How do you detect/avoid Memory leaks in your (Unma

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.

Answer 1

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/

Answer 2

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...

Answer 3

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.

Answer 4

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 <stdlib.h>
#include <crtdbg.h>

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.

Answer 5

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!

Answer 6

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.