1UNIX(7) Linux Programmer's Manual UNIX(7)
2
6 unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local inter‐
7 process communication
8
10 #include <sys/socket.h>
11 #include <sys/un.h>
12 unix_socket = socket(PF_UNIX, type, 0);
14 error = socketpair(PF_UNIX, type, 0, int *sv);
15
18 The PF_UNIX (also known as PF_LOCAL) socket family is used to communi‐
19 cate between processes on the same machine efficiently. Unix sockets
20 can be either anonymous (created by socketpair(2)) or associated with a
21 file of type socket. Linux also supports an abstract namespace which
22 is independent of the file system.
23 Valid types are: SOCK_STREAM, for a stream-oriented socket and
25 SOCK_DGRAM, for a datagram-oriented socket that preserves message
26 boundaries (as on most Unix implementations, Unix domain datagram sock‐
27 ets are always reliable and don't reorder datagrams); and (since kernel
28 2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that preserves
29 message boundaries and delivers messages in the order that they were
30 sent.
31 Unix sockets support passing file descriptors or process credentials to
33 other processes using ancillary data.
34
37 A Unix address is defined as a filename in the filesystem or as a
38 unique string in the abstract namespace. Sockets created by socket‐
39 pair(2) are anonymous. For non-anonymous sockets the target address can
40 be set using connect(2). The local address can be set using bind(2).
41 When a socket is connected and it doesn't already have a local address
42 a unique address in the abstract namespace will be generated automati‐
43 cally.
44 #define UNIX_PATH_MAX 108
46 struct sockaddr_un {
48 sa_family_t sun_family; /* AF_UNIX */
49 char sun_path[UNIX_PATH_MAX]; /* pathname */
50 };
51 sun_family always contains AF_UNIX. sun_path contains the zero-termi‐
53 nated pathname of the socket in the file system. If sun_path starts
54 with a null byte ('' '), then it refers to the abstract namespace main‐
55 tained by the Unix protocol module. The socket's address in this
56 namespace is given by the rest of the bytes in sun_path. Note that
57 names in the abstract namespace are not zero-terminated.
58
61 For historical reasons these socket options are specified with a
62 SOL_SOCKET type even though they are PF_UNIX specific. They can be set
63 with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
64 as the socket family.
65 SO_PASSCRED
67 Enables the receiving of the credentials of the sending process
68 ancillary message. When this option is set and the socket is
69 not yet connected a unique name in the abstract namespace will
70 be generated automatically. Expects an integer boolean flag.
71
74 The following paragraphs describe domain-specific details and unsup‐
75 ported features of the sockets API for Unix domain sockets on Linux.
76 Unix domain sockets do not support the transmission of out-of-band data
78 (the MSG_OOB flag for send(2) and recv(2)).
79 The send(2) MSG_MORE flag is not supported by Unix domain sockets.
81 The SO_SNDBUF socket option does have an effect for Unix domain sock‐
83 ets, but the SO_RCVBUF option does not. For datagram sockets, the
84 SO_SNDBUF value imposes an upper limit on the size of outgoing data‐
85 grams. This limit is calculated as the doubled (see socket(7)) option
86 value less 32 bytes used for overhead.
87
90 Ancillary data is sent and received using sendmsg(2) and recvmsg(2).
91 For historical reasons the ancillary message types listed below are
92 specified with a SOL_SOCKET type even though they are PF_UNIX specific.
93 To send them set the cmsg_level field of the struct cmsghdr to
94 SOL_SOCKET and the cmsg_type field to the type. For more information
95 see cmsg(3).
96 SCM_RIGHTS
99 Send or receive a set of open file descriptors from another
100 process. The data portion contains an integer array of the file
101 descriptors. The passed file descriptors behave as though they
102 have been created with dup(2).
103 SCM_CREDENTIALS
106 Send or receive Unix credentials. This can be used for authen‐
107 tication. The credentials are passed as a struct ucred ancil‐
108 lary message.
109 struct ucred {
111 pid_t pid; /* process ID of the sending process */
112 uid_t uid; /* user ID of the sending process */
113 gid_t gid; /* group ID of the sending process */
114 };
115 The credentials which the sender specifies are checked by the
117 kernel. A process with effective user ID 0 is allowed to spec‐
118 ify values that do not match its own. The sender must specify
119 its own process ID (unless it has the capability CAP_SYS_ADMIN),
120 its user ID, effective user ID, or saved set-user-ID (unless it
121 has CAP_SETUID), and its group ID, effective group ID, or saved
122 set-group-ID (unless it has CAP_SETGID). To receive a struct
123 ucred message the SO_PASSCRED option must be enabled on the
124 socket.
125
128 SCM_CREDENTIALS and the abstract namespace were introduced with Linux
129 2.2 and should not be used in portable programs. (Some BSD-derived
130 systems also support credential passing, but the implementation details
131 differ.)
132
135 In the Linux implementation, sockets which are visible in the filesys‐
136 tem honour the permissions of the directory they are in. Their owner,
137 group and their permissions can be changed. Creation of a new socket
138 will fail if the process does not have write and search (execute) per‐
139 mission on the directory the socket is created in. Connecting to the
140 socket object requires read/write permission. This behavior differs
141 from many BSD-derived systems which ignore permissions for Unix sock‐
142 ets. Portable programs should not rely on this feature for security.
143 Binding to a socket with a filename creates a socket in the file system
145 that must be deleted by the caller when it is no longer needed (using
146 unlink(2)). The usual Unix close-behind semantics apply; the socket
147 can be unlinked at any time and will be finally removed from the file
148 system when the last reference to it is closed.
149 To pass file descriptors or credentials over a SOCK_STREAM, you need to
151 send or receive at least one byte of non-ancillary data in the same
152 sendmsg() or recvmsg() call.
153 Unix domain stream sockets do not support the notion of out-of-band
155 data.
156
158 ENOMEM Out of memory.
159 ECONNREFUSED
161 connect(2) called with a socket object that isn't listening.
162 This can happen when the remote socket does not exist or the
163 filename is not a socket.
164 EINVAL Invalid argument passed. A common cause is the missing setting
166 of AF_UNIX in the sun_type field of passed addresses or the
167 socket being in an invalid state for the applied operation.
168 EOPNOTSUPP
170 Stream operation called on non-stream oriented socket or tried
171 to use the out-of-band data option.
172 EPROTONOSUPPORT
174 Passed protocol is not PF_UNIX.
175 ESOCKTNOSUPPORT
177 Unknown socket type.
178 EPROTOTYPE
180 Remote socket does not match the local socket type (SOCK_DGRAM
181 vs. SOCK_STREAM)
182 EADDRINUSE
184 Selected local address is already taken or filesystem socket
185 object already exists.
186 EISCONN
188 connect(2) called on an already connected socket or a target
189 address was specified on a connected socket.
190 ENOTCONN
192 Socket operation needs a target address, but the socket is not
193 connected.
194 ECONNRESET
196 Remote socket was unexpectedly closed.
197 EPIPE Remote socket was closed on a stream socket. If enabled, a SIG‐
199 PIPE is sent as well. This can be avoided by passing the
200 MSG_NOSIGNAL flag to sendmsg(2) or recvmsg(2).
201 EFAULT User memory address was not valid.
203 EPERM The sender passed invalid credentials in the struct ucred.
205 Other errors can be generated by the generic socket layer or by the
207 filesystem while generating a filesystem socket object. See the appro‐
208 priate manual pages for more information.
209
211 recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili‐
212 ties(7), socket(7)
213Linux Man Page 2004-05-27 UNIX(7)