Leaks with CRT

I'm a big fan of the Windows instrumented debug heap, but today I want to talk about the CRT support, which can sometimes provide a bit more context.

You can make a build-time decision to use the CRT debug heap. A nice thing about these is that the function calls to the debug heap disappear when _DEBUG is not defined, which is presumably how your release build works.

This adds a debug allocator on top of the release allocator, and enables a bunch of new functionality. The most useful ones are these:

• _CrtMemCheckpoint. This captures the current state of the debug heap; you can use it to compare its state at different points in time.
• _CrtMemDumpAllObjectsSince. This dumps all objects since a checkpoint you captured (or the beginning of the program is you pass NULL).
• _CrtSetDumpClient. This allows you to have a hook to print out an object when dumping them. You just get an address though, so it can be tricky to implement.
• _CrtSetReportMode. This specifies where to write out memory dumps. The most common values are _CRTDBG_MODE_DEBUG (debugger window) and _CRTDBG_MODE_FILE (a file or stream).
• _CrtSetReportFile. This specifies what file the memory dumps go to. You can use a file handle (like from CreateFile), or one of _CRTDBG_FILE_STDERR or _CRTDBG_FILE_STDOUT.
• _CrtCheckMemory. This checks integrity, and is used to detect memory corruption. You can also set the _CRTDBG_CHECK_ALWAYS_DF flag with _CrtSetDbgFlag to have this checked on every memory operation (but calling it by hand may help you narrow down the offending code). Still, handy to know about.

Here's how you would put the above functions together.

_CrtMemState startState{};
_CrtMemCheckpoint(&startState);

// Allocate things and presumably deallocate them again!

// Set the memory dump to print to `stderr` and write objects we've allocated and not freed since starting.
_CrtSetReportMode(_CRT_WARN, _CRTDBG_MODE_FILE);
_CrtSetReportFile(_CRT_WARN, _CRTDBG_FILE_STDERR);
_CrtMemDumpAllObjectsSince(&startState);


For the official doc and step-by-step instructions, see Find memory leaks with the CRT library.

Happy leak tracking!

Tags:  cpp debugging

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