How To Fix Vector Overflow Bad Access

At one indicate or another, y'all volition come across a crash caused by EXC_BAD_ACCESS. In this quick tip, you will learn what EXC_BAD_ACCESS is and what it is caused by. I will also give you lot a few tips to fix bugs that are acquired past EXC_BAD_ACCESS.

ane. What Is EXC_BAD_ACCESS?

In one case you comprehend the underlying cause of EXC_BAD_ACCESS, you lot'll better understand its cryptic name. There'due south a elementary explanation and a more technical explanation. Allow'southward kickoff with the simple caption first.

Keeping It Simple

Whenever you encounter EXC_BAD_ACCESS, information technology means that y'all are sending a message to an object that has already been released. This is the virtually common scenario, but in that location are exceptions as we'll discuss in a moment.

What It Really Means

The technical explanation is a bit more than complex. In C and Objective-C, you constantly deal withpointers. A pointer is nil more than than a variable that stores the memory accost of some other variable. When you lot send a message to an object, the pointer that points to the object you're sending the message to needs to be dereferenced. This ways that you take the retentiveness accost the pointer is pointing to and access the value of that block of memory.

When that block of memory is no longer mapped for your application or, put differently, that block of retention isn't used for what yous think information technology'south used, it'south no longer possible to access that chunk of retentiveness. When this happens, the kernel sends an exception (EXC), indicating that your awarding cannot access that block of retention (BAD ACCESS).

In summary, when you run into EXC_BAD_ACCESS, it means that you effort to send a message to a cake of retention that can't execute that message.

In some cases, however, EXC_BAD_ACCESS is caused by a corrupt pointer. Whenever your awarding attempts to dereference a decadent arrow, an exception is thrown by the kernel.

2. Debugging EXC_BAD_ACCESS

Debugging EXC_BAD_ACCESS can be tricky and frustrating. All the same, now that EXC_BAD_ACCESS is no longer an enigma for y'all, it should be less daunting.

The commencement affair y'all need to empathise is that your application doesn't necessarily crash the moment the block of memory is no longer attainable by your application. That's what frequently makes debugging EXC_BAD_ACCESS then difficult.

The aforementioned is truthful for corrupt pointers. Your awarding won't crash considering a arrow went decadent. Information technology also won't crash if you pass a decadent pointer around in your application. When your application attempts to dereference the corrupt arrow, however, thing go wrong.

Zombies

While zombies accept gained in popularity over the past few years, they take been around in Xcode for more than than a decade. The namezombie may sound a bit dramatic, but it's actually a keen name for the feature that'southward going to help the states debug EXC_BAD_ACCESS. Permit me explain how it works.

In Xcode, you can enable zombie objects, which means deallocated objects are kept around as zombies. Put differently, deallocated objects are kept alive for debugging purposes. There's no magic involved. If y'all send a message to a zombie object, your application will all the same crash as a result of EXC_BAD_ACCESS.

Why is this useful? What makes EXC_BAD_ACCESS difficult to debug is that yous don't know what object your application was trying to access. Zombie objects solve this problem in many cases. Past keeping deallocated objects live, Xcode tin can tell y'all what object yous were trying to admission, making the search for the problem that much easier.

Enabling zombies in Xcode is very easy. Annotation that this may differ depending on the version of Xcode that you're using. The following approach applies to Xcode half-dozen and 7. Click the agile scheme in the top left and cullEdit Scheme.

SelectRun on the left and open up theDiagnosticstab at the top. To enable zombie objects, tick the checkbox labeledEnable Zombie Objects.

Enable zombie objects in the current scheme

If y'all now come across EXC_BAD_ACCESS, the output in Xcode's Console will give yous a much better idea of where to start your search. Take a look at the post-obit example output.

2015-08-12 06:31:55.501 Debug[2371:1379247] -[ChildViewController respondsToSelector:] message sent to deallocated instance 0x17579780

In the above case, Xcode is telling u.s. that a message of respondsToSelector: was sent to a zombie object. However, the zombie object is no longer an instance of theChildViewController class. The block of memory that was previously allocated to the ChildViewController instance is no longer mapped for your application. This should give you a pretty good idea of what the root cause of the problem is.

Unfortunately, zombie objects won't be able to save your 24-hour interval for every crash caused past EXC_BAD_ACCESS. If zombie objects don't practise the trick, so it'southward time for some proper analysis.

Analyze

If zombie objects don't solve your problem, then the root cause may be less trivial. In that case, you need to take a closer look at the code that's beingness executed when your awarding crashes. This can be cumbersome and time-consuming.

To help you lot observe problems in your lawmaking base, you could ask Xcode to analyze your code to help you find problematic areas. Note that Xcode analyzes your projection, which ways that information technology will bespeak out every potential trouble it encounters.

To tell Xcode to analyze your project, chooseAnalyze from Xcode'sProduction menu or pressShift-Command-B. It will take Xcode a few moments, but when information technology'due south finished you should see a list of problems in theResult Navigator on the left. Issues found by the analysis are highlighted in blue.

When you lot click an upshot, Xcode takes you to the block of lawmaking that needs your attending. Notation that Xcode is only making a suggestion. In some cases, it is possible that the issue is not relevant and doesn't need fixing.

Xcode will tell you exactly what is wrong with your code

If you can't find the issues that'southward causing EXC_BAD_ACCESS, and so it's important to carefully scrutinize every issue Xcode found during the analysis of your projection.

Determination

EXC_BAD_ACCESS is a common frustration among developers and it is something that is inherent to manual memory management. Issues related to retentiveness management have become less frequent since the introduction of ARC (Automatic Reference Counting), but they take by no means disappeared.

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Bart Jacobs

Bart Jacobs runs Code Foundry, a mobile development company based in Belgium and writes virtually iOS and Swift evolution on Cocoacasts.