barcode maker vb.net 4: You Go Squish Now! Debugging on the iPhone in Objective-C

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CHAPTER 4: You Go Squish Now! Debugging on the iPhone
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notice that the program didn t crash. This is where memory problems can be awful to track down. Depending on what DoSomething() is actually doing and when it was called after mTestCpp has been deleted, it could have stomped over memory that had been allocated to a different object. First, disable NSZombieEnabled (refer to the previous section to see how to find it) by deselecting the check box next to it. Now turn Enable Guard Malloc. NOTE: Unfortunately, Enable Guard Malloc can be used only with the simulator. It cannot be used to debug on your device. Make sure you set your target to build for the simulator. Then go to the Run menu and turn on Enable Guard Malloc, right at the bottom of the menu, as shown in Figure 4-13.
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Figure 4-13. Activating Enable Guard Malloc for the simulator
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TIP: If Enable Guard Malloc is grayed out in the Run menu, it is probably because the build target is still set to Device instead of Simulator. Make sure the target is set to Simulator, and try again. Build and run the app again. This time the app should halt in the debugger. If everything went according to plan, then the debugger should have halted at this line:
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void TestCPPClass::DoSomething() {
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CHAPTER 4: You Go Squish Now! Debugging on the iPhone
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++mSomeNum; }
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// Debugger should have halted here
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The debugger is halting here this time because it s at this point that Enable Guard Malloc has caught something trying to write to memory that has been freed. In this function, mSomeNum is a member variable of an object that has been deleted. By trying to increment it, what s actually happening is that some 4-byte chunk of memory that no longer belongs to the mTestCpp object is being incremented. CAUTION: Running with Enable Guard Malloc turned on will most likely cause any remotely complex app to run extremely slowly, because it does extra processing for every memory allocation and free. Turn it on only when you need it to track down a problem. In this example, because DoSomething() is being called immediately after the object was deleted, chances are this isn t going to do anything dangerous. However, if the call to DoSomething() was made after other memory had been allocated on the heap, this code might now be incrementing memory that belongs to some other object. Memory stomp! Luckily, Enable Guard Malloc caught the problem as soon as something tried to access memory it didn t own anymore. However, it s up to you to figure out why this object was deleted before you thought you were finished with it. The best way I know to do this is to put a breakpoint in the object s destructor and watch where it gets hit. From there you can usually track it back to problem. CAUTION: Enable Guard Malloc won t find all of your memory stomps. It will find instances only where memory that has been marked as freed is changed.
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Watching Variables
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I ve covered a few tools that are available to you for tracking down some memory bugs. However, sometimes a stomp will happen without causing any of the memory violations that the previous tools will detect. In this case, the bug can be extremely tricky to track down. The first step in the process is determining which variable (or variables) is being stomped. That is left as an exercise for you, because that s just good, old-fashioned debugging. If you know that a particular variable is being stomped (or even just changed and you don t know why), one of the most useful tools for debugging this is a variable watch. Xcode allows you to put a watch on any given variable. Setting a watch on a variable halts execution any time something changes the value stored by the variable. Watches can be incredibly useful if something unexpected is changing the value of one of your variables in memory. It can also be useful if something is stomping memory and you know what memory is being stomped, but you don t know from where.
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