Delphi LeakCheck

This is a repository for the Delphi LeakCheck library.

Basic description

LeakCheck is a memory manager extension that adds leak checking functionality. Main difference from the default memory manager is multi-platform implementation of leak checking with DUnit and DUnitX integration.

Main features

  • Multi-platform leak checking
  • Testing framework integration (DUnit and DUnitX), compares true allocations not just allocation size
  • Allocation stack tracing (with symbols)
  • Allocation snapshots (not thread safe)
  • Independent object structure scanning (with object cycle detection, complete allocation visualization and graph generation - with aid of Graphviz)
  • Complex leak ignoring options (including object graph ignore from entry point)
  • Minimal external dependencies (even for stack tracing - for basic implementations)
  • Interception of virtual (and interface) calls on freed objects


  • Add LeakCheck as the first unit in the project
  • Optionally add LeakCheck.Utils unit to your project if you want to enable class detection on Posix (or if you want to use any of its utility functions, on mobile you need to add External\Backtrace\Source to your search path)
  • Enable ReportMemoryLeaksOnShutdown
  • Run the app


  • Follow the steps above

DUnit :

  • Add External\DUnit to your search path so our modifications of the TestFramework unit can be found (and the memory manager can plug into DUnit)
  • Add LeakCheck.DUnit to your project (this is where the the memory manager plugs into DUnit) - there will be compile errors if you do the step above incorrectly.
  • Enable leak reporting in your DUnit runner
  • Run the tests (I recommend using TestInsight to run the tests, but note that you have to enable leak reporting manually in the TestInsight.DUnit.RunRegisteredTests set result.FailsIfMemoryLeaked to True.)


  • Add DUnitX.MemoryLeakMonitor.LeakCheck to your project (it will register the default memory monitor)
  • Run the tests

I recommend checking the latest DUnitX sources with extended reporting options (commit DUnitX/918608a) but other versions should be supported.

Tested on

  • Win32
  • Win64
  • Android

Detailed description

Main goal of this library is providing pure pascal implementation of leak checking so it can be used across platforms, it also provides simple cross platform implementation of some basic functionality for synchronization and ansi char handling that is used internally.

It can be used as a replacement of FastMM full debug mode but keep in mind that it is implemented to be safe and platform independent not as fast as possible so its use is recommended for testing only.

It can create memory snapshots

So you can detect leaks between snapshot and last allocation. This is what DUnit integration does. In test results, you'll be able to see detailed leak information including class names, reference count, string and memory dumps.

It is configurable

You can specify (using TTypeKind) what type of leak to report but only few kinds are supported:

  • tkLString, tkUString - ignore appropriate string type leaks
  • tkClass - ignore object leaks (see bellow)
  • tkUnknown - ignore other types of leaks

In addition to ignoring all classes, each class type can be inspected and ignored separately by assigning InstanceIgnoredProc (this can be useful to ignore globally allocated objects from RTL or other sources, like System.Rtti collections of objects and alike). This can also be used to ignore unreleased closures (anonymous method pointers). (See LeakCheck.Utils for more details).

It plugs into default ReportMemoryLeaksOnShutdown

You can use ReportMemoryLeaksOnShutdown on any platforms.

The output varies across platforms:

On Windows message box containing the leaks is shown, this behavior can be changed by defining NO_MESSAGEBOX conditional in which case the output will be sent into the IDE Event log (by OutputDebugString)

On Android the output is sent to logcat on WARN level using leak tag. You can use adb logcat -s leak:* to see it in console (adb can be found under platform-tools directory of your Android SDK installation). It is highly recommended to ignore tkUnknown leaks on Android (ARC) since the RTL will allocate data for weak references that are mistreated for leaks, System will release them during finalization.

It can detect reference cycles

LeakCheck.DUnitCycle and DUnitX.MemoryLeakMonitor.LeakCheckCycle units implement cycle detection scanner that if given a reference will scan its reference graph to find any instances creating a reference cycle thus preventing the instance from freeing. This works only for managed fields ie. interfaces (and objects on NextGen) or if used together with UseExtendedRtti (enabled manually) any field with RTTI is supported. It can scan inside other objects, records, TValues, anonymous method closures, static and dynamic arrays (which mean it supports any of the System.Generics.Collections types).

