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# CFMessagePortGetInvalidationCallBack(_:)

Returns the invalidation callback function for a CFMessagePort object.

```
func CFMessagePortGetInvalidationCallBack(_ ms: CFMessagePort!) -> CFMessagePortInvalidationCallBack!
```

## Parameters

`ms`

The message port to examine.

## Return Value

The callback function invoked when `ms` is invalidated. `NULL` if no callback has been set with [`CFMessagePortSetInvalidationCallBack(_:_:)`](/documentation/CoreFoundation/CFMessagePortSetInvalidationCallBack(_:_:)).

## Discussion

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