---
title: mbuf_attachcluster
framework: Kernel
role: symbol
role_heading: Function
platforms: [macOS 10.4+]
path: kernel/1535766-mbuf_attachcluster
---

# mbuf_attachcluster

## Declaration

```occ
errno_t mbuf_attachcluster(mbuf_how_t how, mbuf_type_t type, mbuf_t *mbuf, caddr_t extbuf, void (*extfree)(caddr_t, u_int, caddr_t), size_t extsize, caddr_t extarg);
```

## Parameters

- `how`: Blocking or non-blocking.
- `type`: The type of the mbuf if mbuf is non-NULL; otherwise ignored.
- `mbuf`: Pointer to the address of the mbuf; if NULL, an mbuf will be allocated, otherwise, it must point to a valid mbuf address. If the user-supplied mbuf is already attached to a cluster, the current cluster will be freed before the mbuf gets attached to the supplied external buffer. Note that this routine may return a different mbuf_t than the one you passed in.
- `extbuf`: Address of the external buffer.
- `extfree`: Free routine for the external buffer; the caller is required to defined a routine that will be invoked when the mbuf is freed.
- `extsize`: Size of the external buffer.
- `extarg`: Private value that will be passed to the free routine when it is called at the time the mbuf is freed.

## Return Value

Return Value 0 on success EINVAL - Invalid parameter ENOMEM - Not enough memory available

## Discussion

Discussion Attach an external buffer as a cluster for an mbuf. If mbuf points to a NULL mbuf_t, an mbuf will be allocated for you. If mbuf points to a non-NULL mbuf_t, the user-supplied mbuf will be used instead. The caller is responsible for allocating the external buffer by calling mbuf_alloccluster().