How do you detect avoid Memory leaks in your Unmanaged code

Question

In unmanaged C C   code  what are the best practices to detect memory leaks  And coding guidelines to avoid   As if it s that simple     We have used a bit of a silly way in the past  having a counter increment for every memory allocation call and decrement while freeing  At the end of the program  the counter value should be zero   I know this is not a great way and there are a few catches   For instance  if you are freeing memory which was allocated by a platform API call  your allocation count will not exactly match your freeing count  Of course  then we incremented the counter when calling API calls that allocated memory    I am expecting your experiences  suggestions and maybe some references to tools which simplify this

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The best defense against leaks is a program structure which minimizes the use of malloc. This is not only good from a programming perspective, but also improves performance and maintainability. I'm not talking about using other things in place of malloc, but in terms of re-using objects and keeping very explicit tabs on all objects being passed around rather than allocating willy-nilly like one often gets used to in languages with garbage collectors like Java.

For example, a program I work on has a bunch of frame objects representing image data. Each frame object has sub-data, which the frame's destructor frees. The program keeps a list of all frames that are allocated, and when it needs a new one, checks a list of unused frame objects to see if it can re-use an existing one rather than allocate a new one. On shutdown, it just iterates through the list, freeing everything.

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A nice malloc  calloc and reallloc replacement is rmdebug  it s pretty simple to use  It is much faster to then valgrind  so you can test your code extensively  Of course it has some downsides  once you found a leak you probably still need to use valgrind to find where the leak appears and you can only test mallocs that you do directly  If a lib leaks because you use it wrong  rmdebug won t find it   http   www hexco de rmdebug

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Are you counting the allocs and frees by interpolating your own syscall functions which record the calls and then pass the call to the real function   This is the only way you can keep track of calls originating from code that you  haven t written   Have a look at the man page for ld so  Or ld so 1 on some systems   Also do Google LD PRELOAD and you ll find some interesting articles explaining the technique over on www itworld com

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Detect   Debug CRT  Avoid   Smart pointers  boehm GC

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As a C   Developer here s some simply guidelines    Use pointers only when absolutely necessary If you need a pointer  doublecheck if a SmartPointer is a possibility Use the GRASP Creator pattern    As for the detection of memory leaks personally I ve always used Visual Leak Detector and find it to be very useful

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If you re using Visual Studio it might be worth looking at Bounds Checker   It s not free  but it s been incredibly helpful in finding leaks in my code   It doesn t just do memory leaks either  but also GDI resource leaks  WinAPI usage errors  and other stuff   It ll even show you where the leaked memory was initialized  making it much easier to track down the leak

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For Linux    Try Google Perftools  There are a lot of tools that do similar alloc free counting  the pros of Goolge Perftools    Quite fast  in comparison to valgrind  very fast  Comes with nice graphical display of results Has other useful capabilities  cpu-profiling  memory-usage profiling

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At the top of this list  when I read it  was valgrind  Valgrind is excellent if you are able to reproduce the leak on a test system  I ve used it with great success   What if you ve just noticed that the production system is leaking right now and you have no idea how to reproduce it in test  Some evidence of what s wrong is captured in the state of that production system  and it might be enough to provide an insight on where the problem is so you can reproduce it   That s where Monte Carlo sampling comes into the picture  Read Raymond Chen s blog article     The poor man s way of identifying memory leaks    and then check out my implementation  assumes Linux  tested only on x86 and x86-64   http   github com tialaramex leakdice tree master

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If you re using MS VC    I can highly recommend this free tool from the codeproject  leakfinder by Jochen Kalmbach   You simply add the class to your project  and call  InitAllocCheck ACOutput XML  DeInitAllocCheck     before and after the code you want to check for leaks   Once you ve build and run the code  Jochen provides a neat GUI tool where you can load the resulting  xmlleaks file  and navigate through the call stack where each leak was generated to hunt down the offending line of code    Rational s  now owned by IBM  PurifyPlus illustrates leaks in a similar fashion  but I find the leakfinder tool actually easier to use  with the bonus of it not costing several thousand dollars

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Microsoft VC   in debug mode shows memory leaks  although it doesn t show where your leaks are   If you are using C   you can always avoid using new explicitly  you have vector  string  auto ptr  pre C  11  replaced by unique ptr in C  11   unique ptr  C  11  and shared ptr  C  11  in your arsenal   When new is unavoidable  try to hide it in a constructor  and hide delete in a destructor   the same works for 3rd party APIs

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If your C C   code is portable to  nix  few things are better than Valgrind

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At least for MS VC    the C Runtime library has several functions that I ve found helpful in the past   Check the MSDN help for the  Crt  functions

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I would recommend using Memory Validator from software verify  This tool proved itself to be of invaluable help to help me track down memory leaks and to  improve the memory management of the applications i am working on    A very complete and fast tool

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In C    use RAII  Smart pointers like std  unique ptr  std  shared ptr  std  weak ptr are your friends

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Visual Leak Detector is a very good tool  altough it does not supports the calls on VC9 runtimes  MSVCR90D DLL for example

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There are various replacement  malloc  libraries out there that will allow you to call a function at the end and it will tell you about all the unfreed memory  and in many cases  who malloced  or new ed  it in the first place

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Working on Motorola cell phones operating system  we hijacked memory allocation library to observe all memory allocations  It helped to find a lot of problems with memory allocations   Since prevention is better then curing  I would recommend to use static analysis tool like Klockwork or PC-Lint

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I ve been using DevStudio for far too many years now and it always amazes me just how many  programmers don t know about the memory analysis tools that are available in the debug run time libraries  Here s a few links to get started with   Tracking Heap Allocation Requests - specifically the section on Unique Allocation Request Numbers   CrtSetDbgFlag   CrtSetBreakAlloc  Of course  if you re not using DevStudio then this won t be particularly helpful

