UNIX(4) NetBSD Kernel Interfaces 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 pro- vides 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 length of UNIX-domain address, required by bind(2) and connect(2), 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 NetBSD kernel ignores any user-set value in the sun_len member of the structure. 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 comprises 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). Supported file descriptors may be sent in control messages of type SCM_RIGHTS holding an array of int file descriptor objects; see cmsg(3) for details on assembling and examining control messages. A file descriptor received this way 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. A UNIX-domain socket supports the following socket options for use with setsockopt(2) and getsockopt(2) at the level SOL_LOCAL: LOCAL_CONNWAIT (int) May be enabled with setsockopt(2) on a SOCK_SEQPACKET or SOCK_STREAM socket. If enabled, connect(2) will block until a peer is waiting in accept(2). LOCAL_CREDS (int) May be enabled with setsockopt(2) on a SOCK_DGRAM, SOCK_SEQPACKET, or a SOCK_STREAM socket. This option provides a mechanism for the receiver to receive the credentials of the process as a recvmsg(2) 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 struc- ture, defined in <sys/socket.h> as follows: struct sockcred { pid_t sc_pid; /* process id */ uid_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 struc- ture for a specified number of groups. The cmsghdr fields have the following values: cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups)) cmsg_level = SOL_SOCKET cmsg_type = SCM_CREDS LOCAL_PEEREID (struct unpcbid) May be queried with getsockopt(2) to get the PID and effective user and group IDs of a SOCK_STREAM or SOCK_SEQPACKET peer when it did connect(2) or bind(2). The returned structure is struct unpcbid { pid_t unp_pid; /* process id */ uid_t unp_euid; /* effective user id */ gid_t unp_egid; /* effective group id */ }; as defined in <sys/un.h>.
EXAMPLES
The following code fragment shows how to bind a socket to pathname: const char *pathname = "/path/to/socket"; struct sockaddr_un addr; int ret; memset(&addr, 0, sizeof(addr)); addr.sun_family = AF_LOCAL; if (strlen(pathname) >= sizeof(addr.sun_path)) goto too_long; strncpy(addr.sun_path, pathname, sizeof(addr.sun_path)); ret = bind(s, (const struct sockaddr *)&addr, SUN_LEN(&addr)); if (ret != 0) goto bind_failed; ...
COMPATIBILITY
The sun_len field exists only in system derived from 4.4BSD. On systems which don't have the SUN_LEN() macro, the following definition is recom- mended: #ifndef SUN_LEN #define SUN_LEN(su) sizeof(struct(sockaddr_un)) #endif
SEE ALSO
socket(2), CMSG_DATA(3), intro(4) Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication Tutorial. (see /usr/share/doc/reference/ref3/sockets) Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve Miller, and Chris Torek, Advanced 4.4BSD IPC Tutorial. (see /usr/share/doc/reference/ref3/sockets-advanced)
HISTORY
The sc_pid field was introduced in NetBSD 8.0. NetBSD 10.99 June 28, 2022 NetBSD 10.99
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