---
title: sf_listen_func
framework: Kernel
role: symbol
role_heading: Type Alias
platforms: [macOS 10.4+]
path: kernel/sf_listen_func
---

# sf_listen_func

## Declaration

```occ
typedef errno_t (*sf_listen_func)(void *cookie, socket_t so);
```

## Parameters

- `cookie`: Cookie value specified when the filter attach was called.
- `so`: The socket the filter is attached to.

## Return Value

Return Value Return: 0 - The caller will continue with normal processing of listen. EJUSTRETURN - The caller will return with a value of 0 (no error) from that point without further processing the listen command. The protocol will not see the call. Anything Else - The caller will stop processing and return this error.

## Discussion

Discussion sf_listen_func is called before performing listen on a socket.