Tommy Lahitte
Debugging

Track Down Memory Corruption with Stomp2Malloc

Heap corruption bugs are some of the hardest to reproduce; they often crash far from the actual fault. Unreal's Stomp2Malloc allocator places every allocation on a guarded memory page, so use-after-free and buffer overruns crash immediately at the source.

memorycorruptionstomp2mallocdebuggingheap

If you're seeing intermittent crashes with no consistent callstack, or crashes that only happen in optimized builds, memory corruption is often the culprit. Stomp2Malloc makes those bugs deterministic.

Enabling Stomp2Malloc

bash
UnrealEditor.exe YourProject.uproject -Stomp2Malloc
Use this only for targeted debugging sessions. Stomp2Malloc significantly increases memory usage and reduces performance.

What It Catches

  • Buffer overruns: writing past the end of an allocation
  • Buffer underruns: writing before the start of an allocation
  • Use-after-free: accessing memory after it has been freed
  • Double frees: calling free() on the same pointer twice
  • Invalid memory access: reading/writing arbitrary addresses

How It Works

  • Each allocation is placed on its own virtual memory page
  • Guard pages (inaccessible) are placed immediately before and after each allocation
  • Any access outside the allocated region triggers an immediate access violation
  • The crash happens at the exact offending instruction, not somewhere downstream

Practical Workflow

  • Reproduce the crash with -Stomp2Malloc to get a clean callstack
  • The callstack will now point directly to the offending code
  • Compare with the crash without -Stomp2Malloc to confirm it is the same bug
  • Fix the underlying issue, then remove the flag for production runs

Stomp2Malloc is particularly valuable during QA regression testing when a build starts showing flaky crashes that are hard to reproduce on demand.

Related Tips

Control Log Verbosity Per Category at Launch

Drowning in log noise or missing critical messages? The -LogCmds launch argument lets you set verbosity per log category without recompiling, from silencing entire subsystems to enabling Verbose output for one specific category.

Profile Memory Usage by Category with LLM

Unreal's Low Level Memory Tracker (LLM) breaks down live memory consumption by stat group: textures, meshes, audio, RHI, and more. Two commands and a launch flag give you a live memory readout in the viewport.

DX12 Launch Arguments for GPU Crash Debugging

When Unreal crashes on the GPU side with no useful callstack, two launch arguments (-d3ddebug and -gpucrashdebugging) enable the D3D validation layer and NVIDIA Aftermath/Breadcrumbs to capture the exact operation that caused the crash.