UNIX

NAME

SYNOPSIS

unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);  

DESCRIPTION

Valid types are: SOCK_STREAM, for a stream-oriented socket and 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 sockets support passing file descriptors or process credentials to other processes using ancillary data.  

Address Format

#define UNIX_PATH_MAX    108

struct sockaddr_un {
    sa_family_t sun_family;               /* AF_UNIX */
    char        sun_path[UNIX_PATH_MAX];  /* pathname */
};

sun_family always contains AF_UNIX.

Three types of address are distinguished in this structure:

*

pathname: a Unix domain socket can be bound to a null-terminated file system pathname using bind(2). When the address of the socket is returned by getsockname(2), getpeername(2), and accept(2), its length is sizeof(sa_family_t) + strlen(sun_path) + 1, and sun_path contains the null-terminated pathname.

*

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 by getsockname(2), getpeername(2), and accept(2), its length is sizeof(sa_family_t), and sun_path should not be inspected.

*

abstract: an abstract socket address is distinguished by the fact that sun_path[0] is a null byte ('\0'). All of the remaining bytes in sun_path define the "name" of the socket. (Null bytes in the name have no special significance.) The name has no connection with file system pathnames. The socket's address in this namespace is given by the rest of the bytes in sun_path. When the address of an abstract socket is returned by getsockname(2), getpeername(2), and accept(2), its length is sizeof(struct sockaddr_un), and sun_path contains the abstract name. The abstract socket namespace is a non-portable Linux extension.

Socket Options

SO_PASSCRED

Enables the receiving of the credentials of the sending process 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.

Sockets API

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

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 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.

ERRORS

EADDRINUSE

Selected local address is already taken or file system socket object already exists.

ECONNREFUSED

connect(2) called with a socket object that isn't listening. This can happen when the remote socket does not exist or the filename 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 the missing setting of AF_UNIX in the sun_type field of passed addresses or the socket being 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.

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 vs. SOCK_STREAM)

ESOCKTNOSUPPORT

Unknown socket type.

Other errors can be generated by the generic socket layer or by the file system while generating a file system socket object. See the appropriate manual pages for more information.  

VERSIONS

NOTES

Binding to a socket with a filename creates a socket in the file system 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 file system 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 non-ancillary data in the same sendmsg(2) or recvmsg(2) call.

Unix domain stream sockets do not support the notion of out-of-band data.  

EXAMPLE

SEE ALSO

COLOPHON