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UNIX(4) | Device Drivers Manual | UNIX(4) |
NAME¶
unix — UNIX-domain protocol familySYNOPSIS¶
#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 theSOCK_STREAM
and SOCK_DGRAM
socket types and uses file system pathnames for addressing.
ADDRESSING¶
UNIX-domain addresses are variable-length file system 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]; };
NUL
character to be used with
SUN_LEN(), but the terminating NUL
is not part of the address.
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 file system 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 theSOCK_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) 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.
UNIX domain sockets support a number of socket options which can be set with
setsockopt(2) and tested with
getsockopt(2):
LOCAL_CREDS
- This option may be enabled on a
SOCK_DGRAM
or aSOCK_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 structure, defined in <sys/socket.h> as follows:struct sockcred { 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 */ };
cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups)) cmsg_level = SOL_SOCKET cmsg_type = SCM_CREDS
LOCAL_CONNWAIT
- Used with
SOCK_STREAM
sockets, this option causes the connect(2) function to block until accept(2) has been called on the listening socket.
SEE ALSO¶
socket(2), intro(4) An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7. An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.July 15, 2001 | Debian |