UNIX(4) | Device Drivers Manual | UNIX(4)

# 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) mechanisms. The UNIX-domain family supports the `SOCK_STREAM`, `SOCK_SEQPACKET`, 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) causes a socket file to be created in the filesystem. This file is not removed when the socket is closed—unlink(2) 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) or sendto(2) must be writable.

# PROTOCOLS

The UNIX-domain protocol family is comprised of simple transport protocols that support the `SOCK_STREAM`, `SOCK_SEQPACKET`, and `SOCK_DGRAM` abstractions. `SOCK_STREAM` and `SOCK_SEQPACKET` sockets also support the communication of UNIX file descriptors through the use of the msg_control field in the msg argument to sendmsg(2) and recvmsg(2).

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). Per-process descriptor flags, set with fcntl(2), 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.

# SEE ALSO

socket(2), netintro(4)

S. Sechrest, An Introductory 4.4BSD Interprocess Communication Tutorial, 4.4BSD Programmer's Supplementary Documents (PSD).

S. J. Leffler, R. S. Fabry, W. N. Joy, P. Lapsley, S. Miller, and C. Torek, An Advanced 4.4BSD Interprocess Communication Tutorial, 4.4BSD Programmer's Supplementary Documents (PSD).

August 21, 2021 | Debian