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NAME
unix - sockets for local interprocess communication
SYNOPSIS
#include <sys/socket.h>
#include <sys/un.h>
unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);
DESCRIPTION
The AF_UNIX (also known as AF_LOCAL) socket family is used to communicate between processes on the same
machine efficiently. Traditionally, UNIX domain sockets can be either unnamed, or bound to a filesystem
pathname (marked as being of type socket). Linux also supports an abstract namespace which is
independent of the filesystem.
Valid socket types in the UNIX domain are: SOCK_STREAM, for a stream-oriented socket; SOCK_DGRAM, for a
datagram-oriented socket that preserves message boundaries (as on most UNIX implementations, UNIX domain
datagram sockets are always reliable and don't reorder datagrams); and (since Linux 2.6.4)
SOCK_SEQPACKET, for a connection-oriented socket that preserves message boundaries and delivers messages
in the order that they were sent.
UNIX domain sockets support passing file descriptors or process credentials to other processes using
ancillary data.
Address format
A UNIX domain socket address is represented in the following structure:
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
char sun_path[108]; /* pathname */
};
The sun_family field always contains AF_UNIX. On Linux sun_path is 108 bytes in size; see also NOTES,
below.
Various systems calls (for example, bind(2), connect(2), and sendto(2)) take a sockaddr_un argument as
input. Some other system calls (for example, getsockname(2), getpeername(2), recvfrom(2), and accept(2))
return an argument of this type.
Three types of address are distinguished in the sockaddr_un structure:
* pathname: a UNIX domain socket can be bound to a null-terminated filesystem pathname using bind(2).
When the address of a pathname socket is returned (by one of the system calls noted above), its length
is
offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
and sun_path contains the null-terminated pathname. (On Linux, the above offsetof() expression
equates to the same value as sizeof(sa_family_t), but some other implementations include other fields
before sun_path, so the offsetof() expression more portably describes the size of the address
structure.)
For further details of pathname sockets, see below.
* unnamed: A stream socket that has not been bound to a pathname using bind(2) has no name. Likewise,
the two sockets created by socketpair(2) are unnamed. When the address of an unnamed socket is
returned, its length is sizeof(sa_family_t), and sun_path should not be inspected.
* abstract: an abstract socket address is distinguished (from a pathname socket) by the fact that
sun_path[0] is a null byte ('\0'). The socket's address in this namespace is given by the additional
bytes in sun_path that are covered by the specified length of the address structure. (Null bytes in
the name have no special significance.) The name has no connection with filesystem pathnames. When
the address of an abstract socket is returned, the returned addrlen is greater than
sizeof(sa_family_t) (i.e., greater than 2), and the name of the socket is contained in the first
(addrlen - sizeof(sa_family_t)) bytes of sun_path. The abstract socket namespace is a nonportable
Linux extension.
Pathname sockets
When binding a socket to a pathname, a few rules should be observed for maximum portability and ease of
coding:
* The pathname in sun_path should be null-terminated.
* The length of the pathname, including the terminating null byte, should not exceed the size of
sun_path.
* The addrlen argument that describes the enclosing sockaddr_un structure should have a value of at
least:
offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
or, more simply, addrlen can be specified as sizeof(struct sockaddr_un).
There is some variation in how implementations handle UNIX domain socket addresses that do not follow the
above rules. For example, some (but not all) implementations append a null terminator if none is present
in the supplied sun_path.
When coding portable applications, keep in mind that some implementations have sun_path as short as 92
bytes.
Various system calls (accept(2), recvfrom(2), getsockname(2), getpeername(2)) return socket address
structures. When applied to UNIX domain sockets, the value-result addrlen argument supplied to the call
should be initialized as above. Upon return, the argument is set to indicate the actual size of the
address structure. The caller should check the value returned in this argument: if the output value
exceeds the input value, then there is no guarantee that a null terminator is present in sun_path. (See
BUGS.)
Socket options
For historical reasons, these socket options are specified with a SOL_SOCKET type even though they are
AF_UNIX specific. They can be set with setsockopt(2) and read with getsockopt(2) by specifying
SOL_SOCKET as the socket family.
