this method.

I also got errors printing the log message.

To work around the bug(s), I suggest you disable GC during your invocation of -findString:fromSelection:, as you've already discovered.

Also, be sure to make copies of the values of interest from selection before re-enabling GC. Don't just copy selection either.

If you conduct searches from multiple places in your code I also suggest you extract a separate method to perform the search. Then you can invoke that one to conduct the searches for you without duplicating the GC disable/enable nesting.

Answer 3

Judging from your screen shot it doesn't seem like you have NSZombie turned on. Probably the reason why it doesn't help you. Here's how you turn it on:

How to enable NSZombie in Xcode?

The screenshot you provided was otherwise very useful, but you really need NSZombie to figure out this kind of errors. Well, unless it's obvious, which it isn't from the code you posted.

---

EDIT: I read the comment that you're using garbage collection. I'm an iOS developer, so I have very limited experience with garbage collection in Objective-C, but as far as I understand NSZombie doesn't work in a garbage collected environment.

I'm not sure it should be possible to get EXC_BAD_ACCESS in a garbage collected environment, unless you create your own pointer and try to call methods on it without having created an object and I don't see why you would do that.

I've heard that some frameworks doesn't work well with garbage collection, but I wouldn't think PDFKit was among them. Anyway, the solution might be to not use garbage collection. Perhaps you should file a bug report with Apple.

Answer 4

Did you try retaining the searchview stringvalue object before using it?

As you say it happens when you type fast and it happens for async calls, it is possible that the object stringValue is pointing to is being released between the time your query object is pointing to it, and the time you use it int the search.

You could try something like this to see if the problem persists:

- (IBAction) search:(id)id
{
    NSString *query = [[self.searchView stringValue] retain]; // get from textfield
    selection = [document findString: query fromSelection:NULL withOptions:NSCaseInsensitiveSearch]; 
    if (selection != nil)
    {
        [self.pdfView setCurrentSelection:selection];
        [self.pdfView scrollSelectionToVisible:self.searchView];
    } 

    [query release];

}

Of course there is also the possibility that document is relased. How do you declare it? is it a property with retain? Can it be released by the time you are searching?

EDIT:

I see that you posted the code with the second parameter as NULL, but in your screenshot, this value is nil.

The documentation says you should use NULL when you want to start the search from the beginning.

http://developer.apple.com/library/mac/#documentation/GraphicsImaging/Reference/QuartzFramework/Classes/PDFDocument_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40003873-RH2-DontLinkElementID_1

And as the compiler interprets nil and NULL differently, this could be leading to some weird behavior internally.

Answer 5

keep PDFSelection *selection as a member variable and pass it in fromSelection: instead of nil.

It is possible that PDFDocument keeps the returned PDFSelection instance to improve the performance.