You need to manually register TLeakCheckCycleMonitor (or any descendants with more scanning options) as TestFramework.MemLeakMonitorClass to integrate it into your DUnit tests.

You need to manually register TDUnitXLeakCheckCycleMemoryLeakMonitor (or any descendants with more scanning options) into the DUnitX's IoC container to integrate it into your DUnitX tests.

  function : IMemoryLeakMonitor
    result := TDUnitXLeakCheckGraphMemoryLeakMonitor.Create;

Cycle detector scanner can optionaly use extended RTTI information (see TScanFlags), so scanning for object (not just using interfaces) cycles in non ARC environment is possible, but note that RTTI generation is required, for objects not supporting extended RTTI (ie. closures), there is a fallback mechanism to classic solution. When using extended RTTI, the reference scanner may output field names holding the references as well (and add it as named edges for Graphviz, see bellow).

This extension isn't tied to the memory manager itself and doesn't need it to run. So it can be used to generate object graphs in any application at any point of its execution, just give it appropriate entry point reference.

It can generate visual cycle representation

LeakCheck.Cycle supports format flags in the TCycle.ToString method that can be set to generate Graphviz DOT format output. Combined with TLeakCheckCycleGraphMonitor that appends another needed information, test cycle output can be copied to a file (make sure to copy only the graph specification not the entire error message) and converted using Graphviz (dot -O -T png to visual representation. You can also select different memory monitor TLeakCheckGraphMonitor which will generate full reference tree not just cycles.

Cycle visualization

It can acquire stack traces

If registered, LeakCheck memory manager may acquire stack traces when some block is allocated which is then displayed in leak output either as pure addresses or formatted (if trace formatter is registered). Multiple stack tracers and formatters are available. Make sure to configure your compiler/linker properly.

It is configurable via the include file also GetStackTraceProc has to be set to one of the predefined implementations or custom one. To get pretty-printed stack traces make sure to also register GetStackTraceFormatterProc. Keep in mind that different implementations have some limitations and requirements:

The safest solution is WinApi based stack tracer and MAP file based formatter (MAP file needs to be enabled in the linker settings and symbols generation must be enabled in the compiler to get line numbers etc.).

JCL implementation offers better stack traces (RAW) on Win32 and offers more options while formatting the output (debug symbols, MAP file, etc.) but needs external caching.

Android implementation cannot show symbols right away but the formatter allows you to feed the output directly to addr2line utility which will then output the symbols and line numbers.

It can detect virtual (and interface) calls made on freed object instances

Fake VMT is assigned to an object (and if enabled to all interface entries as well) when it is freed. If virtual (or interface) call is made on a dangling pointer of such object, the call is reported and AV is raised.

Ligthweight version may be used that will set fake VMT without reporting but will most likely raise an AV during the call.


Memory manager setup

program SomeApp;

  {$IFDEF WIN32}
  // LeakCheck can be used together with another memory manager (if
  // LEAKCHECK_DEFER is defined)
  {$ENDIF }
  // LeakCheck must be the first unit (if no supervising memory manager is used)
  // Following any platform dependent unit
  // LeakCheck utility unit that adds some reporting enhancements on mobile and
  // defines several useful functions (not required)
  ReportMemoryLeaksOnShutdown := True;
  // Now you will get leak report when your program terminates (if leaks
  // are found).

Getting leak report

// You can use following function at any point and you'll receive complete
// report of all unreleased (and not ignored) pointers.

// This may not sound really helpful but you can create memory snapshot
// and report everything in between current allocation and given snapshot.
procedure DetectLeak;
  Snapshot: TLeakCheck.Snapshot;
  Report: LeakString;
  Report := TLeakCheck.GetReport(Snapshot.Snapshot);
    // Don't forget to do this. We cannot use string in the LeakCheck
    // low-level functions so we need a helper.