SO_PASSCRED
Enables the receiving of the credentials of the sending process in an ancillary message. When
this option is set and the socket is not yet connected a unique name in the abstract namespace
will be generated automatically. Expects an integer boolean flag.
Autobind feature
If a bind(2) call specifies addrlen as sizeof(sa_family_t), or the SO_PASSCRED socket option was
specified for a socket that was not explicitly bound to an address, then the socket is autobound to an
abstract address. The address consists of a null byte followed by 5 bytes in the character set [0-9a-f].
Thus, there is a limit of 2^20 autobind addresses. (From Linux 2.1.15, when the autobind feature was
added, 8 bytes were used, and the limit was thus 2^32 autobind addresses. The change to 5 bytes came in
Linux 2.3.15.)
Sockets API
The following paragraphs describe domain-specific details and unsupported features of the sockets API for
UNIX domain sockets on Linux.
UNIX domain sockets do not support the transmission of out-of-band data (the MSG_OOB flag for send(2) and
recv(2)).
The send(2) MSG_MORE flag is not supported by UNIX domain sockets.
The use of MSG_TRUNC in the flags argument of recv(2) is not supported by UNIX domain sockets.
The SO_SNDBUF socket option does have an effect for UNIX domain sockets, but the SO_RCVBUF option does
not. For datagram sockets, the SO_SNDBUF value imposes an upper limit on the size of outgoing datagrams.
This limit is calculated as the doubled (see socket(7)) option value less 32 bytes used for overhead.
Ancillary messages
Ancillary data is sent and received using sendmsg(2) and recvmsg(2). For historical reasons the
ancillary message types listed below are specified with a SOL_SOCKET type even though they are AF_UNIX
specific. To send them set the cmsg_level field of the struct cmsghdr to SOL_SOCKET and the cmsg_type
field to the type. For more information see cmsg(3).
SCM_RIGHTS
Send or receive a set of open file descriptors from another process. The data portion contains an
integer array of the file descriptors. The passed file descriptors behave as though they have
been created with dup(2).
SCM_CREDENTIALS
Send or receive UNIX credentials. This can be used for authentication. The credentials are
passed as a struct ucred ancillary message. Thus structure is defined in <sys/socket.h> as
follows:
struct ucred {
pid_t pid; /* process ID of the sending process */
uid_t uid; /* user ID of the sending process */
gid_t gid; /* group ID of the sending process */
};
Since glibc 2.8, the _GNU_SOURCE feature test macro must be defined (before including any header
files) in order to obtain the definition of this structure.
The credentials which the sender specifies are checked by the kernel. A process with effective
user ID 0 is allowed to specify values that do not match its own. The sender must specify its own
process ID (unless it has the capability CAP_SYS_ADMIN), its user ID, effective user ID, or saved
set-user-ID (unless it has CAP_SETUID), and its group ID, effective group ID, or saved set-group-
ID (unless it has CAP_SETGID). To receive a struct ucred message the SO_PASSCRED option must be
enabled on the socket.
Ioctls
The following ioctl(2) calls return information in value. The correct syntax is:
int value;
error = ioctl(unix_socket, ioctl_type, &value);
ioctl_type can be:
SIOCINQ
For SOCK_STREAM socket the function returns the amount of queued unread data in the receive
buffer. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned. SIOCINQ
is defined in <linux/sockios.h>. Alternatively, you can use the synonymous FIONREAD, defined in
<sys/ioctl.h>. For SOCK_DGRAM socket, the returned value is the same as for Internet domain
datagram socket; see udp(7).
ERRORS
EADDRINUSE
The specified local address is already in use or the filesystem socket object already exists.
ECONNREFUSED
The remote address specified by connect(2) was not a listening socket. This error can also occur
if the target pathname is not a socket.
ECONNRESET
Remote socket was unexpectedly closed.
EFAULT User memory address was not valid.
EINVAL Invalid argument passed. A common cause is that the value AF_UNIX was not specified in the
sun_type field of passed addresses, or the socket was in an invalid state for the applied
operation.
EISCONN
connect(2) called on an already connected socket or a target address was specified on a connected
socket.
