---
title: perform
framework: corefoundation
role: symbol
role_heading: Instance Property
path: corefoundation/cfrunloopsourcecontext/perform
---

# perform

A perform callback for the run loop source. This callback is called when the source has fired.

## Declaration

```swift
var perform: ((UnsafeMutableRawPointer?) -> Void)!
```

## Parameters

- `msg`: The Mach message received on the Mach port. The pointer is to a mach_msg_header_t structure. A version 0 format trailer (mach_msg_format_0_trailer_t) is at the end of the Mach message.
- `size`: Size of the Mach message in msg, excluding the message trailer.
- `allocator`: The allocator object that should be used to allocate a reply message.
- `info`: The info member of the doc://com.apple.corefoundation/documentation/CoreFoundation/CFRunLoopSourceContext1 structure that was used when creating the run loop source.

## Return Value

Return Value An optional Mach message to be sent in response to the received message. The message must be allocated using allocator. Return NULL if you want an empty reply returned to the sender.

## Discussion

Discussion You only need to provide this callback if you create your own version 1 run loop source. CFMachPort and CFMessagePort run loop sources already implement this callback to forward the received message to the CFMachPort’s or CFMessagePort’s own callback function, which you do need to implement.

## See Also

### Callbacks

- [cancel](corefoundation/cfrunloopsourcecontext/cancel.md)
- [equal](corefoundation/cfrunloopsourcecontext/equal.md)
- [hash](corefoundation/cfrunloopsourcecontext/hash.md)
- [schedule](corefoundation/cfrunloopsourcecontext/schedule.md)