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Never used it myself  but my C friends tell me Purify

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Mtrace appears to be the standard built-in one for linux  The steps are     set up the environment variable MALLOC TRACE in bash MALLOC TRACE  tmp mtrace dat export MALLOC TRACE   Add  include  lt mcheck h gt  to the top of you main source file Add mtrace    at the start of main and muntrace    at the bottom  before the return statement  compile your program with the -g switch for debug information run your program display leak info with mtrace your prog exe name  tmp mtrace dat   I had to install the mtrace perl script first on my fedora system with yum install glibc utils nbsp  nbsp

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I   m amazed no one mentioned DebugDiag for Windows OS  It works on release builds  and even at the customer site   You just need to keep your release version PDBs  and configure DebugDiag to use Microsoft public symbol server

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I d like to offer something I ve used at times in the past  a rudimentary leak checker which is source level and fairly automatic  I m giving this away for three reasons    You might find it useful  Though it s a bit krufty  I don t let that embarass me  Even though it s tied to some win32 hooks  that should be easy to alleviate    There are things of which you must be careful when using it   don t do anything that needs to lean on new in the underlying code  beware of the warnings about cases it might miss at the top of leakcheck cpp  realize that if you turn on  and fix any issues with  the code that does image dumps  you may generate a huge file   The design is meant to allow you to turn the checker on and off without recompiling everything that includes its header   Include leakcheck h where you want to track checking and rebuild once   Thereafter  compile leakcheck cpp with or without LEAKCHECK  define d and then relink to turn it on and off   Including unleakcheck h will turn it off locally in a file   Two macros are provided   CLEARALLOCINFO   will avoid reporting the same file and line inappropriately when you traverse allocating code that didn t include leakcheck h   ALLOCFENCE   just drops a line in the generated report without doing any allocation   Again  please realize that I haven t used this in a while and you may have to work with it a bit   I m dropping it in to illustrate the idea   If there turns out to be sufficient interest  I d be willing to work up an example  updating the code in the process  and replace the contents of the following URL with something nicer that includes a decently syntax-colored listing   You can find it here   http   www cse ucsd edu  tkammeye leakcheck html

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If you are using Visual Studio  Microsoft provides some useful functions for detecting and debugging memory leaks   I would start with this article  https   msdn microsoft com en-us library x98tx3cf v vs 140  aspx  Here is the quick summary of those articles   First  include these headers    define  CRTDBG MAP ALLOC  include  lt stdlib h gt   include  lt crtdbg h gt    Then you need to call this when your program exits    CrtDumpMemoryLeaks      Alternatively  if your program does not exit in the same place every time  you can call this at the start of your program    CrtSetDbgFlag    CRTDBG ALLOC MEM DF    CRTDBG LEAK CHECK DF      Now when the program exits all the allocations that were not free d will be printed in the Output Window along with the file they were allocated in and the allocation occurrence   This strategy works for most programs   However  it becomes difficult or impossible in certain cases   Using third party libraries that do some initialization on startup may cause other objects to appear in the memory dump and can make tracking down your leaks difficult   Also  if any of your classes have members with the same name as any of the memory allocation routines  such as malloc    the CRT debug macros will cause problems   There are other techniques explained in the MSDN link referenced above that could be used as well

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Paul Nettle s mmgr is a long time favourite tool of mine   You include mmgr h in your source files  define TEST MEMORY  and it delivers a textfile full of memory problems that occurred during a run of your app

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Valgrind is a nice option for Linux  Under MacOS X  you can enable the MallocDebug library which has several options for debugging memory allocation problems  see the malloc manpage  the  ENVIRONMENT  section has the relevant details   The OS X SDK also includes a tool called MallocDebug  usually installed in  Developer Applications Performance Tools   that can help you to monitor usage and leaks

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Memory debugging tools are worth their weight in gold but over the years I ve found that two simple ideas can be used to prevent most memory resource leaks from being coded in the first place    Write release code immediatly after writing the acquisition code for the resources you want to allocate   With this method its harder to  forget  and in some sense forces one to seriously think of the lifecycle of resources being used upfront instead of as an aside  Use return as sparringly as possible   What is allocated should only be freed in one place if possible  The conditional path between acquisition of resource and release should be designed to be as simple and obvious as possible

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Most memory profilers slow my large complex Windows application to the point where the results are useless   There is one tool that works well for finding leaks in my application  UMDH - http   msdn microsoft com en-us library ff560206 28VS 85 29 aspx

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I think that there is no easy answer to this question   How you might really approach this solution depends on your requirements   Do you need a cross platform solution   Are you using new delete or malloc free  or both    Are you really looking for just  leaks  or do you want better protection  such as detecting buffer overruns  or underruns    If you are working on the windows side  the MS debug runtime libraries have some basic debug detection functionality  and as another has already pointed out  there are several wrappers that can be included in your source to help with leak detection   Finding a package that can work with both new delete and malloc free obviously gives you more flexibility   I don t know enough about the unix side to provide help  although again  others have   But beyond just leak detection  there is the notion of detecting memory corruption via buffer overruns  or underruns    This type of debug functionality is I think more difficult than plain leak detection   This type of system is also further complicated if you are working with C   objects because polymorhpic classes can be deleted in varying ways causing trickiness in determining the true base pointer that is being deleted   I know of no good  free  system that does decent protection for overruns   we have written a system  cross platform  and found it to be pretty challenging

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General Coding Guideline     Resources should be deallocated at the same  layer   function class library  where they are allocated  If this is not possible  try to use some automatic deallocation  boost shared pointer

[c++] How do you detect/avoid Memory leaks in your (Unmanaged) code?

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