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
provides local (on-machine) interprocess communication through the normal
socket(2)
mechanisms. The UNIX-domain family supports the
SOCK_STREAM 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 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 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) and recvmsg(2).
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). 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.
The effective credentials (i.e., the user ID and group list) the
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) caller) are those of the client when it called
connect(2); the credentials presented to the client (the
connect(2) caller) are those of the server when it called
listen(2).
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) or
listen(2))
under different effective credentials.
SEE ALSO
An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7.
An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.