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# waitForFence(_:)

Encodes a command that instructs the GPU to pause the compute pass until another pass updates a fence.

```
func waitForFence(_ fence: any MTLFence)
```

## Parameters

`fence`

A fence that the pass waits for before it runs any of its commands.

## Discussion

You can synchronize memory operations of a compute pass that access resources with an [`MTLFence`](/documentation/Metal/MTLFence).
This method instructs the GPU to wait until another pass updates `fence` before running the compute pass.
The fence indicates when the pass can access those resources without a race condition.

For more information about synchronization with fences, see:

- [Resource synchronization](/documentation/Metal/resource-synchronization)
- [Synchronizing passes with a fence](/documentation/Metal/synchronizing-passes-with-a-fence)

### Reuse a fence by waiting first and updating second

When encoding a compute pass that reuses a fence, wait for other passes
to update the fence before repurposing that fence to notify subsequent passes with an update:

1. Call the [`waitForFence(_:)`](/documentation/Metal/MTLComputeCommandEncoder/waitForFence(_:)) method before encoding commands that need to wait for other passes.
2. Call the [`updateFence(_:)`](/documentation/Metal/MTLComputeCommandEncoder/updateFence(_:)) method after encoding commands that later passes depend on.

The GPU driver evaluates the fences that apply to the pass and the commands that depend on those fences when your app commits the enclosing [`MTLCommandBuffer`](/documentation/Metal/MTLCommandBuffer).

> Warning:
> Don’t update a fence and then wait for the same fence within a pass because it can create a GPU deadlock.

---

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