C: Find memory corruption & leaks

Well, we all face this problem at some time in our programming lives. Did our code mess up the memory? Did I screw up any of the pointer addresses? Did my code overflow or underflow memory? Does my code leak memory? Well, is there a direct solution to this one? Perhaps, yes. Perhaps, not! It depends on how good or bad you are with pointer manipulation. At this time, lemme confess a very important piece, I am _not_ bad with my pointer manipulations. Yet to any of us, including the likes of me, memory corruption is a very plausible problem we tend face.

So, I thought how about figuring out these corruptions within the code. Well, I am sure valgrind does exactly this but it is kinda fun when we do it ourselves. 🙂 So here is my simple yet effective-enough approach (not necessarily performant though):

 (Q) How to find memory underflow/overflow:
 (A) Pad the requested memory with signature & other debug information.
 (Q) How to find memory leaks?
 (A) Keep track of all allocations & frees.

Simple, yeah? 🙂

Header & Footer Padding

So, the idea is to pad the original request with additional debug information. I chose to pad the original request using this format:

padded-memory
padded-memory

 

So, suppose we called our wrapper to allocate 1024 bytes from function foo() at line number 30, the padded memory should resemble this (not accounting to byte ordering):

example-padding
example-padding

 

Now this gives us all the edge we want. If there was an overflow of, say, 2 bytes (with 0xFFFF) then during the free() phase the padded memory would look like this:

corrupt-memory
corrupt-memory

 

So, obviously, in the free() I check if the value on the padded header & footer is still intact. If there is a mess, like in the above example, we have found our culprit address!!! Then all we gotta do is to code-review and backtrace it. 🙂

Track the allocations

Now for memory leakage, we have to track the allocations. The idea is to maintain a list of allocated (but not yet freed) memory addresses. In other words:

  • During allocation, insert into the list
  • During free, delete from the list.

So, obviously, at the final step if this list has any data then these are leaked memory! :p

Have fun using this code, if required. The code is available (with a test code) at my github page.

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