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Debugging Memory Corruption in Game Development


October 16, 2008 Article Start Previous Page 2 of 6 Next
 

Investigating Corruption

If you suspect that memory corruption is occurring, then your first step is to try to determine if this is actually some form of corruption, and what type it is.

Is it actually corruption?

Just because a value in memory looks rather unusual, does not mean that it was not generated by the code that owns that memory. The unusual value might simply be the result of an error in your logic. It could have been quite legally copied from somewhere else. It could be the result of computations involving incorrect data, perhaps data that was already corrupt.

To determine this, you need to determine if the code that you might think is writing to that location actually is writing to that location, and see what values it is writing. Ideally, you would add assertions at all location that you think might legally be writing to that location, and check the range of values that are being written (make sure the “corrupt” value is outside this range.)

Who owns that memory location?

Memory corruption usually occurs when some piece of code is using an area of memory that it should not. The corrupt memory then causes problems in some code

There are two primary ways in which this can happen: corrupting a legal area, and using an area illegally.

Consider a piece of code A, that uses and area of memory A(m). If another piece of code, B, also happens to have a pointer to A(m), and writes some data to that, then code B is corrupting memory A(m). This is the normal form of memory corruption.

Now consider if the code A is legally using memory location A(m). Code B is illegally also using some location within (or overlapping) A(m). Code B appears to work correctly, but then code A makes a legal update to A(m), causing code B to manifest a bug. It appears that code A is corrupting memory B(m). However, the fault here is with code B. It has the appearance of corruption, yet may mislead you to thinking that the problem is with code A.

It is important to determine who actually owns the memory location that is being corrupted. Is the “legal” use actually legal? Can you demonstrate that code B actually owns those memory locations? If you can quickly determine that code B does not actually own that memory, then the tracking down of code A is irrelevant, which can save you substantial time.

Repeatable, Fixed Location

If the corruption is consistent, meaning it happens in the same location and under the same conditions then you are (relatively speaking) in luck. Debugging in this case is a matter of somehow watching that location, and tracking down the cause of the corruption. Since the corruption happens under the same conditions, you should be able either to trap it immediately, or quickly narrow down the possibilities.

Intermittent, Fixed Location

If the corruption happens in the same memory location, yet is intermittent, then this makes tracking down the corruption more difficult. Since you do not know when the corruption occurs, you cannot be as focused in your search, and must rely more on general observation as to the nature of the corruption when tracking down the cause.

Intermittent, Variable Location

If the corruption happens in varying location, and at unpredictable times, then your debugging options are often limited to making observations about the corruption after it has occurred.

Determine the location of the corruption

If the memory corruption is the immediate cause of the bug, such as with an address error due to a corrupt pointer, then you may be able to immediately determine the effect of the corruption simply by seeing what address was being accessed at the time of the bug.

If the memory corruption is an intermediate cause, then you will track down the address of the corruption in the process of analyzing the immediate cause of the bug, and any intermediate causes that lie between the root cause and the symptoms.

Hardware Breakpoints

If your target platform has some kind of break-on-access breakpoint, then use this as your first line of investigation when debugging memory corruption with a know address. Simply set the debugger to execute a breakpoint when a memory location changes, then when the location happens, see what code is executing.

This technique can work very well if the location that is being corrupted contains data that is relatively static. However, if the location contains some dynamic variable that changes hundreds of times per frame, then you may have some difficult in finding the single write access that is causing the problem.

In that case, you may be able to augment your write-access breakpoint with a conditional check that verifies that the data being written to the corrupt location is in the valid range.

Sometimes memory is corrupted with vales that are within the valid range, but nonetheless are wrong. Your options here are more limited:

- Repeatedly run the code, and each time the breakpoint trips, look at the call stack until you see something that you do not recognize as code that can legally write to this location.

- If the legal places that write to this location are known and relatively limited, then update them to first write to some separate location. First ensue the corruption does not also affect that separate location, and then update the breakpoint condition to check the value written matches the stored value.


Article Start Previous Page 2 of 6 Next

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