---
name: "unix"
section: "4"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/unix/4"
---

# unix(4)

UNIX-domain protocol family

## NAME

**unix** – UNIX-domain protocol family

## SYNOPSIS

**#include \<sys/types.h\>**

**#include \<sys/un.h\>**

## DESCRIPTION

The UNIX-domain protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal [socket(2)](/socket/2.md) mechanisms. The  UNIX-domain family supports the SOCK\_STREAM and SOCK\_DGRAM socket types and uses filesystem pathnames for addressing.

## ADDRESSING

UNIX-domain addresses are variable-length filesystem pathnames of at most 104 characters. The include file ⟨*sys/un.h*⟩ defines this address:

```
struct sockaddr_un {
	u_char	sun_len;
	u_char	sun_family;
	char	sun_path[104];
};
```

Binding a name to a UNIX-domain socket with [bind(2)](/bind/2.md) causes a socket file to be created in the filesystem. This file is *not* removed when the socket is closed—[unlink(2)](/unlink/2.md) must be used to remove the file.

The UNIX-domain protocol family does not support broadcast addressing or any form of “wildcard” matching on incoming messages.  All addresses are absolute- or relative-pathnames of other UNIX-domain sockets. Normal filesystem access-control mechanisms are also applied when referencing pathnames; e.g., the destination of a [connect(2)](/connect/2.md) or [sendto(2)](/sendto/2.md) must be writable.

## PROTOCOLS

The  UNIX-domain protocol family is comprised of simple transport protocols that support the SOCK\_STREAM and SOCK\_DGRAM abstractions. SOCK\_STREAM sockets also support the communication of  UNIX file descriptors through the use of the *msg\_control* field in the *msg* argument to [sendmsg(2)](/sendmsg/2.md) and [recvmsg(2)](/recvmsg/2.md).

Any valid descriptor may be sent in a message. The file descriptor(s) to be passed are described using a  *struct* *cmsghdr* that is defined in the include file ⟨*sys/socket.h*⟩. The type of the message is SCM\_RIGHTS, and the data portion of the messages is an array of integers representing the file descriptors to be passed. The number of descriptors being passed is defined by the length field of the message; the length field is the sum of the size of the header plus the size of the array of file descriptors.

The received descriptor is a  *duplicate* of the sender's descriptor, as if it were created with a call to [dup(2)](/dup/2.md). Per-process descriptor flags, set with [fcntl(2)](/fcntl/2.md), are  *not* passed to a receiver. Descriptors that are awaiting delivery, or that are purposely not received, are automatically closed by the system when the destination socket is closed.

The effective credentials (i.e., the user ID and group list) the of a peer on a SOCK\_STREAM socket may be obtained using the LOCAL\_PEERCRED socket option. This may be used by a server to obtain and verify the credentials of its client, and vice versa by the client to verify the credentials of the server. These will arrive in the form of a filled in *struct* *xucred* (defined in *sys/ucred.h*). The credentials presented to the server (the [listen(2)](/listen/2.md) caller) are those of the client when it called [connect(2)](/connect/2.md); the credentials presented to the client (the [connect(2)](/connect/2.md) caller) are those of the server when it called [listen(2)](/listen/2.md). This mechanism is reliable; there is no way for either party to influence the credentials presented to its peer except by calling the appropriate system call (e.g., [connect(2)](/connect/2.md) or [listen(2)](/listen/2.md)) under different effective credentials.

## SEE ALSO

[socket(2)](/socket/2.md), [intro(4)](/intro/4.md) “An Introductory 4.3 BSD Interprocess Communication Tutorial”, *PS1*, 7. “An Advanced 4.3 BSD Interprocess Communication Tutorial”, *PS1*, 8.