Adding call stack to your report

// Register stack trace to obtain the call stack (call addresses)
TLeakCheck.GetStackTraceProc := WinApiStackTrace;
// Pure pointers aren't really helpful so convert it to helpful format:
// First enable symbol generation and MAP file generation or symbol linking
// (based on what formatter you want to use) then assign the formatter.
TLeakCheck.GetStackTraceFormatterProc := MapStackTraceFormatter;

// More complex may look like this

  TLeakCheck.GetStackTraceProc := WinApiStackTrace;
  TLeakCheck.GetStackTraceProc := JclRawStackTrace;
  TLeakCheck.GetStackTraceFormatterProc := MapStackTraceFormatter;
  TLeakCheck.GetStackTraceProc := BacktraceStackTrace;
  TLeakCheck.GetStackTraceFormatterProc := PosixProcStackTraceFormatter;

Or just add any of the LeakCheck.Setup.* units to simplify this process.


Sometimes it is useful to ignore public caches or other stuff when RegisterExpectedMemoryLeak is just not enough.

// Ignore some basic types
TLeakCheck.IgnoredLeakTypes := [
  // Ignore all class leaks
  // Ignore all ansi string leaks
  // Ignore all unicode string leaks
  // Ignore all other leaks

// In addition classes can be ignored in more complex way by assigning
// TLeakCheck.InstanceIgnoredProc. This can be custom defined function
// or any of the LeakCheck.Utils ignore functions.
// Is not evaluated if tkClass is ignored globally in which case all classes
// are already ignored.
TLeakCheck.InstanceIgnoredProc := IgnoreRttiObjects;

// If you need to ignore multiple objects you can use helper functions
TLeakCheck.InstanceIgnoredProc := IgnoreMultipleObjects; // Helper ignore function
// Register all ignore functions you need
    // Ignores Rtti unit objects (obtained through TRttiContext)
    // Classes created by using anonymous function use
    //Custom attributes allocated by Rtti
    //Ignore all descendants of TMyObject

// Additionaly you can ignore managed fields in your objects by calling
// IgnoreManagedFields or IgnoreAllManagedFields from inside your ignore
// function or use TIgnore<T>.AnyAndFields or TIgnore<T>.AnyAndAllFields.

// To ignore complete object graph use IgnoreGraphLeaks from the
// TLeakCheck.Cycle unit.

// You can also use TLeakCheck's MarkNotLeaking (ignores all allocations
// from and including given snapshot), BeginIgnore and EndIgnore
// (disables/enables allocations from being recorded as a leak) low-level
// functions but be careful when you do!

DUnit integration

program MyTests;

  TestFramework, // Don't forget to use the modified version

  // Configure ignoring and stack tracing if you want
  // Don't forget to enable leak reporting in your runner

// You may create more complete leak report by assigning
// MemLeakMonitorClass.
  MemLeakMonitorClass := TLeakCheckCycleMonitor;

DUnitX integration

program MyTests;


  // Configure ignoring and stack tracing if you want

// You may create more complete leak report by creating other
// IMemoryLeakMonitor.
program MyTests;


procedure Run;
  // Configure ignoring and stack tracing if you want
    function : IMemoryLeakMonitor
      result := TDUnitXLeakCheckGraphMemoryLeakMonitor.Create;

  // It is not a good idea to add anonymous methods to main unit functions

Delphi support

  • Delphi XE7
  • Delphi XE6 (Android not tested since XE6 doesn't support Lollipop)
  • Delphi XE5 (Android not tested)
  • Delphi XE

Delphi between XE and XE6 should also work but was not tested. If you find any problems, please create an issue.

Support for older version will not be added, some other versions may work but I will not test them nor will I add any patches that would add support to older versions.


Although this library was tested on fairly large projects it comes with no guarantees, use at your own risk.

This is a low level library at a very early stage of development so you may expect errors if you use it incorrectly or if you have memory issues in your application that the default memory manager survives somehow. But I'd like to get any input about issues you may run into, but please don't say it crashed my application, include stack traces or other technical information that may help me figure out the problem or better yet, submit patches.

Known issues

On Android (or actually any platform that needs libicu) the program may cause SEGFAULT during System unit finalization. This is caused by System allocating some memory before the memory manager is initialized and not freeing it properly (there are more comments in the LeakCheck source code). This should now be prevented in cases when the application doesn't leak any memory.

Thanks to

  • FastMM team
  • DUnit team
  • Stefan Glienke - for creation of TestInsight


Unless stated explicitly otherwise (in the code), the sources of this library is licensed under Apache License 2.0 (