--- name: "unix" section: "4" platform: "linux" package: "freebsd-manpages" version: "6.0-5" release: "Debian 4.0 (Etch)" 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 \** **#include \** ## 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 file system pathnames for addressing. ## ADDRESSING UNIX-domain addresses are variable-length file system 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 file system. This file is *not* removed when the socket is closed — [unlink(2)](/unlink/2.md) must be used to remove the file. The length of UNIX-domain address, required by [bind(2)](/bind/2.md) and [connect(2)](/connect/2.md), can be calculated by the macro **SUN\_LEN**() defined in ⟨sys/un.h⟩. The *sun\_path* field must be terminated by a NUL character to be used with **SUN\_LEN**(), but the terminating NUL is *not* part of the address. 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 file system 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) 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. UNIX domain sockets support a number of socket options which can be set with [setsockopt(2)](/setsockopt/2.md) and tested with [getsockopt(2)](/getsockopt/2.md): - LOCAL\_CREDS This option may be enabled on a SOCK\_DGRAM or a SOCK\_STREAM socket. This option provides a mechanism for the receiver to receive the credentials of the process as a [recvmsg(2)](/recvmsg/2.md) control message. The *msg\_control* field in the *msghdr* structure points to a buffer that contains a *cmsghdr* structure followed by a variable length *sockcred* structure, defined in ⟨sys/socket.h⟩ as follows: ``` struct sockcred { id_t sc_uid; /* real user id */ uid_t sc_euid; /* effective user id */ gid_t sc_gid; /* real group id */ gid_t sc_egid; /* effective group id */ int sc_ngroups; /* number of supplemental groups */ gid_t sc_groups[1]; /* variable length */ }; ``` The **SOCKCREDSIZE**() macro computes the size of the *sockcred* structure for a specified number of groups. The *cmsghdr* fields have the following values: ``` cmsg_len = sizeof(struct cmsghdr) + SOCKCREDSIZE(ngroups) cmsg_level = SOL_SOCKET cmsg_type = SCM_CREDS ``` - LOCAL\_CONNWAIT Used with SOCK\_STREAM sockets, this option causes the [connect(2)](/connect/2.md) function to block until [accept(2)](/accept/2.md) has been called on the listening socket. ## 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.