What causes a ParallelFor data race on a shared array in Unreal?

ParallelFor runs iterations on multiple worker threads, so multiple iterations writing the same TArray slot, calling Add, or resizing the array race and corrupt memory.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix a ParallelFor Data Race on a Shared Array in Unreal

Quick answer: Have each iteration write only its own pre-sized index, preallocate the array before the loop, and use atomics or per-thread buffers for any aggregation.

You parallelized a loop with ParallelFor and call Results.Add(value) inside it, getting crashes or wrong counts. TArray::Add is not thread-safe and multiple threads collide. Here is how to make the loop safe.

How to fix it

1. Preallocate and write by index

Call Results.SetNum(Count) before the loop, then inside write Results[Index] = value; so each iteration touches only its own slot with no contention.

2. Never Add or resize inside

Avoid Add, Emplace, or anything that resizes the array during the loop; those reallocate and race across threads.

3. Aggregate with atomics or reductions

For sums or counters, use std::atomic / FPlatformAtomics::InterlockedAdd, or accumulate per-thread partials and combine them after the loop.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unreal Engine error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Reproduce it once with full context and the fix writes itself. The hunt is the expensive part.