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“Unexpected Memory Leak” after “MyTestApplication.exe: Memory Leak Detected”

Posted by jpluimers on 2021/07/28

Sometimes after a regular FastMM memory leak test report you get a dialog like this:

---------------------------
Unexpected Memory Leak
---------------------------
An unexpected memory leak has occurred. The unexpected small block leaks are:

61 - 68 bytes: Unknown x 1
---------------------------
OK   
---------------------------

The normal FastMM memory leak reporting dialog looks like this:

---------------------------
MyTestApplication.exe: Memory Leak Detected
---------------------------
...
Note: Memory leak detail is logged to a text file in the same folder as this application. To disable this memory leak check, undefine "EnableMemoryLeakReporting".
---------------------------
OK   
---------------------------

Searching for “Unexpected Memory Leak” in RTL or FastMM code did not reveal results, but that might be me not doing a proper search.

One big problem is that the regular memory leak dialog is being suppressed by setting SuppressMessageBoxes to True (see for instance my blog post Application shutdown: wait for all threads to terminate or not? or [WayBack] delphi – Generating a FASTMM report WITHOUT the shutdown dialog – Stack Overflow).

However the “Unexpected Memory Leak” message box is always shown.

Research

Location of the error message caption

  • GETMEM.INC has an identifier LeakMessageTitle:
    LeakMessageTitle: _PAnsiChr = 'Unexpected Memory Leak';
  • Unlike FastMM4.pas, GETMEM.INC does not take into account SuppressMessageBoxes.
  • Like FastMM4.pas, GETMEM.INC does take into account ReportMemoryLeaksOnShutdown.

More on GETMEM.INC: [WayBack] delphi – Reporting memory leaks on shutdown with a console application – Stack Overflow

Versioned FastMM4.pas.

Complicating the hunt

Not all allocations go via the memory manager. A complicating factor is that:

  • ALL TMonitor.Create allocations go through SysAllocMem:
class function TMonitor.Create: PMonitor;
begin
  if CacheLineSize = 0 then
    AtomicExchange(CacheLineSize, GetCacheLineSize);
  if (CPUCount > 1) and (FDefaultSpinCount = 0) then
    AtomicExchange(FDefaultSpinCount, 1000);
  if CacheLineSize > SizeOf(Result^) then
    Result := SysAllocMem(CacheLineSize)
  else
    Result := SysAllocMem(SizeOf(Result^));
  Result.FSpinCount := FDefaultSpinCount;
end;
  • ALL TMonitor.Destroy deallocations go through SysFreeMem:
procedure TMonitor.Destroy;
begin
  if (MonitorSupport <> nil) and (FLockEvent <> nil) then
    MonitorSupport.FreeSyncObject(FLockEvent);
  SysFreeMem(@Self);
end;

Debugging this is easiest to set a breakpoint in the FastMM4.pas finalization section enabling a breakpoint group that has breakpoints on these methods inside GETMEM.INC:

  • function SysGetMem(Size: NativeInt): Pointer;
  • function SysFreeMem(P: Pointer): Integer;
  • function SysReallocMem(P: Pointer; Size: NativeInt): Pointer;
  • function SysAllocMem(Size: NativeInt): Pointer;

For inspecting for instance an asm construct like TSmallPoolBlockPoolHeader[edx], use a conversion PSmallBlockPoolHeader(Pointer(EDX))^,r

Poor mans shotgun approach

This will hide all the SysAllocMem related leaks:

unit FastMM4WrapperUnit;

interface

{$ifdef UseFastMM4}
uses
  FastMM4,
  FastMM4Messages;
{$endif UseFastMM4}

implementation

initialization
//
finalization
// Disable GETMEM.INC reports that neglect SuppressMessageBoxes and NoErrMsg; this will effectively hide the leak as GETMEM.INC also does not write the leaks to a log file
{$ifdef UseFastMM4}
  if SuppressMessageBoxes then
  begin
    {$WARN SYMBOL_PLATFORM OFF} NoErrMsg := True {$WARN SYMBOL_PLATFORM ON}; // no ShowMessage from the RTL (except for memory leaks)
    ReportMemoryLeaksOnShutdown := False; // No ShowMessage from the RTL in the GETMEM.INC teardown
  end;
{$endif UseFastMM4}
end.

IsConsole: needs linker option

Note that I tried embedding this in the then portion, but enabling IsConsole fails:

    IsConsole := True; // can only force run-time error messages to be written to the console if the "Project/Options/Linking/Generate console application" is set.

The reason is that this does not allocate a console handle, so you really need to ensure the linker has allocated a console for you by enabling Project/Options/Linking/Generate console application (or option DCC_ConsoleTarget in the .dproj file)

For more details, see my post console – When is System.IsConsole true in Delphi? – Stack Overflow.

–jeroen

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