ENOENT The pathname in the remote address specified to connect(2) did not exist.
ENOMEM Out of memory.
ENOTCONN
Socket operation needs a target address, but the socket is not connected.
EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried to use the out-of-band data option.
EPERM The sender passed invalid credentials in the struct ucred.
EPIPE Remote socket was closed on a stream socket. If enabled, a SIGPIPE is sent as well. This can be
avoided by passing the MSG_NOSIGNAL flag to sendmsg(2) or recvmsg(2).
EPROTONOSUPPORT
Passed protocol is not AF_UNIX.
EPROTOTYPE
Remote socket does not match the local socket type (SOCK_DGRAM versus SOCK_STREAM)
ESOCKTNOSUPPORT
Unknown socket type.
Other errors can be generated by the generic socket layer or by the filesystem while generating a
filesystem socket object. See the appropriate manual pages for more information.
VERSIONS
SCM_CREDENTIALS and the abstract namespace were introduced with Linux 2.2 and should not be used in
portable programs. (Some BSD-derived systems also support credential passing, but the implementation
details differ.)
NOTES
In the Linux implementation, sockets which are visible in the filesystem honor the permissions of the
directory they are in. Their owner, group, and permissions can be changed. Creation of a new socket
will fail if the process does not have write and search (execute) permission on the directory the socket
is created in. Connecting to the socket object requires read/write permission. This behavior differs
from many BSD-derived systems which ignore permissions for UNIX domain sockets. Portable programs should
not rely on this feature for security.
Binding to a socket with a filename creates a socket in the filesystem that must be deleted by the caller
when it is no longer needed (using unlink(2)). The usual UNIX close-behind semantics apply; the socket
can be unlinked at any time and will be finally removed from the filesystem when the last reference to it
is closed.
To pass file descriptors or credentials over a SOCK_STREAM, you need to send or receive at least one byte
of nonancillary data in the same sendmsg(2) or recvmsg(2) call.
UNIX domain stream sockets do not support the notion of out-of-band data.
BUGS
When binding a socket to an address, Linux is one of the implementations that appends a null terminator
if none is supplied in sun_path. In most cases this is unproblematic: when the socket address is
retrieved, it will be one byte longer than that supplied when the socket was bound. However, there is
one case where confusing behavior can result: if 108 non-null bytes are supplied when a socket is bound,
then the addition of the null terminator takes the length of the pathname beyond sizeof(sun_path).
Consequently, when retrieving the socket address (for example, via accept(2)), if the input addrlen
argument for the retrieving call is specified as sizeof(struct sockaddr_un), then the returned address
structure won't have a null terminator in sun_path.
In addition, some implementations don't require a null terminator when binding a socket (the addrlen
argument is used to determine the length of sun_path) and when the socket address is retrieved on these
implementations, there is no null terminator in sun_path.
Applications that retrieve socket addresses can (portably) code to handle the possibility that there is
no null terminator in sun_path by respecting the fact that the number of valid bytes in the pathname is:
strnlen(addr.sun_path, addrlen - offsetof(sockaddr_un, sun_path))
Alternatively, an application can retrieve the socket address by allocating a buffer of size
sizeof(struct sockaddr_un)+1 that is zeroed out before the retrieval. The retrieving call can specify
addrlen as sizeof(struct sockaddr_un), and the extra zero byte ensures that there will be a null
terminator for the string returned in sun_path:
void *addrp;
addrlen = sizeof(struct sockaddr_un);
addrp = malloc(addrlen + 1);
if (addrp == NULL)
/* Handle error */ ;
memset(addrp, 0, addrlen + 1);
if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
/* handle error */ ;
printf("sun_path = %s\n", ((struct sockaddr_un *) addrp)->sun_path);
This sort of messiness can be avoided if it is guaranteed that the applications that create pathname
sockets follow the rules outlined above under Pathname sockets.
EXAMPLE
See bind(2). For an example of the use of SCM_RIGHTS see cmsg(3).
SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7), udp(7)
COLOPHON
This page is part of release 4.04 of the Linux man-pages project. A description of the project,
information about reporting bugs, and the latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.